We analyse ultradeep JWST observations of the galaxy JADESGSz90 at z  9.4327 and derive detailed stellar and interstellar medium ISM properties of this luminous MUV20.43 highredshift system. Complementary information from NIRCam imaging and NIRSpec both low and mediumresolution spectroscopy reveal a compact system Re 110 pc characterised by a steeply rising star formation history which is reflected in the inferred young stellar age t  3 Myr lightweighted high starformation rate surface density SigmaSFR  72 M yr1 kpc2 high ionisation parameter logU  1.5 low metallicity 12logOH  7.5 and low carbonoveroxygen abundance CO  0.64. Leveraging the detection of N iii1750 we derive nitrogenoveroxygen abundance NO  0 higher than the plateau followed by lowredshift galaxies of similar metallicity possibly revealing the imprint from very massive stars on the ISM enrichment and favouring a topheavy Initial Mass Function IMF scenario. Massive stars powering a hard radiation field are also required to explain the restframe UV line ratios though the presence of the highexcitation Ne vlambda3426 emission line possibly hints at additional ionization from an AGN. We also report the tentative detection of Lyalpha emission in the G140M spectrum shifted by 450 kms redward of the systemic redshift. Combined with a modelling of the Lyalpha spectral break we rule out the presence of very high column densities of neutral gas pertaining to local absorbers as well as any extended surrounding ionised bubble suggesting that JADESGSz90 has not yet significantly contributed to cosmic Reionization.
['Astrophysics - Astrophysics of Galaxies']
JADES The starformation and chemical enrichment history of a luminous galaxy at z9.43 probed by ultradeep JWSTNIRSpec spectroscopy
{'JADES: The star-formation and chemical enrichment history of a luminous galaxy at z ∼ 9 . 43 probed by ultra-deep JWST /NIRSpec spectroscopy': "Mirko Curti 1, ⋆ , Joris Witstok 2 , 3 , Peter Jakobsen 4 , 5 , Chiaki Kobayashi 6 , Emma Curtis-Lake 6 , Kevin Hainline 7 , Xihan Ji 2 , 3 , Francesco D'Eugenio 2 , 3 , 8 , Jacopo Chevallard 9 , Roberto Maiolino 2 , 3 , 10 , Jan Scholtz 11 , 12 , Stefano Carniani 13 , Santiago Arribas 14 , William M. Baker 2 , 3 , Rachana Bhatawdekar 15 , Kristan Boyett 16 , 17 , Andrew J. Bunker 9 , Alex Cameron 9 , Phillip A. Cargile 18 , Stéphane Charlot 19 , Daniel J. Eisenstein 18 , Zhiyuan Ji 7 , Benjamin D. Johnson 18 , Nimisha Kumari 20 , Michael V. Maseda 21 , Brant Robertson 22 , Maddie S. Silcock 6 , Sandro Tacchella 2 , 3 , Hannah Übler 2 , 3 , Giacomo Venturi 13 , Christina C. Williams 23 , Christopher N. A. Willmer 7 , and Chris Willott 24  \n(A ffi liations can be found after the references)", 'ABSTRACT': 'We analyse ultra-deep JWST observations of the galaxy JADES-GS-z9-0 at z = 9 . 4327, and derive detailed stellar and interstellar medium (ISM) properties of this luminous (MUV = -20.43) high-redshift system. Complementary information from NIRCam imaging and NIRSpec (both lowand medium-resolution) spectroscopy reveal a compact system (R e ∼ 110 pc) characterised by a steeply rising star formation history, which is reflected in the inferred young stellar age (t ∼ 3 Myr, light-weighted), high star-formation rate surface density ( Σ SFR ∼ 72 M ⊙ yr -1 kpc -2 ), high ionisation parameter (log(U) ∼ -1 . 5), low metallicity (12 + log(O / H) ∼ 7 . 5), and low carbon-over-oxygen abundance ([C / O] = -0 . 64). Leveraging the detection of N iii ] λ 1750 we derive nitrogen-over-oxygen abundance ([N / O] ∼ 0) higher than the plateau followed by low-redshift galaxies of similar metallicity, possibly revealing the imprint from (very) massive stars on the ISM enrichment and favouring a top-heavy Initial Mass Function (IMF) scenario. Massive stars powering a hard radiation field are also required to explain the rest-frame UV line ratios, though the presence of the high-excitation [Ne v ] λ 3426 emission line possibly hints at additional ionization from an AGN. We also report the tentative detection of Ly α emission in the G140M spectrum, shifted by ∼ 450 km / s redward of the systemic redshift. Combined with a modelling of the Ly α spectral break, we rule out the presence of very-high column densities of neutral gas pertaining to local absorbers, as well as any extended surrounding ionised bubble, suggesting that JADES-GS-z9-0 has not yet significantly contributed to cosmic Reionization.  \nKey words. galaxies: high-redshift - galaxies: evolution - galaxies: abundances', '1. Introduction': "The identification and characterisation of the earliest galaxies ever formed in the history of the Universe is one of the topics at the forefront of the current astrophysical research, and one of the main motivations behind the concept and development of the James Webb Space Telescope ( JWST ). Within already the first two cycles of operations, early results from extensive imaging and spectroscopic campaigns have not only pushed further the limits of the known redshift frontier (e.g. Robertson et al. 2023b; Curtis-Lake et al. 2023; Arrabal Haro et al. 2023; Hainline et al. 2024b; Castellano et al. 2024; Carniani et al. 2024), but marked unprecedented progress in the study of the physical properties of the early galaxy population, in terms of their number density (Harikane et al. 2024; Chemerynska et al. 2023; Robertson et al. 2023a; McLeod et al. 2024), star-formation histories (SFH) (Dressler et al. 2023; Endsley et al. 2023; Looser et al. 2023; Tacchella et al. 2023b; Clarke et al. 2024), interstellar medium (ISM) conditions (Sanders et al. 2023; Cameron et al. 2023b; Reddy et al. 2023; Calabro et al. 2024), incidence, growth, and impact of supermassive black holes (Harikane et al. 2023; Greene et al. 2024; Kokorev et al. 2024; Maiolino et al. 2023a; Scholtz et al. 2023; Übler et al. 2023; Matthee et al. 2024)  \nOne of the key advances provided by JWST / NIRSpec resides in the possibility to simultaneously cover rest-frame UV and rest-optical spectra of galaxies at z > 6. Even prior to the advent of the JWST , observations of rest-frame UV spectra in z > 6 galaxies had exhibited large equivalent widths (EW), high-ionization emission lines, as seen in none but the most extreme galaxies in the local Universe (Berg et al. 2019; Izotov et al. 2024), suggesting that extreme radiation fields characterise galaxies in the Epoch of Reionization (EoR) (Stark et al. 2015a,b; Mainali et al. 2017; Senchyna et al. 2017; Hutchison et al. 2019). Leveraging the wide spectral coverage of JWST / NIRSpec, it finally became feasible to combine diagnostic features pertaining to both spectral regions, to enable a more indepth characterisation of the underlying ionising spectrum (aiding in deciphering whether this originates from metal-poor stellar populations or requires instead the hardness typical of active galaxies), as well as of the conditions of the ionised gas which produces the bright emission lines as seen in high-z galaxy spectra, in terms of its density, ionization structure, and chemical enrichment. Despite the intrinsic weakness of rest-UV features still hampering the analysis of large galaxy samples, observations of some of the brightest sources have already revealed peculiar (and sometimes unexpected) ionization and chemical enrichment patterns (e.g. Bunker et al. 2023b; Maiolino et al. 2023b; Cameron et al. 2023a; Isobe et al. 2023b; Topping et al. 2024; D'Eugenio et al. 2023; Schaerer et al. 2024). Indeed, chemical abundances  \nprovide some of the most relevant observational constraints for galaxy formation and evolution models (Maiolino & Mannucci 2019). The di ff erent pathways in which heavy elements are produced by stars of di ff erent masses in fact are reflected in the differential timescales regulating the enrichment of the interstellar medium (ISM). Therefore, relative abundance ratios among different chemical species are powerful probes of the past history of mass assembly and star formation in galaxies.  \nIn a simple framework of galactic chemical evolution, α -elements like oxygen or neon are primarily produced by massive stars ( M ⋆ > 8 M ⊙ ) and returned to the ISM relatively quickly through core-collapse supernovae (SNe), on timescales of approximately 10 Myr. Although carbon is also generated in massive stars, the main production channel is associated with intermediate-mass asymptotic giant branch (AGB) stars (M ≈ 1-4 M ⊙ ) with lifetimes spanning from about 100 Myr to 10 Gyr (Kobayashi et al. 2011, 2020). Consequently, young, metal-poor galaxies with formation timescales of less than ∼ 100 Myr are expected to showcase a C / O abundance consistent with the predicted yields of core-collapse SNe, whereas C / O levels are expected to rise as galaxies evolve and become more metal-rich. The C / O abundance ratio is, therefore, a very valuable tracer of early galaxy formation due to its variations within relatively short timescales, and it is generally inferred from the ratios of rest-frame UV emission lines of carbon (C iii ] λλ 1907,1909, C iv λ 1550) and oxygen (O iii ] λ 1666). Complementary information is provided by the ratio between nitrogen and oxygen abundance (N / O) which, in the average population of local galaxies, is observed to follow a plateau at low N / O and low O / H representative of 'primary' nitrogen production mechanisms (i.e. with a yield independent of metallicity, e.g. Matteucci 1986; Chiappini et al. 2006), while increasing at higher metallicity due to the onset of the CNO cycle in low- and intermediate-mass stars and also the 'secondary' nitrogen production (i.e. where the nitrogen yield depends on the amount of carbon and oxygen already present within the star, e.g. Vincenzo & Kobayashi 2018).  \nWhile limited for decades almost exclusively up to intermediate redshifts ( z ≲ 3), detailed studies of chemical abundances in early galaxies have seen an unprecedented development following the advent of the JWST . This has enabled not only the characterisation of the metallicity scaling relations for the highredshift galaxy population (e.g. Nakajima et al. 2023; Curti et al. 2024; Langeroodi & Hjorth 2023), but has also allowed us to investigate the history of chemical enrichment in some of the earliest systems ever observed. Although some of the analysed galaxies appear in agreement with the expected behaviour as predicted by standard galactic chemical evolution models for young galaxies (e.g. Jones et al. 2023; Arellano-Córdova et al. 2022), others have shown peculiar patterns in their C / Oand N / O abundances, possibly revealing the signatures of enrichment processes occurring on short timescales in the earliest phases of galaxy formation, and which are not commonly observed in the typical galaxy population at lower redshift. This includes evidence for super-solar nitrogen enrichment in z > 5 galaxies (e.g. Isobe et al. 2023b; Ji et al. 2024b), and has been observed not only in the extremely luminous GN-z11 at z = 10 . 6 (Bunker et al. 2023b; Cameron et al. 2023a), but even at higher redshift in the galaxy GHz2 Castellano et al. (2024). Such observations have been interpreted as the possible seeds of forming globular clusters (Senchyna et al. 2023; Marques-Chaves et al. 2024; Watanabe et al. 2024), or as the e ff ect of enrichment confined within the small volume of the Broad Line Region (BLR) of AGN (Maiolino et al. 2023b). Furthermore, possible evidence for super-solar C / O has been instead reported in a galaxy at  \nz ∼ 12 . 5 (D'Eugenio et al. 2023, GS-z12), and interpreted as the footprint of chemical enrichment from SNe explosions of the first populations of overly massive, extremely metal-poor (or even metal-free Population III) stars.  \nOne remarkable example of high-redshift system whose rich emission line spectrum has been unveiled by JWST / NIRSpec is JADES-GS-z9-0 (hereafter GS-z9-0), a luminous galaxy spectroscopically confirmed at z ∼ 9 . 43 in the HUDF (Bunker et al. 2023a), and one of the most distant objects for which it is possible to simultaneously probe rest-frame UV and optical spectra from Ly α to [O iii ] λ 5007, as the latter leaves the NIRSpec coverage at z ≳ 9 . 55. This source was originally identified as a robust high-redshift galaxy candidate within the CANDELS GOODSS field on the basis on its red J125 - H160 color by Oesch et al. (2014) (the source was known as GS-z10-1 in that work), who reported a photometric redshift of z phot = 9 . 9 ± 0 . 5. Initial modelling of SED and morphology delivered log( M ⋆ / M ⊙ ) ≈ 9 and a size of r e ≈ 0 . 5 kpc, with an inferred star-formation rate (SFR) surface density of ≈ 1 -20M ⊙ yr -1 kpc -2 (Oesch et al. 2014; Holwerda et al. 2015), contributing to the preJWST characterisation of the UV luminosity function at z ∼ 9 -10 (Bouwens et al. 2019, e.g.). Based on its robust photometric redshfit and luminosity, it was included as a bona-fide z ∼ 10 candidate in the HST-selected sample to be followed-up with JWST / NIRSpec in one of the first observational programmes of the JWST Advanced Deep Extragalactic Survey (JADES) (PID 1210, Eisenstein et al. 2023b; Bunker et al. 2023a), and some of its properties have been already discussed in early, sample-based papers of the collaboration (e.g. Cameron et al. 2023b; Curti et al. 2024; Laseter et al. 2023; Boyett et al. 2024; Scholtz et al. 2023). It has been then re-observed with NIRSpec in November 2023, in the framework of the JADES Origins Field (JOF) programme (PID 3215, Eisenstein et al. 2023a).  \nIn this paper, we leverage the unprecedented depth provided by the combined programmes 3215 and 1210 in both NIRSpecMSA PRISM and medium resolution gratings, together with complementary NIRCam imaging, to analyse in more detail its physical properties and chemical enrichment patterns. We outline the data processing and spectral fitting in Section 2 and discuss the ionization mechanisms powering GS-z9-0 in Section 3. Section 4 presents the derivation of chemical abundances, and in Section 5 we discuss possible scenarios of chemical enrichment. Finally, in Section 6, we discuss the possible contribution of GSz9-0 to reionization based on the tentative detection of Ly α in emission and the modelling of the Ly α damping wing. Our conclusions are summarised in Section 7. Throughout this work, we assume a Planck Collaboration et al. (2020) cosmology, with H 0 = 67 . 4 km s -1 Mpc -1 , Ω M = 0 . 315 and ΩΛ = 0 . 685. We also assume the Solar abundances of Asplund et al. (2009).", '2.1. Observations and data reduction': "We analyse JWST / NIRSpec observations carried out in two di ff erent programmes, namely PID 1210 (PI N. Lützgendorf, Bunker et al. 2023a) and PID 3215 (PI D. Eisenstein and R. Maiolino, Eisenstein et al. 2023a), as the target of the present study was included in both NIRSpec / MSA mask configurations, with an almost identical relative slitlet position. NIRCam imaging covering GS-z9-0 in both wide- and medium-band filters are also available. In particular, F090W, F115W, F150W, F200W, F277W, F335M, F356W, F410M, and F444W images were taken as part of the medium-depth JADES programme ID  \nFig. 1: False-colour RGB image of the galaxy GS-z9-0. The location of the NIRSpec slitlet from the 3215 MSA mask design for the first nod and visit is overplotted. Only one of the five total ( ∼ 0.1 arcsec dithered) visits is shown.  \n<!-- image -->  \n1286, whereas F182M and F210M as part of the FRESCO programme (PID 1895, PI Oesch, Oesch et al. 2023). Figure 1 shows a composite RGB image of GS-z9-0, with the position of the NIRSpec slitlet from the first visit and first nod of PID 3215 overplotted.  \nIn virtue of its allocation over multiple programmes, GS-z90 has collected a total exposure time with NIRSpec / MSAof72 . 3 hours in PRISM / CLEAR, 44 . 3 hours in G395M / F290LP, and 16 . 3 hours in G140M / F070LP, as a result of combining 72 + 114, 18 + 96, and 18 + 24 individual integrations from both 1210 and 3215 programmes (19 groups / int, 2 integrations of 1400 s each per exposure, with a three-nodding pattern repeated over threedithered and five-dithered pointings in 1210 and 3215, respectively). We note that the last visit (visit 5) of PID 3215 was affected by short circuits, reducing the exposure time compared to the original allocated time by 8400 s for PRISM and G140M observations (i.e. 6 integrations lost out of the requested 120), and by 33600 s for G395M (24 integrations lost, equivalent to the full visit). The total exposure time in G235M / F170LP is instead of 7 hours, resulting from solely the 1210 programme as such grating / filter combination has not been repeated in 3215. In this paper, we leverage primarily the ultra-deep combined 1210 + 3215 data for PRISM, G140M, and G395M configurations, noting also that the C iii ] λ 1909 emission line covered by G235M falls unfortunately in the gap between the two NIRSpec detectors. A summary of the observing modes and total exposure times is provided in Table 1.  \nThe data reduction of the 3215 data follows the same recipe of 1210, as described in other papers of the JADES collaboration (e.g., Bunker et al. 2023a; D'Eugenio et al. 2024). In brief, we adopt a 3-nodding scheme for background subtraction, apply path-loss corrections appropriate for point sources (taking into account the intra-shutter position of the source in each nod and dither configuration), and extract the 1-d spectrum using a  \nTable 1: Summary of NIRSpec observations of JADES-GS-z9-0. † exposure time reduced from nominal allocated time due to short circuits a ff ecting visit 5 of PID 3215  \nfull-shutter (5 pixels) window from each individual integration. For the current analysis, we have combined the individual subspectra from 1210 and 3215 for PRISM, G140M, and G395M by inverse-variance-weighted averaging over the surviving entries in each wavelength bin following five passes of iterative three-sigma-clipping aimed at flagging anomalous noise spikes representing outliers to the statistical noise in the data. The final, combined 2D and 1D PRISM spectra are displayed in Figure 2.", '2.2. Emission line fitting': "We performed emission line fitting separately for low-resolution PRISM and medium-resolution gratings spectra. Formally, the error on each parameter of the fit is evaluated exploiting the output pipeline error spectrum. However, we also perform additional tests to assess the robustness of low-significance detections in both gratings and PRISM spectra by leveraging the large number of individual integrations in G140M (42) G395M (114), and PRISM (186) provided by the combined 1210 and 3215 datasets. More specifically, we generate 300 bootstrapped spectra by randomly sampling (with replacement) over the set of individual sub-spectra, after five passes of iterative 3 σ -clipping have removed strong outliers at each wavelength bin. We then repeat the fitting process on each individual bootstrapped combined (averaged) spectrum, and take the square root of the variance of the resulting distribution as the error on the parameter. Such empirical bootstrapped uncertainties are expected to be more conservative than those estimated by assuming the output error spectrum from the pipeline, in that they implicitly take into account all possible sources of noise, including the correlated error among wavelength bins present in resampled NIRSpec spectra, which might not be fully accounted for by the pipeline (see also Maseda et al. 2023; Hainline et al. 2024a). We report the extracted emission line fluxes and equivalent widths for both our PRISM and gratings fitting in Table 2, where the signal-to-noise-ratio (S / N) quoted on the emission line fluxes is reported on the basis of both uncertainty estimates. Overall, we find bootstrapped-based S / N to be generally lower than those based on the pipeline error spectrum, and this is particularly true for medium-resolution grating spectra, whereas the pipeline error for the PRISM spectrum appears intrinsically more conservative in virtue of its attempt to rescale the noise to account for the correlation induced by the spectral resampling. A more detailed discussion about the noise model in NIRSpec spectra will be presented in a forthcoming paper (Jakobsen et al., in preparation). We also compare our results with those obtained via the joint continuum and emission lines modelling with ppxf (Cappellari 2017) as described in D'Eugenio et al. (2024), finding consistent results. Further details on the results of our fitting procedure are given below.  \nFig. 2: Top panel : 2-d (top) and 1-d (bottom) PRISM spectra of GS-z9-0 at z = 9 . 4327, as obtained combining observations from both JADES 1210 and 3215 programmes. The pipeline error spectrum is reported in magenta. The main emission line features detected in the spectrum are marked in di ff erent colours, depending on whether they are also covered by G140M / F070LP (blue), G395M / F290LP (red), or are observed only in the PRISM data (green). The forecepho photometry extracted from available NIRCam wide- and medium-band imaging is also reported, showing good consistency in the flux calibration with the pathlosses corrected spectrum.  \n<!-- image -->", '2.2.1. Rest-frame UV spectrum': "To begin with, we focus on the rest-frame UV region in the PRISM spectrum. We first perform a fit to the Calzetti et al. (2000) region down to 2600Å (rest-frame) but excluding the region blueward of 1450Å (rest-frame) to minimise the impact of the Ly α damping wing (we further discuss the modelling of the Ly α spectral break in Section 6). We model the underlying continuum with a power-law, and include the following emission lines in the fitting procedure: C iv λ 1550, He ii λ 1640, O iii ] λ 1666, N iii ] λ 1750, C iii ] λ 1909.  \nEach individual line is assumed as spectrally unresolved and is modelled with a single Gaussian whose width is allowed to vary within ten percent of the line spread function (LSF) modelled from 1210 data by de Graa ff et al. (2024), which takes into account galaxy size and relative NIRSpec intra-shutter position. Emission line doublets (e.g C iii ] λλ 1907,1909, O iii ] λ 1661 , 1666) or multiplets (e.g. N iii ] λλ 1747-1754) are also assumed unresolved in the PRISM spectra, whereas multiple components are included when fitting the gratings spectra. To model the N iii ] λλ 1747-1754 multiplet in the PRISM, we set the line centroid to the average between the two brightest transitions in the multiplet, i.e. at 1751.83Å. To aid the modelling of the line complex involving O iii ] λ 1666 and He ii λ 1640 (which are partially blended at the PRISM resolution), their ratio is fixed to that measured from the G140M spectrum (see below). Furthermore, considering the proximity of the Si iii ] λλ 1883,1892 doublet to C iii ] λ 1909, and that such emission line has been observed in z ∼ 2 . 5 galaxies (with relative intensity of ∼ 20 -30% that  \nof C iii ], e.g. Steidel et al. 2016), we also include an additional Gaussian component 1 to account for the Si iii ] doublet in our fitting procedure.  \nThe results of fitting the rest-frame UV region of the PRISM spectrum are shown in the left-hand panel of Figure 3: C iv λ 1550, He ii λ 1640, O iii ] λ 1666, and C iii ] λ 1909 are detected above the continuum level at ≳ 5 σ significance. From the same fit, we constrain the UV slope to β UV = -2 . 54 ± 0 . 02. Although formally undetected ( ∼ 1 . 8 σ significance), and despite its small contribution to the total flux of the complex, we note that including the Si iii ] λλ 1883,1892 component (hatched green) provides a better match to the 'blue wing' of the C iii ] line profile. Higher resolution observations of the C iii ] complex are needed to assess the real significance of the Si iii ] emission 2 , and we here note that none of the main results of the analysis depends on the inclusion (or not) of such component in the fitting procedure.  \nIn addition, we report marginal detection (marked in green in Figure 3) of the N iii ] λλ 1747-1754 multiplet. The N iii ] λλ 1747-1754 arises in a region not contaminated by other emission lines, and its possible detection is interesting given the implications of this transition in the determination of the N / O abundance. The line is formally detected at 2 . 7 σ assuming the error spectrum from the pipeline, whereas at 3 σ adopting the bootstrapping approach. We discuss possible scenarios of chem-  \nnrichment in GS-z9-0 based on the detection of such emission line in Section 5.  \nExtending our fitting analysis to longer wavelengths, we also report the marginal detection of [Ne v ] λ 3426 (at ∼ 3 σ with both pipeline error and bootstrapping, Figure 4). To produce such very high-ionization (97 . 11 eV) emission line requires an extremely hard photoionization source. The presence of [Ne v ] in galaxy spectra has been attributed to actively accreting black holes in AGN hosts, stellar continuum from an extremely hot ionizing spectrum including Wolf-Rayet stars, or energetic radiative shocks from supernovae (Gilli et al. 2010; Izotov et al. 2012a; Mignoli et al. 2013; Zeimann et al. 2015; Backhaus et al. 2022; Cleri et al. 2023) We discuss the implications of possible [Ne v ] λ 3426 detection in the determination of the dominant ionising source of GS-z9-0 in Section 3. However, we note that we do not detect any significant emission above the continuum level at the location of the [Ne iv ] λ 2424 emission line in the PRISM spectrum. One possible explanation for the absence of [Ne iv ] λ 2424 (in the presence of both [Ne iii ] and [Ne v ]) is that, despite the lower ionization energy of Ne 3 + compared to Ne 4 + , the strength of the line is hampered on the one hand by its lower emissivity, while on the other by the relatively higher energy required to collisionally excite the ion.  \nMoving to the fit of the G140M spectrum, we report the detection of both O iii ] λ 1666 and He ii λ 1640 at ∼ 7 σ and ∼ 5 σ significance based on the pipeline errors, respectively; based on the more conservative bootstrapping approach described above, both lines are instead detected at 3 . 5 σ (right-hand panel of Figure 3). During the fit, we fix the velocity and width of He ii λ 1640 line to that of O iii ] λ 1666, in order to restrict the fit only to the He ii nebular component. However, we do not find any clear evidence for a residual broad component that could be associated with stellar winds features. The O iii ] λ 1661 is not detected, however the ratio of O iii ] λ 1666 / O iii ] λ 1661 is fixed by atomic physics to 2.93. N iii ] λ 1750 is also undetected, and its 3 σ upper-limit is consistent with the expectations given the lower sensitivity of the G140M observations.  \nWhen fitting C iv λλ 1549,1551, we leave the velocity and width of the two components of the doublet (spectrally resolved in G140M) free to vary, to account for possible resonant scattering through highly ionised gas that could impact the line profile (see e.g. Leitherer et al. 2011; Berg et al. 2019; Senchyna et al. 2022; Topping et al. 2024). In general, the C iv λλ 1549,1551 spectral feature is challenging to interpret due to its complex composite profiles, with possible contributions from narrow nebular emission, broad stellar emission, stellar photospheric absorption, and interstellar medium absorption and scatter. We detect (at 4 . 4 σ assuming the pipeline error, at 3 . 2 σ with bootstrapping) only one of the two C iv lines in the doublet, which we interpret as the 'red' C iv λ 1551 component (right-hand panel of Figure 3). If such interpretation is correct, the line appears shifted by ∼ 120 km s -1 with respect to the systemic redshift of the galaxy inferred from strong rest-frame optical lines, possibly indicative of resonant scattering through outflowing gas attenuating the blue component of the doublet, which has a relative oscillator strength ∼ 2 × higher than C iv λ 1551. On the contrary, interpreting such emission line as C iv λ 1548 would require a redshift > 400 km s -1 . Moreover, this would worsen the tension with the total flux and equivalent width of the total C iv doublet as inferred from the PRISM.", '2.2.2. Rest-frame optical spectrum': "We fit the rest-frame optical lines in both PRISM and G395M spectra modelling the underlying local continuum with a power law. Emission lines are assumed as unresolved and modelled with individual Gaussians, the only exception being the [O ii ] λλ 3726,3729 doublet which is marginally resolved and modelled with two components when fitting the G395M spectrum. As shown in Figure 5, rest-frame optical lines are well detected, with high signal-to-noise, as already reported e.g. in Bunker et al. (2023a) based on observations from programme PID 1210. This includes not only a robust detection of the [O iii ] λ 4363 auroral line, of specific interest for deriving chemical abundances (Section 4, see also Laseter et al. 2023), but also (more marginal) detections of high-order Balmer lines like H9 λ 3835 and H10 λ 3797, which, in addition to well detected H β , H γ , and H δ , provide information to constrain the amount of nebular attenuation over a wide wavelength range (Section 2.3).  \nWe derive a fiducial redshift for GS-z9-0 from the combined 1210 + 3215 spectrum by averaging over the brightest, isolated rest-frame optical lines, finding zG395M = 9 . 432681 ± 0 . 000069 and zprism = 9 . 43774 ± 0 . 00020 (mean and error on the mean), respectively. These values are consistent with those determined individually from 1210 and 3215 spectra, while revealing a significant discrepancy between G395M and PRISM, possibly caused by o ff sets in the wavelength calibration introduced by a non proper correction of the relative intra-shutter position in the determination of the wavelength solution (D'Eugenio et al. 2024).  \nWe also note that existing flux calibration o ff sets between PRISM and grating spectra, as well as o ff sets as a function of wavelength and of the location on the MSA detectors, have been reported from the analysis of large galaxy samples observed with NIRSpec (e.g. Bunker et al. 2023a; D'Eugenio et al. 2024). We test this for GS-z9-0 by comparing the inferred flux for restoptical lines redward of [Ne iii ] λ 3869 (to avoid further uncertainties on the continuum modelling in the PRISM spectrum introduced by the presence of a 'Balmer jump'), and find that the gratings' fluxes are higher by ∼ 10-12 percent compared to those measured from the PRISM, a value slightly larger than the average reported in JADES data release papers (Bunker et al. 2023a; D'Eugenio et al. 2024). Therefore, to avoid introducing systematics associated with uncertain scaling factors, throughout this paper we consider only line ratios computed within the same spectral configuration (i.e. we do not use gratings-to-prism line ratios), and avoid to adopt UV-to-optical line ratios where possible to infer the physical properties of interest. As a general criterion, we primarily adopt line ratios from medium resolution grating spectra where available, especially in the rest-frame optical (while testing the consistency of our results by comparing them with those inferred from PRISM-based fitting), whereas we resort to line ratios measured from the PRISM when no detections are available from the gratings (as in the case of some rest-UV emission lines). In general, we note that, despite the o ff -set in the absolute flux calibration between PRISM and gratings, we find that line ratios among the same set of lines derived from either configuration generally agree within their respective uncertainties.", '2.3. Photometry and full spectral fitting': "We performed forcepho (Johnson et al. in preparation) fitting of the GS-z9-0 system to forward model the light distribution and extract the photometry and morphological parameters from all the available NIRCam images in individual medium- and wide-  \nTable 2: Emission line fluxes and rest-frame equivalent widths measured in both PRISM and gratings (G140M and G395M) spectra. The signal-to-noise on the lines is estimated either from the pipeline error spectrum, and by repeating the fit on 300 realisations of the spectra generated via randomly bootstrapping (with replacement) over the 186, 42, and 114 individual integrations available for the PRISM, G140M, and G395M configurations, respectively. 3 σ upper limits are reported for non-detections. † C iv λλ 1549,1551, O iii ] λλ 1661,1666, [O ii ] λλ 3726,3729 are modelled as unresolved doublets in the PRISM spectrum.  \n<!-- image -->  \nFig. 3: Rest-frame UV emission lines in GS-z9-0. The best-fit to the continuum and lines is shown in red. Detections at ≥ 4 σ are marked in yellow, while marginal detections ( ∼ 3 σ ) are highlighted in green. Vertical purple lines mark the expected location of the emission lines based on the systemic redshift of the source. The bottom panels report the residuals of the fit and the 1 σ uncertainty from the pipeline error spectrum. Left panel : zoom-in on the region of the PRISM spectrum between 1 . 5 µ mand 2 . 1 µ m. Beside clear detections of C iv , He ii , O iii ], and C iii ], marginal detection of N iii ] λ 1750 is also highlighted. Right panel : zoom-in on the region of the G140M spectrum around C iv , He ii , and O iii ] emission. The C iv λ 1548 , 51 doublet is resolved, but only the redder line is detected at > 3 σ significance. The C iv complex is blueshifted by ∼ 117km s -1 compared the systemic redshift, whereas no velocity o ff set is seen for O iii ] λ 1666 and He ii λ 1640.  \n<!-- image -->  \nband filters, namely F090W, F115W, F150W, F182M, F200W, F210M, F277W, F335M, F356W, F410M, F444W. The forcepho setup follows that adopted in Tacchella et al. (2023a) and Baker et al. (2023), and we model the galaxy with a single Sérsic component. The extracted forcepho photometry for is reported in Table 3. Based on the morphological parameters derived from the  \nforcepho fitting, we infer a high compactness for this source, with an e ff ective radius as small as R e = 110 ± 9 pc.  \nWe then model the full SED of the galaxy by simultaneously fitting PRISM spectrum and photometry with di ff erent codes, namely beagle (Chevallard & Charlot 2016) and bagpipes (Carnall et al. 2018). We note that, given the excellent agreement  \nTable 3: Photometry extracted from forcepho fitting of the available NIRCam images in both wide and medium band filters. Units are in nJy, and quoted errors only report formal statistical uncertainties.  \nFig. 4: Tentative detection of the very high-ionization [Ne v ] λ 3426 emission line (ionization potential 97 . 11 eV) in the PRISM spectrum of GS-z9-0. The line is formally detected at ∼ 3 σ from both the pipeline error spectrum and the bootstrapping approach described in Section 2.2.  \n<!-- image -->  \nbetween the pathlosses corrected PRISM spectrum and the extracted forcepho photometry (see Figure 2), no significant rescaling of the spectrum is required in the procedure. We summarise the results of SED fitting from both codes in Table 4 and Figure 6. The resulting best-fit spectra (corresponding to the minimum chi-square model) are shown in the top panels of Figure 6, whereas in the bottom panels we report the posterior PDFs for stellar mass, stellar age, ionization parameter, and metallicity (median, 16th, and 84th percentiles are marked with dashed black lines).  \nThe beagle setup mimics that employed in previous studies (e.g. D'Eugenio et al. 2023; Hainline et al. 2024a): in brief, we set an upper-mass cut-o ff for a Chabrier (2003) IMF to 300 M ⊙ , and model the SFH as a delayed-exponential with a burst occurring in the last 10 Myr. The metallicity of stars is tied to that of the nebular gas, and we implement dust attenuation following the prescriptions of Charlot & Fall (2000). Overall, the best-fit spectrum provides a good match to the observed continuum and spectral shape, while struggling to match the intensity of some of the rest-frame UV emission lines such as He ii and C iv . We discuss further on the possible mechanisms powering nebular emission lines in Section 3. The beagle fit favours a recent history of star-formation for GS-z9-0, (log(sSFR / yr -1 ) = -7 . 52, with an inferred light-weighted age of ∼ 3 Myr and an age of the oldest stars ∼ 12 Myr), a scenario consistent with the chemical abundance patterns discussed in Section 5. We infer a low metallicity (0 . 046 ± 0 . 002 Z ⊙ ), in good agreement with that derived from the T e-method (Section 4), and a relatively high ionization parameter, log(U) = -1 . 45 + 0 . 05 -0 . 05 . For comparison, adopting for instance the photoionization models presented in Berg et al. (2019) we would derive a relatively lower log(U) = -1 . 78 ± 0 . 10 based on the [O iii ] λ 5007 / [O ii ] λλ 3726,3729 ratio and given the measured T e-metallicity.  \nWe explored the impact of systematics associated with the adoption of di ff erent stellar population synthesis models and star-formation histories on our inferred physical properties by fitting the data also with bagpipes . In this setup, we employ the Binary Population and Stellar Synthesis (BPASS) v2.2.1 (Stanway & Eldridge 2018) templates including the evolution of binary systems, an upper mass cut-o ff of 300 M ⊙ for a Kroupa IMF, dust extinction modelled by a Calzetti et al. (2000) law, and IGM attenuation models from Inoue et al. (2014). Nebular emission (in form of lines and continuum) is included by processing the BPASS stellar templates through the CLOUDY photoionization code (Ferland et al. 2017). We leave the gas metallicity and ionization parameter as free parameters in the fit, but inform our priors (especially for metallicity) by exploiting the information empirically derived from emission lines (see Section 4); in particular, we adopt uniform priors on log(Z / Z ⊙ ) ∈ [0.01, 0.25] and log(U) ∈ [-3, -1]. Finally, we assume a so-called non-parametric SFH following the recipe of Leja et al. (2019), defining six age bins counted backwards from the epoch of observations (the first two spanning 0-3 and 3-10 Myr, respectively). The priors on ∆ log(SFR) values among adjacent bins are modelled as a Student'st distribution with scaling factor σ = 0 . 3 ('continuity' model). The top-right inset panel of Figure 6 shows the inferred SFH, reporting the SFR in each of the temporal bins considered, as a function of lookback time. The bagpipes fit confirms the very recent history of mass assembly for GS-z9-0, predicting the majority of star-formation to have occurred within the last two bins 3 , with a total stellar mass of log(M ⋆ ) = 8 . 18 + 0 . 06 -0 . 06 M ⊙ , and a mass-weighted age of 32 + 20 -9 Myr, while the star-formation rate averaged over the past 10 Myr is SFR = 5 . 40 + 0 . 2 -0 . 2 M ⊙ yr -1 , in agreement with the values inferred both by beagle ( = 4 . 34 + 0 . 10 -0 . 08 M ⊙ yr -1 ), and by applying the calibration for low metallicity systems from Reddy et al. (2022); Shapley et al. (2023b) to the measured H β flux ( = 5 . 46 ± 1 . 04 M ⊙ yr -1 ). Assuming the latter as fiducial value, and given the compactness of the system (R e ∼ 110pc), this translates into a high star-formation rate surface density of Σ SFR = 72 ± 14 M ⊙ yr -1 kpc -2 , and into a stellar mass surface density of Σ M ⋆ = 1 . 85 ± 0 . 3 × 10 3 M ⊙ pc -2 . We note bagpipes prefers an even more extreme ionization parameter log(U) = -1 . 06 compared to beagle (log(U) = -1 . 46), whereas the derived metallicity is fully consistent. A possible source of the discrepancy between the two inferred estimates of log(U) stems from its definition: in beagle , the ionization parameter is defined at the Strömgren radius (Equation 7 in Gutkin et al. 2016), whereas bagpipes follows the prescription of Byler et al. (2017) and defines log(U) at the inner radius of the illuminated cloud, hence its value is expected to be systematically higher.  \nBoth SED fitting codes infer very low dust attenuation ( A V = 0 . 050 + 0 . 008 -0 . 006 and A V = 0 . 004 + 0 . 005 -0 . 002 from beagle and bagpipes , respectively), in agreement with the measured 'decrement'among Balmer lines ratios. Having access to spectrally resolved H β , H γ , and H δ in both G395M and PRISM observations (with additional detections of higher order H9 and H10 lines in G395M),  \nFig. 5: Rest-frame optical emission lines in GS-z9-0 from G395M medium resolution spectrum. The best-fit to continuum and emission lines is overlaid in red, with detected lines highlighted (in green 'marginal' ∼ 3 σ detections). The left-hand panel shows a zoom-in on the region between [O ii ] λλ 3726,3729 and H δ , highlighting the detection of both strong lines and fainter Balmer lines like H9 and H10. The middle panel shows the region around H γ and the [O iii ] λ 4363 auroral line, while the right-hand panel highlights the high S / N detections of the [O iii ] λ 4959,5007 and H β complex.  \n<!-- image -->  \nwe can in fact obtain an independent constraint on the amount of nebular attenuation. In Figure 7, we compare the observed H γ / H β , H δ / H β , H9 / H β , and H10 / H β ratios with the theoretical values expected for case B recombination, assuming a temperature of 20000 K and density n e = 600 cm -3 , consistent with the values measured directly from emission lines as detailed in Section 4. The ratios measured independently from G395M and PRISM spectra are in excellent agreement, and are consistent within their uncertainties with negligible-to-no dust attenuation. In fact, the best-fit A V derived from simultaneously fitting all the available Balmer line ratios with a Gordon et al. (2003) SMC attenuation curve and RV = 2 . 505 are A V G395M = -0 . 07 ± 0 . 07 and A V PRISM = 0 . 01 ± 0 . 07, respectively.  \nFinally, by converting the UV luminosity density at 1500Å rest-frame as measured in the PRISM spectrum, we derive a UV magnitude of MUV = -20.43, consistent with previous determinations based on the 1210 spectrum (Boyett et al. 2024).", '3. Ionization source': "The ratios among di ff erent emission lines (either collisionally excited or produced by recombination) can be modelled to constrain the nature of the dominant source of photoionization in galaxies. In the case of GS-z9-0, we can leverage high S / N detections of several emission lines in both rest-frame optical and rest-frame UV regimes to explore a variety of di ff erent diagnostic diagrams. Rest-frame optical diagrams for this source (e.g. [O iii ] / [O ii ] vs ([O iii ] + [O ii ]) / H β ) have been explored already in Cameron et al. (2023b) as based on the data from the 1210 programme. In those diagrams, GS-z9-0 is observed to occupy the region belonging to local analogues of high-z galaxies characterised by low-metallicity and high ionization parameter.  \nMore recently, Scholtz et al. (2023) explored some restframe UV diagnostics with the aim of selecting robust type-2 AGN candidates within JADES. According to the criteria outlined in Scholtz et al. (2023), and following, in particular, a possible ( ∼ 4 σ ) detection of [Ne iv ] λ 2424 in the G235M spectrum (a high-ionization emission line which requires very hard ionising continua to be powered, e.g. Brinchmann 2022), GS-z9-0 is classified as a type-2 AGN. As mentioned already in Section 2.2, this emission line, however, is formally undetected in our PRISM  \nspectrum (we quote a 2 . 4 σ significance based on the pipeline error spectrum, and only 1 . 6 σ based on bootstrapping).  \nHere, we compare several UV-based diagnostics with predictions from suites of photoionization models assuming di ff erent input ionising continua. These are presented in Figure 8. More specifically, we focus our model predictions by exploring range of values in the physical properties matching (or in broad agreement with) those independently inferred for GS-z9-0 from the emission lines (e.g. for O / H, C / O). This is done in an attempt to limit the degeneracies induced by comparing model grids from di ff erent ionising sources under very di ff erent physical conditions, even when they do not match those (empirically and independently) estimated for an individual galaxy.  \nMore specifically, to model emission line ratios as predicted by ionization from star-formation, we adopt an updated version of the model grids described in Gutkin et al. (2016); Plat et al. (2019) for the two cases of constant star-formation history (upper panels of Figure 8) and single burst of di ff erent ages (lower panels of 8), respectively. We assume an upper-mass cut-o ff of the IMF = 300 M ⊙ , a dust-to-metal mass ratio ξ d = 0.1, and the gas density ne = 10 2 cm -3 as fiducial values. In the first scenario (constant star-formation) we assume a maximum stellar age of 100 Myr, whereas for the single stellar population scenario, we explore two bursts of 1 and 3 Myr age, respectively (more reflective of the SFH inferred in Section 2.3). We then explore a range of values in ionization parameter (spanning between -3 < log(U) < -1) and gas-phase metallicity (0 . 015 < Z / Z ⊙ < 0 . 7). In the upper panels, we explore also the variation in C / O abundance relative to solar ([C / O] ∈ [-1, 0], where di ff erent [C / O] values are marked by di ff erent symbols), whereas in the middle panels [C / O] is fixed to -1 (in better agreement with that inferred from direct measurements as described in Section 4). To model line ratios produced by the narrow-line-region (NLR) of AGN instead, we adopt the models from Feltre et al. (2016), fixing ξ d = 0.3 and ne = 10 3 cm -3 (more consistent with the typical densities of the NLR of AGN), while varying ionization parameter, metallicity, and slope of the ionising continuum ( α ∈ [-2, -1.2]). For AGN grids, the C / O abundance is fixed to the solar value.  \nIn Figure 8 we show the location of GS-z9-0 on two di ff erent diagrams based on rest-frame UV lines, namely  \n¯  \n¯  \n1e-20  \n<!-- image -->  \n¯  \n¡  \n¯  \nFig. 6: SED fitting to the PRISM spectrum of GS-z9-0. The left-hand panel shows the fit performed with beagle , while the righthand panel shows the fit adopting bagpipes with a non-parametric star-formation history. In both cases, the upper panel shows the best-fit (minimum chi-square) model spectrum superimposed on the observed spectrum (in grey) and the forcepho photometry (red points). The shaded areas mark the wavelength intervals masked during the fit (in particular, the region around the Ly α break). The inferred non-parametric star-formation history is depicted in the inset panel for the bagpipes fit. The lower panels report the marginalised posterior PDFs from the two fits for stellar mass, stellar age, ionization parameter, and metallicity (relative to solar), respectively.Fig. 7: Nebular attenuation inferred from Balmer decrements. The ratios between di ff erent Balmer lines and H β as measured from both G395M and PRISM spectra are compared with the theoretical values set by Case B recombination (solid black bars) for T e = 20,000 K and n e = 600 cm -3 (consistent with electron temperature and density derived in Section 4). The ratios measured from both PRISM and gratings suggest negligible dust attenuation, with best-fit A V values consistent with zero within 1 σ uncertainty.  \n<!-- image -->  \nC iii ] λλ 1907,1909 / He ii λ 1640 vs O iii ] λ 1666 / He ii λ 1640 (left-hand panel, hereafter C3He2-O3He2) and C iii ] λλ 1907,1909 / He ii λ 1640 vs C iv λ 1550 / C iii ] λλ 1907,1909 (right-hand panel, hereafter C3He2-C43). These diagrams have been recently suggested as some of the most reliable in discriminating the dominant ionising source in galaxy spectra (Mingozzi et al. 2024), on the basis of observations of a sample of local, high-z analogues with full coverage  \nof rest-UV spectrum from the CLASSY survey (Berg et al. 2022): these objects are included in our plots for comparison. In the C3He2-O3He2 diagram, GS-z9-0 occupies the region probed by low-C / O grids at low-to-intermediate metallicity from the SF-models, and appears inconsistent with the model tracks produced by AGN-NLR ionization (while lying at the boundary of the SF-shocks demarcation line from Mingozzi et al. 2024). In the C3He2-C43 diagram instead, GS-z9-0 falls at the intersection between the SF and AGN model grids. Notably, 1 Myr SSP model grids (dot-dashed lines in the bottom panels) span a region of the diagram that overlaps with the AGN / NLR grids, making it harder to disentangle the dominant contribution to ionization. However, we note that, because AGN grids are computed assuming a solar C / O abundance ratio, one could expect line-ratio grids for AGN-like ionization of lower C / O (e.g. 0 . 1 × (C / O) ⊙ ) to be shifted from those shown in the panel by a similar amount as observed between [C / O] = -1 and [C / O] = 0 SF-grids (at fixed other parameters): this in generally would apply to all diagrams that intrinsically involve a dependence on the C / O abundance, and in the case of the C3He2-C43 diagram would make the agreement between GS-z9-0 and AGN-like grids worse.  \nHowever, the spectrum of GS-z9-0 reveals also the possible presence of a very high ionization emission line, which is challenging to explain by standard stellar population models, namely [Ne v ] λ 3426 (Figure 4). Therefore, we can exploit the detection of such emission line in our PRISM spectrum to explore di ff erent diagnostic diagrams: these are shown in the left and middle panels of Figure 9 for [Ne v ] λ 3426 / C iii ] λ 1909 vs C iii ] λ 1909 / He ii λ 1640 (Ne53C3He2) and [Ne v ] λ 3426 / [Ne iii ] λ 3869 vs [O iii ] λ 5007 / H β (Ne53-O3HB), respectively. GS-z9-0 is located in-between SF and AGN grids in the Ne53-C3He2 diagram, but appears in slightly better agreement with AGN-ionization (though still  \nbroadly consistent with SF models of very high ionization parameter log(U) = -1), regardless of the dependence on C / O of the di ff erent grids (we recall that AGN / NLR models from Feltre et al. 2016 are computed assuming solar C / O). However, based on the Ne53-O3HB diagram, GS-z9-0 appears totally consistent with AGN ionization grids, with pure stellar population models struggling to produce significant [Ne v ]. The advantage of the Ne53-O3HB diagram is that it exploits the large di ff erence in minimum energy required to produce [Ne v ] and [Ne iii ] emission lines (which trace di ff erent ionization zones Berg et al. 2021); moreover, it is based on lines closely spaced in wavelength (hence avoiding potential issues associated with wavelength-dependent slit loss correction and potential dust reddening), and it is also not a ff ected by degeneracies in chemical abundances introduced by the use of emission lines of di ff erent elements. Such diagram has also been recently proposed as a possible way to discriminate between the ionization produced by AGN with accreting supermassive black holes (MBH ≥ 10 6 M ⊙ ), AGN with intermediate-mass black holes (IMBH, MBH ≲ 10 5 M ⊙ ), and extreme stellar populations or even Population III stars (Cleri et al. 2023). In the middle panel of Figure 9, we report the empirical demarcation lines for SF, AGN, and IMBH / Pop III stars based on the set of photoionization models presented in Cleri et al. (2023) (dashed black lines). Interestingly, we note that although GS-z9-0 resides in the 'composite' region, its [O iii ] λ 5007 / H β ratio (which is not as high as in local AGN given the low metallicity of the system) places it not far from the region dominated by IMBH / Pop III models. We note that in Cleri et al. (2023), the latter models assume zero metallicity for the stellar population but a slightly pre-enriched gas-phase metallicity of Z = 0.05Z ⊙ (similar to what inferred for GS-z9-0 based on T e measurements, Section 4) from primordial supernovae or stellar mass-loss events.  \nFinally, in the right-hand panel of Figure 9, we plot a di ff erent diagnostic diagram, now leveraging the tentative detection of N iii ] λ 1750 in the PRISM spectrum. In particular, we explore N iii ] λ 1750 / O iii ] λ 1666 vs C iii ] λ 1909 / He ii λ 1640, which introduces explicitly the dependence on the N / O abundance. For this purpose, we employ a set of grids generated with cloudy , with 1 Myr old SSP templates from BPASS as the input spectra to model star-formation, and a canonical SED with an e ff ective big blue bump temperature of TBB = 10 6 K, a UV-to-X-ray slope of -1 . 4, a UV slope of -0 . 5, and an X-ray slope of -1 . 0 to model AGN continuum. We vary the metallicity Z / Z ⊙ between 5 × 10 -4 and 2, and the ionization parameter log(U) between -3 . 5 and -1. We adopt the prescriptions from Groves et al. (2004) to compute the initial N / O (before dust depletion) at every given O / H, while C / O is scaled accordingly assuming solar abundance patterns. More details are given in Ji et al. (2024b). In this diagram, GSz9-0 occupies a region consistent with stellar ionization.  \nSummarising, on the basis of rest-UV diagnostics explored here, the line ratios observed in GS-z9-0 are consistent with ionization from a population of massive and metal-poor stars, although the marginal [Ne v ] λ 3426 detection reported in the present work bring some evidence in support of the AGN scenario (see e.g. the recent observations of such transition in the galaxy GN42437 at z ∼ 6, Chisholm et al. 2024). Nonetheless, radiative shocks driven by stellar winds and SNe explosions have been also proposed as physical mechanisms capable of boosting the [Ne v ] λ 3426 emission in metal-poor star-forming galaxies (Izotov et al. 2012b; Lecroq et al. 2024). In general, one should be probably careful about interpreting these diagnostic diagrams too rigidly. In fact, it is not unlikely that the nebular spectrum of GS-z9-0 is the result of mixing between di ff erent sources of  \nionization, as recently suggested by the analysis of similar spectra of high-redshift galaxies (e.g. GS-9422 at z ∼ 6, Tacchella et al. 2024, GN-z11 at z ∼ 10 . 6, Bunker et al. 2023b; Maiolino et al. 2023b, GHz2 at z ∼ 12 . 3, Castellano et al. 2024), and decoupling their relative contribution would require more detailed modelling and a better understanding of the shape of the ionising continua of young, massive stellar populations.", '4. Chemical abundances': "The simultaneous detection of both nebular and auroral lines in the spectrum of GS-z9-0 enables us to perform a detailed study of chemical abundance patterns in this galaxy, employing the 'direct', T e-method. Throughout this Section, we assume that ionisation comes primarily from star-formation (see Section 3). However, we note that even in the case of possible AGN contribution to ionization as discussed in Section 3, this does not prevent a direct measurement of the abundances via the auroral lines, provided that the proper ionization correction factors (ICF) are included, which in the case of GS-z9-0 we expect to be similar between AGN and stellar spectrum with hard ionizing continuum and / or high ionization parameter.  \nThroughout this Section, unless stated otherwise, we adopt pyneb for chemical abundances derivation, with atomic data and collision strengths tabulated from the chianti database. The errors on all the derived quantities are estimated by randomly perturbing the input emission line fluxes by their uncertainties (assumed Gaussian) and repeating the full procedure 100 times, taking the standard deviation of the distribution of values for each inferred parameter at each step of the procedure as our estimate of the (statistical) uncertainty associated to the fiducial value. Additional systematic uncertainties in the derivation of chemical abundances are discussed throughout individual subsections for each given element, and more generally in Section 4.5.", '4.1. Derivation of oxygen abundance': 'As a first step, we derive the temperature associated with the emitting region of O ++ (hereafter t 3) exploiting the high S / N detection of both [O iii ] λ 4363 and [O iii ] λ 5007 in the G395M grating spectrum. The gas density is simultaneously derived exploiting the [O ii ] doublet ratio, which is marginally resolved in G395M observations, while the temperature of the O + emitting region (hereafter t 2) is assumed in the process to follow the temperature-temperature relation from Izotov (2006), i.e. t 2 = 0 . 693 t 3 + 2810. We infer electron temperatures of t 3 = 20137 ± 1940 K and t 2 = 16765 ± 1345 K, while the gas density is not well constrained ( ne = 650 ± 430 cm -3 ) though its bestfit value is consistent with typical densities measured in highredshift galaxies (Isobe et al. 2023a, e.g.). We assume, therefore, ne = 650 cm -3 in our abundance calculations, noting that varying density between 100 and 1000 cm -3 would produce a di ff erence in the inferred oxygen abundance of only ≈ 0 . 01 dex.  \nWe can then derive the relative ionic abundance of two elements comparing the intensity I ( λ ) in the emission lines of each species while taking into account the di ff erent temperature- and density-dependent volumetric emissivity of the transitions J , i.e.  \nN ( X l ) N ( Y m ) = I ( λ ) l I ( λ ) m J ( λ ) m ( T , n ) J ( λ ) l ( T , n ) . (1)  \nWe compute the abundance of O ++ / H and O + / H from the [O iii ] λ 5007 / H β (assuming t = t 3) and [O ii ] λλ 3726,3729 / H β (as-  \nFig. 8: Rest-frame UV diagnostic diagrams. The location of GS-z9-0 is reported and compared to a sample of local, highz analogues from the CLASSY survey (Berg et al. 2022; Mingozzi et al. 2024). Top panels : C iii ] λλ 1907,1909 / He ii λ 1640 vs O iii ] λ 1666 / He ii λ 1640 (C3He2-O3He2, left-hand panel) and C iii ] λλ 1907,1909 / He ii λ 1640 vs C iv λ 1550 / C iii ] λλ 1907,1909 (C3He2C43, right-hand panel) line ratios are compared with photoionization models by Gutkin et al. (2016) and Feltre et al. (2016) for constant star-formation (solid lines), and NLR of AGNs (dashed lines), respectively. We fix the dust-to-metal ratio to ξ d = 0.1, while varying metallicity and ionization parameter (colour-coded), C / O abundance and slope α of the AGN continuum (di ff erent symbols). GS-z9-0 occupies a region fully consistent with star-formation driven ionization in the C3He2-O3He2 diagram, whereas it sits in a region of overlapping star-formation and AGN grids in the C3He2-C43 diagram. In the C3He2-O3He2 diagram, the solid and dot-dashed black lines represent the empirical demarcation between SF-, AGN-, and shock-driven ionization as proposed by Mingozzi et al. (2024), whereas in the C3He2-C43 diagram we report the demarcation lines between SF, AGN, and composite proposed by Hirschmann et al. (2019). Bottom panels: Same as upper panels, but for single stellar population models from Plat et al. (2019) (3 Myr and 1 Myr old for solid and dashed lines, respectively, with fixed ξ d = 0.3, and C / O = 0 . 1C / O ⊙ ). The grids from 1 Myr stellar populations at low metallicity fully overlap with those of AGN-like ionization of higher metallicity and solar C / O in the C3He2-C43 diagram.  \n<!-- image -->  \ng t = t 2) ratios, respectively, and derive log(O ++ / H) = -4 . 62 ± 0 . 09 and log(O + / H) = -6 . 00 ± 0 . 14.  \nOne question is whether a significant fraction of oxygen could be in the triple-ionised state as possibly suggested by the detection of He ii λ 1640, since He 2 + shares the same ionization potential as O 3 + (54 . 9eV). We do not detect any significant O iv λλ 14021404 emission in the G140M spectrum, and photoionization modelling from Berg et al. (2019) suggests a fractional contribution of O 3 + / O < 0 . 01 given the measured ionization parameter and the relative C 3 + / C ++ ratio (see below). The measured 3σ upper limit on the O iv λλ 14021404 flux < 1 . 82 × 10 -19 erg s -1 cm -2 only provides an upper limit  \non O 3 + / O ++ ≲ 0 . 4. We note that a small peak possibly associated with O iv λλ 14021404 is seen in the PRISM spectrum at ≈ 1400Å, but the low spectral resolution makes it impossible to properly disentangle the O iv contribution from other nearby features like Si iv λ 1394, considering also the uncertainty on the intrinsic shape of the underlying continuum and the presence of the damping wing of the nearby Ly α break.  \nAlternatively, we can exploit an ICF for oxygen based on the relative abundance of single- and double-ionised Helium, following Torres-Peimbert & Peimbert (1977) and leveraging the similar ionization potential of He ++ and O 3 + (see also Izotov et al. 2006; Valerdi et al. 2021; Dors et al. 2020, 2022). First,  \nFig. 9: Alternative diagnostic diagrams for GS-z9-0. The left-hand and middle panels leverage the very high ionization ( ∼ 97eV) [Ne V] λ 3426 emission line, and the same set of models from Gutkin et al. (2016) and Feltre et al. (2016) as in Figure 8. The left-hand diagram is sensitive to the C / O abundance, while the diagnostic considered in the middle panel is not. Here, the location of GS-z9-0 is more in line with AGN-powered line ratios, especially in the [Ne v ] / [Ne iii ] vs [O iii ] λ 5007 / H β diagram, where we show also the empirical demarcation lines between SF, AGN, and IMBH / PopIII models from Cleri et al. (2023). Right-hand panel: N iii ] λ 1750 / O iii ] λ 1666 vs C iii ] λ 1909 / He ii λ 1640 diagram. The location of GS-z9-0 is compared with the set of photoionization models for star-formation (solid lines) and AGN / NLR (dashed lines) ionization from Ji et al. (2024b).  \n<!-- image -->  \nwe estimate the intensity of He I λ 3889 emission by correcting the flux measured in the G395M spectrum by the contribution of the blended H8 Balmer line, which we infer from the expected theoretical ratio to the nearby H δ (i.e. H8 / H δ = 0.406) assuming t 2 and density as measured above, and under the assumption of no dust attenuation as suggested by the analysis in Section 2.3 (see also Figure 7). Then, we compute the relative He + / H + and He ++ / H + abundances from the He I λ 3889 / H β and He ii λ 1640 / H β flux ratios, assuming t 2 and t 3 respectively. The ICF(O) is finally given by the (He + + He ++ ) / He + ratio, which we measure as ICF(O) = 1 . 23, corresponding to a fractional contribution of O 3 + / O ∼ 19 per-cent. We note here that we are neglecting possible additional uncertainties on the ICF(O) determination associated with radiative transfer e ff ects of He I lines (as we assume the simple case B); therefore, we assume a conservative 20 percent error on ICF(O). The total oxygen abundance is hence O / H = ICF(O) × (O ++ / H + O + / H), and for GS-z9-0, this corresponds to 12 + log(O / H) = 7.49 ± 0.11, which we assume as our fiducial value in the following analysis. The fraction of doubly-ionised oxygen over the total is O ++ / O = 0.84 ± 0.01. Neglecting instead any O 3 + contribution (i.e. assuming O / H = O ++ / H + O + / H) would turn into 12 + log(O / H) = 7.40 ± 0.09. Finally, we note that repeating the procedure assuming emission line fluxes measured from the PRISM spectrum delivers a total (ICF-corrected) 12 + log(O / H) = 7.41 ± 0.13, lower but consistent with our fiducial value based on G395M within statistical uncertainties.', '4.2. C/O abundance': "We derive C ++ / O ++ from the C iii ] λ 1909 / O iii ] λ 1666 ratio, assuming the same electron temperature t 3 when calculating the emissivity of the two ions. Because C iii ] λ 1909 is observed only in the PRISM spectrum (while falling in the detector gap in G235M observations), here we adopt the C iii ] λ 1909 / O iii ] λ 1666 ratio as derived from the PRISM, to avoid introducing uncertainties associated with flux calibration di ff erences between PRISM and grating spectra. We also note that the relative temperature associated with C iii ] and O iii ] emission might be di ff erent, as C iii ] is possibly associated with an 'intermediate-ionization zone', which could translate into a higher (lower) C ++ / O ++ abundance by 0 . 15 dex (Garnett 1992; Croxall et al. 2016; Rogers et al.  \n2021; Jones et al. 2023) in case of lower (higher) C ++ temperature, respectively. In principle, it would be possible to derive C ++ / O ++ also from the C iii ] λ 1909 / [O iii ] λ 5007 ratio, however, we prefer to adopt C iii ] λ 1909 / O iii ] λ 1666 to minimise uncertainties on the reddening correction and the choice of the attenuation curve given the short involved wavelength separation, as well as for consistency with the vast majority of literature studies.  \nSince the ionization potential of O 2 + is higher than that of C ++ (54 . 9 eV versus 47 . 9 eV, respectively), systems subject to hard ionising spectra such as GS-z9-0 may have a significant amount of carbon in the C 3 + state. Therefore, we correct the inferred C ++ / O ++ abundance applying an ICF. The C iv λ 1550 emission is detected in both the PRISM spectrum (though possibly blended with other spectral features) and in the G140M grating spectrum (though at lower significance), therefore we can exploit the C iv / C iii ] ratio to estimate the C 3 + / C ++ relative abundance and derive the ICF for C / O or, alternatively, we can compare its strength to that of O iii ] to directly measure a C 3 + / O ++ abundance; the two approaches give consistent results. We note that we have to assume that the C iv emission is purely nebular in origin (as indeed suggested by its narrow line profile in G140M), although the full spectral profile of the C iv λ 1550 doublet can be further complicated by resonant scattering through highly ionised gas, as well as interstellar absorption or contribution from stellar emission, that can either under- or over-estimate the total C iv flux (Berg et al. 2018, 2019; Senchyna et al. 2022). Given the likely non-negligible contribution of O 3 + to the total oxygen abundance, we can not simply assume that C / O = C 3 + / O ++ + C ++ / O ++ . Therefore, we first compute C ++ / H + and C 3 + / H + by multiplying C ++ / O ++ and C 3 + / O ++ by O ++ / H + as derived in Section 4.1, and then we assume that the contribution from single-ionised carbon is negligible, so that the total C / H abundance is = C ++ / H + + C 3 + / H + . Finally, we divide C / H by O / H to infer a total log(C / O) = -0.90 ± 0.12 dex. We note that if we assign the flux of the emission line detected in G140M to the red component of the C iv doublet (i.e., C iv λ 1551), we can compare its strength to that of O iii ] λ 1666 to obtain a gratingbased measurement of the C 3 + / O ++ abundance. This ultimately translates into a total log(C / O) abundance of -0 . 91 ± 0 . 13, fully consistent with the previous estimate.  \nThese values are also consistent, within their uncertainties, with the C / O inferred by assuming an ICF based on the pho-  \nProperty  \nJADES ID  \nJADES-GS-z9-0  \nNIRSpec ID [3215]  \n265801  \nNIRSpec ID [1210]  \n10058975  \nRA  \n53.1124351  \nDec  \n-27.7746258  \nzG395M  \n9.432681  \n±  \n0.000069  \nzPRISM  \n9.43774  \n±  \n0.00020  \nMUV  \n-20.43  \nβ  \nUV  \nlog(  \nR  \ne  \nR  \ne  \n/  \nerg  \n-  \n1  \nHz)  \n25.64  \nMorphology (  \n[arcsec]  \n0.025  \n±  \n0.002  \n[kpc]  \n0.110  \n±  \n0.009  \nPA [degree]  \n-64  \n±  \n3  \naxis ratio  \nq  \n0.44  \n±  \n0.04  \nlog(M  \n⋆  \nSFR [M  \n/  \nM  \n⊙  \n⊙  \n)  \n8.17  \n-  \n1  \n-  \n1  \nyr  \nlog(sSFR  \n/  \nyr  \nAV  \n0.050  \nAge (light-weighted) [Myr]  \n2.80  \nAge (oldest stars) [Myr]  \n12  \nlog(U)  \n-1.46  \nZ  \n/  \nZ  \n⊙  \nlog(M  \n⋆  \nSFR [M  \n/  \nM  \n⊙  \n⊙  \n)  \n8.18  \n-  \n1  \n-  \n1  \n0.046  \n]  \n4.34  \n)  \n-7.53  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n+  \n2  \n-  \n1  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n]  \n5.5  \n)  \n-7.44  \n+  \n0  \n-  \n0  \nyr  \nlog(sSFR  \n/  \nyr  \nAV  \n0.004  \nAge (mass-weighted) [Myr]  \n32  \nlog(U)  \n-1.04  \nZ  \n/  \nZ  \n⊙  \n+  \n20  \n-  \n9  \n+  \n0  \n-  \n0  \n+  \n0  \n-  \n0  \n.  \n02  \n.  \n04  \n.  \n.  \n1  \n08  \n.  \n.  \n.  \n02  \n04  \n008  \n.  \n.  \n006  \n06  \n.  \n03  \n.  \n.  \n.  \n002  \n.  \n.  \n002  \n06  \n.  \n.  \n.  \n06  \n2  \n2  \n.  \n.  \n.  \n07  \n07  \n005  \n.  \n002  \n.  \n.  \n.  \n03  \n06  \n06  \n.  \n06  \nTable 4: Derived physical properties for GS-z9. Fiducial values on the inferred chemical abundances are quoted with both their statistical uncertainties and including (co-added in quadrature) additional systematic uncertainties as discussed in Section 4.5. † upper (lower) limit based on 3 σ upper limits on N iii ] λ 1750 from G140M spectrum.  \n06  \n04  \n0.06", 'SED modelling': 'beagle  \nfitting)  \n-2.54  \n±  \n0.02  \nξ  \nion  \nValue  \ntoionization models presented by Berg et al. (2019), assuming photoionization from star-formation and the ionization parameter self-consistently inferred for GS-z9-0 from the same models (log(U) = -1 . 78), in which case ICF(C ++ / O ++ ) = 1 . 20 ± 0 . 05 4 and C / O = ICF × C ++ / O ++ , providing log(C / O) = -0 . 86 ± 0 . 11. Finally, we estimate the C / O abundance via the equations outlined in Pérez-Montero & Amorín (2017), which deliver log(C / O) = -0 . 80 ± 0 . 12.  \nThroughout the rest of the paper, we assume the C / O derived including both C iii ] λλ 1907,1909 and C iv λ 1550 line fluxes as our fiducial estimate, i.e. log(C / O) = -0 . 90 ± 0 . 11 (statistical uncertainty), corresponding to ∼ 23 per-cent the solar C / Oabundance, or [C / O] = -0 . 64. However, we note that C / O estimates based on the C iii ] λλ 1907,1909 / O iii ] λ 1666 ratio and ICFs from photoionization models generally provide C / Oabundances up to ∼ 0 . 1 dex higher; therefore, we include an additional 0 . 1 dex uncertainty (co-added in quadrature) on the upper value to take into account these systematics.', '4.3. N/O abundance': 'We estimate the N / O ratio by exploiting the detection of N iii ] λ 1750 in emission. Leveraging the fact that the ionization potentials of N ++ and C ++ are basically identical (47 . 448 eV and 47 . 887 eV, respectively), we assume the C ++ / N ++ abundance calculated from the C iii ] λ 1909 / N iii ] λ 1750 ratio (accounting for the emissivity of all five lines of the N iii ] λ 1750 multiplet) as a proxy of the relative C / N enrichment, finding log(C / N) = 0 . 03 ± 0 . 19. Then, we combine this ratio with our fiducial C / O ratio, measured as described in the Section 4.2, to derive a total N / O of log(N / O) = -0 . 93 ± 0 . 24. Based instead on the 3 -σ upper limit on the total flux of the N iii ] λλ 1747-1754 multiplet from the G140M spectrum, we obtain a lower limit on log(C / N) > -0 . 24, and an upper limit on log(N / O) < -0 . 66.', '4.4. Ne/O abundance': "We observe intense emission from Ne in its Ne ++ form and the [Ne iii ] λ 3869 emission line. We derive the Ne ++ / O ++ abundance ratio assuming the t 3 temperature for both ions, and the correct to the total Ne / Oabundance exploiting the ICF presented in Amayo et al. (2021). We note that, given the di ff erent ionization potentials of Ne + and O + , coupled with the charge-transfer recombination rate of the two ions, such ICF is quite uncertain in lowionization, high-metallicity systems. However, in the case of GSz9-0, the fractional contribution of unseen ionization states to the total Ne / O is expected to be small, with ICF(Ne ++ / O ++ ) = 1.02, nonetheless we conservatively assume a 10 per-cent uncertainty on such ICF. We therefore infer a total log(Ne / O) = -0.68 ± 0.06. We note that this value already 'includes' the possible contribution from Ne 4 + : in fact, accounting separately for the marginal detection of [Ne v ] λ 3426 (but without applying any ICF) would deliver log(Ne / O) = -0.73 ± 0.07.  \nforcepho  \nbagpipes", '4.5. Systematics in the abundance measurements': "Despite the high S / N of most of the emission lines detected in the GS-z9-0 spectrum, a number of additional systematics uncertainties a ff ect our chemical abundance measurements beyond those already mentioned in the previous sections. Some of the most relevant are associated with electron temperatures, as briefly discussed here.  \nIn Section 4, we have adopted the temperature inferred from the [O iii ] 4363 / [O iii ] λ 5007 ratio as our fiducial estimate for t 3. However, the O iii ] λ 1666 / [O iii ] λ 5007 is another temperature diagnostics that can be adopted in studies of high-z galaxies where the [O iii ] 4363 emission line is not detected (e.g. Revalski et al. 2024). Exploiting the O iii ] λ 1666 flux measured from G140M (where it is spectrally resolved from He ii λ 1640), we infer t 3 = 24405 ± 1490 K, higher (significant at 1 . 6 σ ) than that inferred from [O iii ] λ 4363. This propagates into a lower inferred O / H by 0 . 15 dex, whereas only in a di ff erence of 0 . 05 dex in C / O, as the C iii ] λ 1909 / O iii ] λ 1666 ratio is only mildly sensitive to temperature. We include further 0 . 1 dex systematic uncertainty on the lower value of log(O / H) to reflect this di ff erence.  \nThe origin of such discrepancy is uncertain. Although similar di ff erences in t 3 as inferred from either O iii ] λ 1666 or [O iii ] 4363 have been reported before (e.g. Berg et al. 2016), these were generally attributed to o ff sets in the flux calibration between O iii ] λ 1666 and [O iii ] λ 5007 (which, before the advent of JWST , were usually observed by instruments on di ff erent facilities), or to the large uncertainty on the relative reddening correction between UV and optical wavelength regimes. In our case, assuming no attenuation correction as suggested by the measured Balmer decrements (see Section 3), this would imply an unrealistic (and never observed in NIRSpec to our knowledge) factor × 1 . 9 o ff set in the flux calibration between G140M and G395M (in the sense that O iii ] λ 1666 is almost a factor of two brighter than it should be) in order to match the t 3 estimates from optical and UV lines.  \nMore physically motivated explanation involves the fact that O iii ] λ 1666 and [O iii ] λ 4363 come from di ff erent energy levels, with di ff erent critical densities and di ff erent collisional excitation rates; hence, their ratio also depends on the gas density, and can therefore be impacted by the presence density inhomogeneities within galaxies. Moreover, when di ff erent temperature estimators rely on collisionally excited lines originating from well-spaced energy levels, the inferred temperatures can also differ by an amount that depends on the variance of the temperature distribution within the nebula, even if the diagnostics pertain to the same ionic species (O ++ in this case). This possibly suggests that rest-frame UV and rest-optical emission lines (even of the same ionic species) could originate from di ff erent regions in the galaxy, and that this e ff ect could be particularly relevant in high-redshift systems like GS-z9-0 characterised by highly ionised gas, where temperature inhomogeneities are expected to be enhanced. This e ff ect has been historically described with the t 2 parameter (see Peimbert & Costero 1969, and more recently Méndez-Delgado et al. 2023), which represents the root mean square deviation from the average temperature. In the case of GS-z9-0, the di ff erence measured between O iii ] λ 1666-based and [O iii ] λ 4363-based temperatures would translate into t 2 ∼ 0 . 24, a value higher than measured in local HII regions. However, given such a large t 2 one would expect a much lower temperature inferred from the nebular continuum ( ∼ 11 , 000K) than observed. In fact, fitting a broken power-law continuum to the 'Balmer jump' region of GS-z9-0 one infers T = 2 . 0 ± 0 . 5 × 10 4  \nK, consistent with the T e-based measurements presented above (see also Ji et al. 2024a).  \nIn a similar manner, another source of systematic uncertainty is associated with the adoption of the same temperature for the emitting region of [O iii ] λ 5007 and C iii ] λ 1909. As C ++ has a lower ionization energy than O ++ , it might be better associated with an 'intermediate' ionization region, probed by ions like S ++ (Berg et al. 2021). Since we do not have access to transitions suitable for directly inferring the temperature of such intermediate zone, we can employ Equation (3) from Berg et al. (2020), which provides Tintermediate = 23150 K and consequently a C ++ / O ++ ratio lower by 0 . 20 dex (which propagates into a total C / O lower by 0 . 13 dex, if still assuming the same t 3 for O ++ and C 3 + ). We note, however, that such temperature-temperature relation can be quite scattered and uncertain, especially at high temperatures and ionization (Hägele et al. 2006; Binette et al. 2012; Berg et al. 2015; Jones et al. 2023); moreover it is mostly calibrated on samples of local HII regions. Therefore, we choose to not adopt it in our fiducial analysis, nonetheless we note that the amount of systematic uncertainty it carries can be significant.  \nA similar argument can also be made for the temperaturetemperature relation employed to infer the abundance of singlyionised oxygen. In Section 4 we have adopted the t 2t 3 relation from Izotov et al. (2006) ; adopting instead the relation calibrated by e.g. Pilyugin et al. (2009), or the theoretical one by Campbell et al. (1986), provides higher t 2 by ∼ 2 , 500 K and ∼ 322 K, respectively. However, this translates to a total ∆ log(O / H) of ≲ 0 . 005 dex, since O + is contributing only ∼ 2 . 5 percent to the total oxygen abundance.  \nFinally, to take into account the abundance of highly ionised ionic species like C 3 + and N 3 + , Berg et al. (2021) proposed a four-zone ionization model to provide a more accurate description of the regions of line emission in galaxies characterised by 'extreme' conditions. In such framework, the temperature of the 'very-high' ionization zone is better probed by the [Ne iii ] λ 3342 / λ 3868 ratio than the classical t 3 (as the ionization potential of Ne 2 + is higher than that of O 2 + ). However, we do not detect [Ne iii ] λ 3342 in our prism spectra, and therefore we have to assume t 3 when computing the emissivity of C 3 + and N 3 + in our carbon and nitrogen abundance calculations. The extent of the associated systematic uncertainty is di ffi cult to assess. Berg et al. (2021) find only a mild (mostly within 1σ uncertainty) impact on the total inferred O / H, C / O, and N / O when adopting either the three- or four-zone ionization model for the galaxies J104457 and J141851, though we note that the fraction of highly ionised species of oxygen, carbon, and nitrogen are minimal in those systems. On the other hand, we note that our derivation of the N / O abundance in GS-z9-0 follows from the measurement of C / N, which is based on emission line ratios with relative weak temperature dependence, whereas for what concerns the C / Oderivation, conservatively assuming a temperature much higher than t 3 (e.g., 25 , 000 K) as representative of the C iv emitting region (as it would be in case C 3 + preferentially occupies a region closer to the ionising source) would translate into a lower total inferred C / O by ∼ 0 . 24 dex compared to our fiducial value. This e ff ect might be even larger for higher states of neon like Ne 4 + that could originate from extremely high-ionization regions (especially in the case of the NLR of AGN); however, given the small relative Ne 4 + / Ne ++ abundance, we verified that even assuming a temperature as high as 25 , 000 K it would not significantly impact the final inferred Ne / O abundance (which would be lower by 0.02 dex compared to our fiducial estimate).  \nTo account for the hereby discussed e ff ects, we include an additional 0 . 15 dex of systematic uncertainty also on the C / O  \nand N / O 5 abundances beyond the formal statistical uncertainties propagated from the error on the measured line fluxes. This, together with the uncertainties on C / O from photionisation-based ICFs discussed already in Section 4.2, translates into the final uncertainties quoted in parenthesis in Table 4.", '5. Possible scenarios for chemical enrichment': "In Figure 10 we report GS-z9-0 (green hexagon marker) on different diagrams which explore trends between various chemical abundance patterns (i.e. C / O, N / O, and Ne / O) and metallicity (probed by oxygen abundance). The solid errorbars report the formal statistical uncertainties on our fiducial estimates of chemical abundances, whereas dashed errorbars account for the systematics discussed in Section 4.5.  \nThe top-left panel shows the C / O versus O / H diagram, with GS-z9-0 compared with previous abundance determinations in both local and high-redshift systems, as well as those measured in Milky Way (MW) disk and halo stars (see caption of Fig. 10 for details and references). The low C / Oabundance in GS-z9-0 is comparable to that observed in metal-poor systems at lower redshift, being in line with some of the most recent determinations in z ≳ 6 galaxies observed with JWST from (Arellano-Córdova et al. 2022, z ∼ 8 . 5), (Jones et al. 2023, z ∼ 6 . 23), (Isobe et al. 2023b, z ∼ 6 . 2), (Topping et al. 2024, z ∼ 6 . 3), (Castellano et al. 2024, z ∼ 12 . 3) (whereas in contrast, the upper limits on GS-z12 from D'Eugenio et al. (2023) suggest significantly super-solar C / O in that system).  \nGiven the very young age of the system (mass-weighted age ∼ 30 Myr as inferred from the SED fitting), we expect chemical yields dominated by core-collapse SNe (CCSNe), whereas low- and intermediate-mass stars in their AGB phase (with lifetimes ≈ 100 Myr) would not have had enough time to significantly contribute to the overall enrichment of the galaxy. Indeed, the datapoint for GS-z9-0 on the C / O versus O / H plane is consistent with the range in C / O predicted by integrating the yields of type II SNe from very-metal-poor stars over a Salpeter IMF (Tominaga et al. 2007, golden shaded region). In the same panel, we also compare our observations with the C / O enrichment pattern predicted by galactic chemical evolution (GCE) model of the solar neighbourhood presented in Kobayashi et al. (2020) (shaded green curve), which follows the time evolution of elemental abundances in a one-zone model with ISM instantaneously mixing but no instantaneous recycling approximation (i.e, the delay of chemical enrichment from di ff erent sources is taken into account), assuming a Kroupa et al. (1993) IMF and solar abundances from Asplund et al. (2009). Our observations agree well with the sequence in C / O of Kobayashi et al. (2020) at the measured oxygen abundance for GS-z9-0, as this system resembles the C / O enrichment pattern of metal-poor stars in the MW.  \nThe position of galaxies on the C / O vs O / H diagram is also sensitive to the amount of metals lost due to outflows. Possible indications of outflowing gas in GS-z9-0 comes from the observed blueshifted C iv λλ 1549,1551 doublet in G140M (Figure ?? . The spread observed in C / O at fixed O / H in samples of metal-poor local dwarfs and z ∼ 2 galaxies can be reproduced in fact not only in terms of di ff erent SFH and IMF slopes, but even accounting for selective oxygen removal in outflows driven by CCSNe (Yin et al. 2011; Berg et al. 2019) According to the formalism from Berg et al. (2019), developed to model the C / O vs O / H pattern observed in local, metal-poor dwarf galaxies, the  \nlow O / H and low C / O measured in GS-z9-0 would indicate negligible oxygen-enhancement in SN-driven outflows. However, we note that in such a framework low C / O abundances are associated with single, long bursts of star-formation characterised by relatively small star-formation e ffi ciency, whose timescales ( ∼ 300 -400Myr) can be hardly reconciled with the inferred age and star-formation history of GS-z9-0.  \nIn the bottom-left panel of Figure 10, we report instead the location of GS-z9-0 on the N / O vs O / H diagram. Based on the N / O derived from the N iii ] λ 1750 detection in the PRISM spectrum as detailed in Section 4.3, GS-z9-0 appears nitrogenenriched compared to the typical range of values observed in local, low-metallicity dwarf galaxies (e.g. Pérez-Montero & Contini 2009; Berg et al. 2012; Vale Asari et al. 2016; Vincenzo & Kobayashi 2018) as predicted by 'primary' nitrogen enrichment, showcasing instead N / O consistent with the solar value. A similar pattern characterised by high N / O abundance at low O / H and low C / O has been already observed in several highredshift ( z > 6) galaxies observed with JWST (e.g., GN-z11, GLASS150008,RXCJ2248-ID, GHZ2 Cameron et al. 2023a; Isobe et al. 2023b; Topping et al. 2024; Castellano et al. 2024; Schaerer et al. 2024), although we note that GS-z9-0 does not reach the same level of N / O enhancement as the aforementioned systems, which are observed to be significantly supersolar in N / O. Low C / N ratios are also often observed in the so-called 'nitrogen-loud quasars' (Batra & Baldwin 2014), as possibly produced by the enrichment from AGB stars or SNe occurring within the small, local volume of the narrow-line or even broad-line region of AGN (Maiolino et al. 2023b). Such a scenario can produce a 'chemical stratification' between central, denser regions and the rest of the galaxy, which is imprinted on the largely di ff erent N / O derived from UV lines (tracing dense, highly ionised regions) and optical lines (tracing the bulk of the galaxy) as observed e.g. in the z ∼ 5 . 5 active system GS-3073 (Ji et al. 2024b, for which UV-based and optical-based measurements are marked by hatched and plain brown octagons in Figure 10). However, in the case of GS-z9-0 the contribution from AGB stars is strongly disfavoured given the inferred age and SFH of the system, and moreover it would be di ffi cult to reconcile with the simultaneous low O / H and C / O.  \nThe N / O pattern seen in GS-z9-0 also resembles that of nitrogen-enhanced metal-poor (NEMP) stars observed in the Galactic halo (Belokurov & Kravtsov 2023). Even higher level of N-enhancement (log(N / O) ≳ 0) is seen in some dwarf stars within globular clusters (Carretta et al. 2005; D'Orazi et al. 2010), which have been suggested to share a similar chemical composition as some of the most extremely nitrogen rich galaxies observed by JWST (e.g. Charbonnel et al. 2023; Senchyna et al. 2023; Marques-Chaves et al. 2024; Topping et al. 2024), carrying signatures of nuclear burning of hydrogen in the CNO process. Despite GS-z9-0 showing a relatively lower N / O compared to such systems, its N-enrichment level compared to the N / O vs O / H plateau, coupled with the concentrated starformation, could still be consistent with proto-globular cluster formation contributing, to some extent, to the observed integrated galaxy spectra. Unfortunately, no direct constraints on the density of the highly ionised gas in GS-z9-0 from UVdiagnostics (e.g. from C iii ] λ 1909 or N iii ] λ 1750 doublet ratios, either because unresolved in the PRISM spectrum, or undetected in R1000 spectra), as well as the putative AGN contribution suggested by the presence of very high-ionization emission lines, prevents us from drawing more definitive conclusions in such sense.  \nFig. 10: Chemical abundance patterns in GS-z9-0. The location of GS-z9-0 is marked by the green hexagon, with solid errorbars reporting formal statistical uncertainties on our fidudcial values and dashed errorbars accounting for additional systematic uncertainties as discussed in 4.5. Dashed black lines in the diagrams mark the solar abundance values. We compare GS-z9-0 with a compilation of recent abundance measurements in high-z galaxies from JWST , i.e. from Ji et al. 2024b (plain marker for opticalbased and hatched marker for UV-based abundances, respectively), Topping et al. 2024, Isobe et al. 2023b and Jones et al. 2023 at z ∼ 6, Arellano-Córdova et al. 2022 at z ∼ 8 . 5, Castellano et al. 2024 at z ∼ 12, as well as upper limits on GN-z11 and GS-z12 from Cameron et al. 2023a and D'Eugenio et al. 2023, respectively. We also include measurements in Milky Way disk and halo stars, local HII regions, local dwarf galaxies, z ∼ 2 galaxies, and Damped Lyman-Alpha systems (DLA). Upper-left panel: C / O versus O / H diagram. The C / O abundance range predicted by type II SNe yields from Tominaga et al. 2007 is marked by the golden hatched region, whereas the C / O vs O / H pattern from Kobayashi et al. 2020 galactic chemical evolution (GCE) models is shown by the green sequence. The C / O level observed in GS-z9-0 is consistent with the enrichment expected from core-collapse SNe for a recently assembled, low-metallicity system. Bottom-left panel: N / O versus O / H diagram, reporting the abundance measurement in GS-z9-0 leveraging the detection of N iii ] λ 1750 in the PRISM spectrum. The measured N / O is higher than the plateau described in the diagram by local galaxies and HII regions at low metallicity, although the nitrogen enhancement observed in this galaxy appears less extreme compared to other systems recently observed by JWST at high-z. The green dashed line marks the upper limit on N / O from the 3 σ upper limit N iii ] λ 1750 flux in the G140M grating. Bottom-right panel: Ne / Oabundance for GS-z9-0, which we observe consistent with the abundance pattern expected at low-metallicity for α -elements.  \n<!-- image -->  \nAdditional references for literature points : MW stars (Gustafsson et al. 1999; Akerman et al. 2004; Fabbian et al. 2009; Nissen et al. 2014); HII regions (Tsamis et al. 2003; Esteban et al. 2004, 2009, 2014; Esteban et al. 2017; García-Rojas et al. 2004, 2005, 2007; Peimbert et al. 2005; García-Rojas & Esteban 2007; López-Sánchez et al. 2007; Toribio San Cipriano et al. 2016; Toribio San Cipriano et al. 2017); local dwarfs (Berg et al. 2016, 2019; Vale Asari et al. 2016; Senchyna et al. 2017); z ∼ 2 galaxies (Fosbury et al. 2003; Erb et al. 2010; Christensen et al. 2012; Bayliss et al. 2014; James et al. 2014; Stark et al. 2014; Steidel et al. 2016; Vanzella et al. 2016; Amorín et al. 2017; Berg et al. 2018; Mainali et al. 2020; Matthee et al. 2021; Rigby et al. 2021; Iani et al. 2023); DLAs (Cooke et al. 2017; Saccardi et al. 2023).  \nIn GS-z9-0 therefore, we possibly observe signatures of chemical enrichment mechanisms beyond that of pure CCSNe, as depicted by the relatively high N / O and low C / N. Several scenarios have been explored to reproduce similar abundance patterns in high-z galaxies, including ejection of CNOcycle processed material in stellar winds from Wolf-Rayet (WR) and Super Massive Stars (SMS) (e.g. Nagele & Umeda 2023; Watanabe et al. 2024). Here, we compare the observed C / O, N / O, and C / N abundances in GS-z9-0 with a set of GCE models from Kobayashi et al. (2020); Kobayashi & Taylor (2023), with updated carbon yields for WR stars. In particular, we assume a single-burst model following similar prescriptions as in Kobayashi & Ferrara (2024), and explore the impact of di ff erent IMF shapes on the nucleosynthetic patterns of the most massive stars, and hence on the observed chemical abundances. In Figure 11, we over-plot the tracks from three di ff erent realisations of the model onto the C / Ovs O / H, N / Ovs O / H, and C / Nvs O / H diagrams, respectively. In particular, the blue track represents the predictions assuming a standard Kroupa et al. (1993) IMF (with high mass-end slope x = 1 . 3), whereas the magenta and red tracks assume a top-heavy IMF ( x = 0) with upper mass limits of 120M ⊙ and 280M ⊙ , respectively. Each model track reaches the observed value of O / H for GS-z9-0 after 4 . 05 Myr.  \nEvolutionary tracks with underlying top-heavy IMF provide the best match to the position of GS-z9-0 in all three diagrams simultaneously, with WR stars producing the required enrichment in N / O (magenta track). We note that the red track, which includes a contribution from pair-instability supernovae (PISNe) of massive progenitors ( > 150M ⊙ ), also provides a good match to the observed abundances, maintaining the C / Oto a level more consistent with that observed in GS-z9-0, while slightly underpredicting (overpredicting) N / O (C / N). Additional abundance measurements of elements like Sulphur, Argon, or Iron would be required to better constrain the possible signatures of PISNe and discriminate their relative contribution.  \nIn addition, we also consider a 'dual burst model' (dashed orange track in Figure 11) that reproduces the abundances in GS-z9-0 without the need to invoke top-heavy IMFs, similarly to what has been developed by Kobayashi & Ferrara (2024) to match the abundance patterns observed in GN-z11. Contrary to 'single burst models', this model assumes a more tailored starformation history, which is halted for a period of ∼ 100Myr between two strong bursts. Such a model produces a quick enhancement of both C / O and N / O via WR stars just after the second burst, which occur within a pre-enriched ISM from the first star-formation episode. While N / O can be regulated (and lowered) by increasing the timescale of star-formation for the second burst (making it 'less extreme'), this model slightly overpredicts the C / N level compared to what observed in GS-z9-0.  \nFinally, we display the measured Ne / Oversus the O / Habundances for GS-z9-0 in the bottom right panel of Figure 10, where we compare our measurement at z ∼ 9 . 4 with the values inferred for the galaxy ERO-4590 at z ∼ 8 . 5 by Arellano-Córdova et al. (2022), as well as with measurements in blue compact dwarfs and extragalactic giant HII regions from Vale Asari et al. (2016), and in HII regions of local galaxies from the CHAOS sample as studied in Berg et al. (2020). As neon is an α -element, its production mechanisms and timescales are expected to closely follow that of oxygen, with no significant trend in Ne / O as a function of metallicity. Indeed, the Ne / O abundance measured in GS-z9-0 is fully in agreement with the low-metallicity tail in the diagram as probed by local blue compact dwarf galaxies. Although a possible evolution in the Ne / O abundance pattern at z > 3 has been suggested on the basis of the observed scal-  \nlation between stellar mass and the [Ne iii ] / [O ii ] line ratio (Shapley et al. 2023a), and Isobe et al. (2023b) find that significantly lower Ne / Oratios in some z > 6 galaxies can be explained by models of CCSNe ejecta including massive ( > 30M ⊙ ) progenitors, here we find no evidence for a deviation in the Ne / O abundance in GS-z9-0 compared to the chemical abundance patterns of local, metal-poor compact galaxies.", '6. Ly α emission and implications for Reionization': "Despite the MSA detector gap cutting the G140M spectrum just blueward of the systemic Ly α wavelength, it still covers the onset of the Gunn & Peterson (1965) absorption trough. Notably, an emission feature stands above the continuum just redward of the systemic redshift Ly α at z = 9 . 4327. If we interpret this feature as Ly α emission, we can put more stringent constraints on the shape of the transmission curve, the escape fraction of Ly α photons, and the possible existence of an ionised bubble around the system. We fit this emission line with a single Gaussian component and model the underlying continuum with a broken linear component as shown in Fig. 12, measuring a flux of 2.18 × 10 -19 erg s -1 cm 2 , significant at 2 . 7 σ (2 . 3 σ based on bootstrapping). The emission line centroid is redshifted by 456 ± 145 km / s with respect to the systemic redshift, and the estimated rest-frame EW of the line above the G140M continuum level is 31 ± 16.  \nBased on the theoretical ratio between (unattenuated) Ly α and H β fluxes (Ly α / H β = 25 . 1 for case B recombination, T = 20,000 K, N e = 650 cm -3 ), and assuming the H β flux measured from the G395M spectrum, we derive an intrinsic Ly α flux of 1.89 × 10 -17 erg s -1 cm 2 , from which we infer that the Ly α emission has been attenuated down to ∼ 1 percent of its intrinsic flux. Such a low Ly α escape fraction, coupled with the presence of emission observed only significantly redward of the systemic wavelength, and the negligible dust attenuation, provides evidence against the presence of a large ionised region surrounding GS-z9-0. Such an ionised bubble in fact would likely allow a higher fraction of Ly α photons to escape (given the intrinsic predicted flux) and, under the assumption of a broad, symmetric intrinsic Ly α profile, would have also caused the line profile to spill across to the blue of the systemic redshift.  \nIn addition to the detection of Ly α emission, we can leverage the robust redshift determination enabled by the bright emission lines detected in GS-z9-0 to remove the redshift-degeneracy in modelling the Ly α break as observed (at high signal-to-noise) in the PRISM spectrum, and get more insights into the properties of the surrounding medium, the possible extent of any ionised region, and the presence (or absence) of large column densities of neutral gas along the line of sight. We model the attenuation due to neutral IGM along the line of sight following the prescriptions from Inoue et al. (2014), and include treatment of the the full optical depth (including damping-wing absorption) of Ly α photons following Dijkstra (2014); Mason & Gronke (2020) (for further details, see Witstok et al. 2024b), under the assumptions of the gas in the IGM having mean cosmic density and T = 1 K, while gas in the ionised bubble is fully ionised (residual neutral fraction x HI = 10 -8 ) with T = 10 4 K. We apply such transmission curve to a power-law spectrum modelling the continuum blueward of 1500Å rest-frame 6 , to avoid the region contaminated by emission lines, and we include the contribution from the Ly α emission matching flux, velocity and width as observed  \n<!-- image -->  \n<!-- image -->  \nFig. 11: Evolution of CNO abundance ratios as a function of oxygen abundance for single starburst models with di ff erent IMFs, based on the framework of galactic chemical evolution models from Kobayashi et al. 2020. Symbols are as in Figure 10, with GS-z9-0 in green and other high-z objects observed by JWST from the literature highlighted in yellow. Shaded regions in the C / O vs O / H and C / N vs O / H panels mark the range allowed by pure CCSNe based on the yields from Tominaga et al. 2007 and Watanabe et al. 2024, respectively. The blue model assumes a 'standard' IMF (with high-mass end slope x = 1 . 3), whereas the magenta and red curves assume a top-heavy IMF (with flat slope) for massive stars in range 30-120M ⊙ , and 100-280M ⊙ , respectively, with the latter including PISNe. For each model, the curve reaches the observed O / H in GS-z9-0 after 4.05 Myr. The combined CNO pattern of GS-z9-0 is better matched by models with underlying top-heavy IMF. In order to reproduce the abundance patterns of GS-z9-0 with a 'standard' IMF, a more tailored 'double burst model' (with two bursts separated by 100 Myr, dashed orange track) is needed, similar to what proposed by Kobayashi &Ferrara 2024 to explain the nitrogen enhancement in GN-z11.  \n<!-- image -->  \nin the G140M spectrum 'a-posteriori' into the model (i.e., after attenuating the intrinsic spectrum), as it could possibly a ff ect  \nFig. 12: Zoom-in on the tentative ( ∼ 2 . 5 σ ) detection of Ly α emission line in the G140M spectrum of GS-z9-0. The emission feature is observed shifted by ∼ 450 km s -1 redward of the systemic Ly α at redshift z = 9 . 4327.  \n<!-- image -->  \nthe observed profile. 7 We then convolve the attenuated model by the PRISM LSF provided by de Graa ff et al. (2024) and resample it to the PRISM wavelength grid before comparing to the data. In order to properly account for the inter-correlation between adjacent wavelength bins in the fitting procedure, the χ 2 goodness-of-fit statistics involves the covariance matrix empirically derived from 2000 bootstrapped realisations of the PRISM spectrum leveraging the 186 available individual exposures, as detailed in Jakobsen et al. (in preparation). In particular,  \nχ 2 = R T [ CovF λ ] -1 R (2)  \nwhere [ CovF λ ] -1 is the inverted covariance matrix and R = F λ -F model is the vector of residuals.  \nHere in particular we explore which constraints can be set on the size of the ionised bubble and the amount of neutral gas along the line of sight in a simple scenario where the intrinsic stellar spectrum (modelled by a 'power-law') is attenuated by a fully neutral IGM. In this model, the fraction of neutral hydrogen is set to unity (i.e. x HI = 1), while the size of the surrounding ionised region R ion (expressed in physical Mpc) and the column density of neutral gas are free to vary. We assume the neutral gas to pertain to the local galaxy ISM or the surrounding circumgalactic medium (CGM), similar to what is observed in DLA systems, hence we fix the redshift of such DLA component to the systemic redshift of GS-z9-0. The Ly α optical depth due to the DLA component is modelled as a Voigt profile (following the approximation of Tasitsiomi 2006) and is solely parametrised by the column density of neutral atomic hydrogen N HI (we assume an ISM temperature T = 10 4 K, and no additional contribution due to a turbulent medium).  \nThe best-fit model (red curve in Figure 13, χ 2 = 54 . 57 for 34 degrees of freedom) predicts log(N HI / cm -2 ) = 19 . 92 and R ion = 0 as formal best-fit values, delivering a good match to the observed spectral profile. We explore the confidence region for the two main parameters by mapping the χ 2 over the R ion = 0 versus N HI parameter space, taking the ∆ χ 2 = ( χ 2 -χ 2 best ) = 1 , 4 , 9 contour levels as the 1 σ , 2 σ and 3 σ confidence intervals, respectively. As shown in the bottom left panel of Figure 13, any  \nlocal (i.e. at systemic redshift) column density of neutral gas is strongly constrained to be lower than log(N HI / cm -2 ) ≲ 20 . 75 at 99 . 7% confidence. Very similar constraints are obtained by employing the multinest algorithm (Feroz et al. 2009) to sample the posterior probability density function of each parameter 8 . The bottom-right panel of Figure 13 shows the corner plot for R ion and log(N HI / cm -2 )), from which we quote an upper limit on log(N HI / cm -2 ) < 20 . 5 as estimated from the 95th percentile of the marginalised posterior distribution.  \nInterestingly, our inferred upper limits are much lower than what other recent observations of z ≳ 10 galaxies have suggested, reporting detection of local column densities of neutral gas as high as log(N HI / cm -2 ) = 22.5 which have been interpreted as evidence for the presence of large reservoirs of cold gas available to sustain early star-formation and galaxy growth (e.g. D'Eugenio et al. 2023; Heintz et al. 2023, 2024; Hainline et al. 2024a; Carniani et al. 2024). Such large column densities in fact would push the onset of the Ly α absorption trough redward than the systemic redshift and significantly soften the observed spectral break: this is illustrated for instance by the light and dark green curves in Figure 13, which show how the transmitted spectrum would look like in the presence of local DLAs of log(N HI / cm -2 ) = 21.5 and 22.5, respectively, resulting in very di ff erent spectral shapes than observed in the PRISM spectrum of GS-z9-0. We note that if we instead leave x HI free to vary between 0 and 1 (hence accounting for the degeneracy between the column density of neutral gas in the CGM / ISM and the hydrogen neutral fraction in the IGM), while setting R ion to zero, the fit strongly favours a solution with x HI = 1 and low N HI , i.e. the transmission curve from the IGM absorption can not be exactly mimicked by local DLA absorption at any column density.  \nFor what concerns the constraints inferred from our simple model on the size of a possible ionised region around GS-z90, the bottom left panel of Figure 13 shows that for any value of log(N HI / cm -2 ) ≲ 20 R ion is constrained to be ≲ 0 . 1pMpc at 1 σ confidence ( ≲ 0 . 4pMpc at 3 σ confidence), whereas only within a rather fine-tuned interval of log(N HI / cm -2 ) values (20 . 3 ≲ log(N HI / cm -2 ) ≲ 20 . 8) the 1 σ confidence region extends up to 0 . 6 pMpc ( with 2 σ and 3 σ intervals extending beyond 1pMpc). Again, similar constraints are obtained from the marginalised posterior PDF for R ion , from which we quote R ion < 0 . 447pMpc as the 95 th percentile of the distribution.  \nWe can compare the constraints on R ion derived from our modelling of the Ly α spectral break with what we could expect for a galaxy like GS-z9-0 on the basis of simple arguments involving the age of the system and its ionizing e ffi ciency. First, we estimate the ionizing photons production e ffi ciency of GSz9-0 leveraging the measured flux of H β . We follow e.g. Saxena et al. (2023); Simmonds et al. (2023); Witstok et al. (2024a) to infer  \nξ ion = 3 × 10 12 erg -1 L H β / L ν, 1500 , (3)  \nmeasuring the UV luminosity density at 1500Å rest-frame and under the assumption of case B recombination and zero escape fraction of Lyman continuum photons ( f esc,LyC = 0, hence yielding a lower limit), finding log( ξ ion / erg -1 Hz) = 25.64, in agreement with typical values for bright (MUV < -20) galaxies in the EoR (e.g. Endsley et al. 2023). Following (e.g. Mason & Gronke 2020; Witstok et al. 2024b), we can then estimate the size of the  \nsurrounding ionised region that would be produced by a galaxy like GS-z9-0 from  \nR ion ( t ) ≈ GLYPH<18> 3 f esc,LyC ˙ N ion t 4 π ¯ nH ( z ) GLYPH<19> (4)  \nwhere ¯ nH (z) is the mean hydrogen number density at redshift z and ˙ N ion the production rate of ionising photons. Assuming that the ionising photon production is powered by a very recent burst of star-formation occurred over the past 5 Myr, as suggested by both the steeply rising SFH derived in Section 2.3 and the chemical abundance patterns discussed in Section 5, and a f esc,LyC = 5% (Finkelstein et al. 2019) would produce an ionised bubble with size R ion ≈ 0 . 1pMpc (0 . 05-0 . 12pMpc if assuming f esc,LyC ranging between 1% and 10%), in good agreement with the 1 σ confidence limits inferred from our modelling of the Ly α break for the majority of allowed N HI values. These values are also in line with what predicted for Ly α emitters (LAEs) with similar physical properties to GS-z9-0 discovered at slightly lower redshift (see e.g. ∼ 7 -8 objects in Witstok et al. 2024b,a), whose (sometimes significantly) larger inferred sizes of ionised bubbles (constrained by large observed Ly α / H β ratios and small velocity o ff sets) requires to include the contribution from either faint neighbour companions or older galaxies, to assume bursty SFHs, or to invoke particularly favourable geometrical configurations.  \nSummarising, we have found indications for GS-z9-0 to be surrounded by a small ionised bubble, as suggested by the low escape fraction of Ly α photons ( ∼ 1%), the relatively large velocity o ff set ( ∼ 450 km / s), and the constraints set by modelling the damping wing of the Ly α break. Despite the high production e ffi ciency of ionising photons powered by its recent SFH, this galaxy is therefore unlikely to have contributed significantly to Reionization at the epoch of observations.  \nFinally, we note that, despite providing a reasonably good match to the data, our baseline model assuming x HI = 1 is not capable of perfectly reproducing the observed spectral shape of GS-z9-0 in the PRISM spectrum, as clear residuals are observed for instance between ∼ 1 . 3-1 . 35 µ m, where the profile softens before turning-over (see Figure 13). Indeed, we stress that in order to place more precise and stringent constraints on the neutral fraction of the IGM it is critical to have a better understanding of the systematics involved in modelling the intrinsic UV continuum and the exact shape of the Ly α emission that emerges from the galaxy, as well as the impact of possible fluctuations in the hydrogen density associated to large scale structures at these redshifts (e.g. Smith et al. 2022). Moreover, a proper modelling of the contribution to attenuation due to local absorbers would require the precise knowledge of the geometry and density profiles of the intervening gas clouds (Gronke 2017; Hutter et al. 2023). A more detailed discussion on the damping wing of the Ly α absorption profile in GS-z9-0, the inferred constraints on x HI , and the implications on the history of cosmic Reionization, is presented in Jakobsen et al., (in preparation).", '7. Summary': 'We have presented ultra-deep JWST / NIRSpec observations of GS-z9-0, a luminous (MUV = -20.43) galaxy at redshift z ∼ 9 . 4327. Combining datasets from JADES programmes PID 1210 and PID 3215 provides a total of 72 hours of observations in PRISM / CLEAR mode, 44 hours in G395M / F290LP mode, and 16 hours in G140M / F070LP mode. Leveraging the high signalto-noise of the spectrum (Figure 2) and the detection of multiple  \n]  \nc  \np  \nM  \np  \n[  \nn  \no  \ni  \nR  \n1.0  \n0.8  \n0.6  \n0.4  \n0.2  \n0.0  \n)  \nc  \np  \nM  \np  \n(  \nn  \no  \nR  \nIonised bubble radius  \ni  \n1.00  \n0.75  \n0.50  \n0.25  \n0.00  \nlog10  \nFig. 13: Model of the Ly α break in the PRISM spectrum. Upper panel : The red line represents the best-fit from a model in which we attenuate a power-law spectrum by applying the transmission curve from a fully neutral IGM (x HI fixed to 1) while fitting for the size of any ionised bubble (R ion ) surrounding GS-z9-0 and for the column density of a local DLA system along the line of sight (N HI ). The residuals from the fit are compared with both the pipeline error spectrum and the square root of the diagonal terms of the covariance matrix described in Section 6 (Jakobsen et al., in preparation). The formal best-fit values provide R ion ∼ 0 and log(N HI / cm -2 ) = 19.9. We note that including attenuation from such a column density of neutral gas placed at the galaxy systemic redshift does not provide significant improvements to the model compared to only applying attenuation from purely neutral IGM (blue dotted line). For comparison, the light and dark green curves show the spectral shape obtained by including DLAs with log(NHI / cm -2 ) = 21 . 5 and 22 . 5, respectively. Bottom-left panel : Contour levels of the χ 2 -map on the log(NHI) versus R ion parameter space, highlighting 1 σ , 2 σ , and 3 σ confidence regions. The column density of any DLA at the systemic redshift is constrained to log(NHI / cm -2 ) < 20 . 75 at 99 . 7% confidence. Bottom-right panel : Joint contours and marginalised posterior probability density function for the same parameters. The quoted upper limits report the 95th percentiles of the posterior distributions.  \n<!-- image -->  \nÂ  \n2  \n1  \n2  \n3  \n¾  \n¾  \n¾  \n19.00  \n20.00  \n21.00  \n22.00  \nlog(NHI  \n=  \ncm  \n¡  \n2  \n)  \n(  \nN  \nHI (cm  \n-2  \n)  \n)  \n<20.5  \n19.2 19.8 20.4 21.0 21.6  \nHI column density  \nlog10  \n(  \nN  \nHI (cm  \n-2  \n)  \n)  \n0.00  \n0.25  \n0.50  \n0.75  \n1.00  \nIonised bubble radius  \nR  \nion (pMpc)  \nemission lines in both rest-frame optical and UV regimes (Figure 3 and 5), we characterise the ISM properties and the starformation history of this source, observed only ∼ 500 Myr after the Big Bang.  \nThe main results discussed in the paper are summarised as follows :  \n- -The system is young (light-weighted age ∼ 3 Myr, massweighted age of ∼ 30 Myr), compact (R e ∼ 110 pc), and ex-  \nmap and confidence region  \n- hibit a steeply rising star-formation history over the past 310 Myr with high star-formation rate surface density ( Σ SFR ∼ 72 M ⊙ yr -1 kpc -2 ), suggesting that the majority of mass assembly has occurred during a recent and concentrated burst of star-formation (Figure 6). Negligible dust attenuation is inferred from both SED fitting and Balmer lines (Figure 7). -Emission line ratios from the rest-frame UV are indicative of a hard radiation field powered by young, massive, and metal-poor stars (Figure 8), in particular when the abundance patterns of the model grids (e.g. in C / O) are matched to those inferred independently from the emission lines. However, the tentative detection of the extremely high-ionization [Ne v ] λ 3426 line (Figure 4) suggests that contribution to ionization from the NLR of an AGN, or from shocks driven by SNe explosions, can not be excluded (Figure 9).\n- -The measured C / O abundance (log(C / O) = -0 . 90, [C / O] = -0 . 64) lies at the lowest envelope of the distribution of local and high-redshift galaxies at similar metallicity (12 + log(O / H) ∼ 7 . 5), in agreement with a rapid, recent history of chemical enrichment dominated by yields of corecollapse Supernovae from massive stars progenitors (Figure 10).\n- -However, marginal detection of N iii ] λ 1750 emission in the PRISM spectrum suggests over-abundant N / O (log(N / O) = -0 . 93, close to the solar value) compared to the average plateau occupied by local, low-metallicity galaxies, similar to (though not as extreme as) nitrogen-enriched galaxies at z > 6 recently observed by JWST (Figure 10).\n- -The observed CNO abundance patterns are well reproduced by \'single-burst\' chemical evolution models including contribution from very massive stars (M ⋆ ≳ 100M ⊙ ), possibly unveiling the short timescale signatures of enrichment from Wolf-Rayet stars or pair-instability Supernovae progenitors, in line with a top-heavy IMF scenario regulating the process of star-formation in the earliest galaxies (Figure 11). More \'fine-tuned\' SFHs (e.g. characterised by two bursts separated by a ∼ 100 Myr period of quiescence) are required to match the CNO abundance patterns assuming a standard IMF.\n- -By modelling the Ly α spectral break, we rule out the presence of very-high column density local absorbers (log(N HI / cm -2 ) ≲ 20 . 75 at 99 . 7% confidence, Figure 13). We also report tentative detection of Ly α emission in the G140M spectrum (Figure 12), redshifted by ∼ 450 km / s from systemic, from which we infer an escape fraction of Ly α photons from IGM attenuation of ∼ 1 percent. Together with the constraints from the Ly α break, and in line with the young age of the system, this suggests the size of any ionised region surrounding GS-z9-0 to be ≲ 0 . 1-0 . 2 pMpc and that, despite its high ionising photons production e ffi ciency, this system have not yet significantly contributed to cosmic Reionization at the time of observations.  \nSources like GS-z9-0 currently represent the best opportunities to directly investigate the physics underlying the early phases of galaxy assembly, their evolution, and the environment they live in. The richness of spectral features and the peculiar chemical abundances observed in this and other high-redshift systems challenge our understanding of the nature of massive stars powering the extreme emission lines seen in the spectra and possibly hint at the role of early accretion onto (super-)massive black holes. In this sense, the present analysis also highlights the wealth of information delivered by deep observations that simultaneously probe rest-frame UV and optical spectra of z ∼ 6-10 galaxies. However, despite the enormous advancements already  \nR  \nion (pMpc) < 0.447  \nbrought by JWST , the intrinsic faintness of most rest-frame UV stellar and nebular features currently hinders their detailed characterisation within the context of the more general high-redshift galaxy population. Alongside the identification and follow-up of new, bright high-redshift candidates, deeper spectroscopy at medium-high resolution probing the UV continuum and emission line features over large samples will therefore be key to gradually move from the study of the most luminous systems towards a broader understanding of the physical properties of galaxies in the early Universe.  \nAcknowledgements. We are grateful to Danielle Berg, Matilde Mingozzi, Anna Feltre, Bethan James, John Chisholm, Evan Skillman, Alessandro Marconi, and Charles Steidel for enlightening conversations. MC acknowledges support by the European Southern Observatory (ESO) Fellowship Programme. JW, FDE, RM, JS, & WB acknowledge support by the Science and Technology Facilities Council (STFC), ERC Advanced Grant 695671 \'QUENCH", and by the UKRI Frontier Research grant RISEandFALL. RM also acknowledges funding from a research professorship from the Royal Society. The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant DNRF140. ECL acknowledges support of an STFC Webb Fellowship (ST / W001438 / 1). JC, AJB & AC acknowledge funding from the \'FirstGalaxies" Advanced Grant from the European Research Council (ERC) under the European Union\'s Horizon 2020 research and innovation programme (Grant agreement No. 789056). SC & GV acknowledge support by European Union\'s HE ERC Starting Grant No. 101040227 - WINGS. SA acknowledges grant PID2021-127718NB-I00 funded by the Spanish Ministry of Science and Innovation / State Agency of Research (MICIN / AEI / 10.13039 / 501100011033). This research is supported in part by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. DJE, BDJ, BR, and CNAW are supported by JWST / NIRCam contract to the University of Arizona NAS5-02015. DJE is also supported as a Simons Investigator. BER acknowledges support from the NIRCam Science Team contract to the University of Arizona, NAS5-02015, and JWST Program 3215. ST acknowledges support by the Royal Society Research Grant G125142. HÜ gratefully acknowledges support by the Isaac Newton Trust and by the Kavli Foundation through a Newton-Kavli Junior Fellowship. The research of CCW is supported by NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation.', 'References': "Akerman, C. J., Carigi, L., Nissen, P. E., Pettini, M., & Asplund, M. 2004, A&A, 414, 931  \n- Amayo, A., Delgado-Inglada, G., & Stasi'nska, G. 2021, MNRAS, 505, 2361 Amorín, R., Fontana, A., Pérez-Montero, E., et al. 2017, Nature Astronomy, 1, 0052  \nArellano-Córdova, K. Z., Berg, D. A., Chisholm, J., et al. 2022, ApJ, 940, L23 Arrabal Haro, P., Dickinson, M., Finkelstein, S. L., et al. 2023, Nature, 622, 707 Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481 Backhaus, B. E., Trump, J. R., Cleri, N. J., et al. 2022, ApJ, 926, 161  \n- Baker, W. M., Tacchella, S., Johnson, B. D., et al. 2023, arXiv e-prints, arXiv:2306.02472\n- Batra, N. D. & Baldwin, J. A. 2014, MNRAS, 439, 771\n- Bayliss, M. B., Rigby, J. R., Sharon, K., et al. 2014, ApJ, 790, 144\n- Belokurov, V. & Kravtsov, A. 2023, MNRAS, 525, 4456\n- Berg, D. A., Chisholm, J., Erb, D. K., et al. 2019, ApJ, 878, L3\n- Berg, D. A., Chisholm, J., Erb, D. K., et al. 2021, ApJ, 922, 170\n- Berg, D. A., Erb, D. K., Auger, M. W., Pettini, M., & Brammer, G. B. 2018, ApJ, 859, 164\n- Berg, D. A., Erb, D. K., Henry, R. B. C., Skillman, E. D., & McQuinn, K. B. W. 2019, arXiv e-prints\n- Berg, D. A., James, B. L., King, T., et al. 2022, ApJS, 261, 31\n- Berg, D. A., Pogge, R. W., Skillman, E. D., et al. 2020, The Astrophysical Journal, 893, 96\n- Berg, D. A., Skillman, E. D., Croxall, K. V., et al. 2015, ApJ, 806, 16\n- Berg, D. A., Skillman, E. D., Henry, R. B. C., Erb, D. K., & Carigi, L. 2016, ApJ, 827, 126\n- Berg, D. A., Skillman, E. D., Marble, A. R., et al. 2012, ApJ, 754, 98\n- Binette, L., Matadamas, R., Hägele, G. F., et al. 2012, A&A, 547, A29\n- Bouwens, R. J., Stefanon, M., Oesch, P. A., et al. 2019, ApJ, 880, 25\n- Boyett, K., Bunker, A. J., Curtis-Lake, E., et al. 2024, arXiv e-prints, arXiv:2401.16934  \nBrinchmann, J. 2022, arXiv e-prints, arXiv:2208.07467  \n- Bunker, A. J., Cameron, A. J., Curtis-Lake, E., et al. 2023a, arXiv e-prints, arXiv:2306.02467\n- Bunker, A. J., Saxena, A., Cameron, A. J., et al. 2023b, A&A, 677, A88 Byler, N., Dalcanton, J. J., Conroy, C., & Johnson, B. D. 2017, ApJ, 840, 44 Calabro, A., Castellano, M., Zavala, J. A., et al. 2024, arXiv e-prints, arXiv:2403.12683\n- Calzetti, D., Armus, L., Bohlin, R. C., et al. 2000, ApJ, 533, 682\n- Cameron, A. J., Katz, H., Rey, M. P., & Saxena, A. 2023a, MNRAS, 523, 3516\n- Cameron, A. J., Saxena, A., Bunker, A. J., et al. 2023b, A&A, 677, A115\n- Campbell, A., Terlevich, R., & Melnick, J. 1986, MNRAS, 223, 811 Cappellari, M. 2017, MNRAS, 466, 798\n- Carnall, A. C., McLure, R. J., Dunlop, J. S., & Davé, R. 2018, MNRAS, 480, 4379\n- Carniani, S., Hainline, K., D'Eugenio, F., et al. 2024, arXiv e-prints, arXiv:2405.18485\n- Carretta, E., Gratton, R. G., Lucatello, S., Bragaglia, A., & Bonifacio, P. 2005, A&A, 433, 597\n- Castellano, M., Napolitano, L., Fontana, A., et al. 2024, arXiv e-prints, arXiv:2403.10238\n- Chabrier, G. 2003, PASP, 115, 763\n- Charbonnel, C., Schaerer, D., Prantzos, N., et al. 2023, A&A, 673, L7\n- Charlot, S. & Fall, S. M. 2000, ApJ, 539, 718\n- Chemerynska, I., Atek, H., Furtak, L. J., et al. 2023, arXiv e-prints, arXiv:2312.05030\n- Chevallard, J. & Charlot, S. 2016, MNRAS, 462, 1415\n- Chiappini, C., Hirschi, R., Meynet, G., et al. 2006, A&A, 449, L27\n- Chisholm, J., Berg, D. A., Endsley, R., et al. 2024, arXiv e-prints, arXiv:2402.18643\n- Christensen, L., Laursen, P., Richard, J., et al. 2012, MNRAS, 427, 1973  \nClarke,  \nL.,  \nShapley, A. E., Sanders, R. L., et al. 2024, arXiv e-prints,  \narXiv:2406.05178  \n- Cleri, N. J., Olivier, G. M., Hutchison, T. A., et al. 2023, ApJ, 953, 10  \nCooke, R. J., Pettini, M., & Steidel, C. C. 2017, MNRAS, 467, 802  \n- Croxall, K. V., Pogge, R. W., Berg, D. A., Skillman, E. D., & Moustakas, J. 2016, ApJ, 830, 4  \nCurti, M., Maiolino, R., Curtis-Lake, E., et al. 2024, A&A, 684, A75  \nCurtis-Lake, E., Carniani, S., Cameron, A., et al. 2023, Nature Astronomy, 7,  \n622  \n- de Graa ff , A., Rix, H.-W., Carniani, S., et al. 2024, A&A, 684, A87\n- D'Eugenio, F., Cameron, A. J., Scholtz, J., et al. 2024, arXiv e-prints, arXiv:2404.06531\n- D'Eugenio, F., Maiolino, R., Carniani, S., et al. 2023, arXiv e-prints, arXiv:2311.09908\n- Dijkstra, M. 2014, PASA, 31, e040\n- D'Orazi, V., Lucatello, S., Gratton, R., et al. 2010, ApJ, 713, L1\n- Dors, O. L., Maiolino, R., Cardaci, M. V., et al. 2020, MNRAS, 496, 3209  \nDors, O. L., Valerdi, M., Freitas-Lemes, P., et al. 2022, MNRAS, 514, 5506  \n- Dressler, A., Vulcani, B., Treu, T., et al. 2023, ApJ, 947, L27\n- Eisenstein, D. J., Johnson, B. D., Robertson, B., et al. 2023a, arXiv e-prints, arXiv:2310.12340\n- Eisenstein, D. J., Willott, C., Alberts, S., et al. 2023b, arXiv e-prints, arXiv:2306.02465\n- Endsley, R., Stark, D. P., Whitler, L., et al. 2023, arXiv e-prints, arXiv:2306.05295\n- Erb, D. K., Pettini, M., Shapley, A. E., et al. 2010, ApJ, 719, 1168  \nEsteban, C., Bresolin, F., Peimbert, M., et al. 2009, ApJ, 700, 654  \n- Esteban, C., Fang, X., García-Rojas, J., & Toribio San Cipriano, L. 2017, MNRAS, 471, 987\n- Esteban, C., García-Rojas, J., Carigi, L., et al. 2014, MNRAS, 443, 624\n- Esteban, C., Peimbert, M., García-Rojas, J., et al. 2004, MNRAS, 355, 229\n- Fabbian, D., Nissen, P. E., Asplund, M., Pettini, M., & Akerman, C. 2009, A&A, 500, 1143\n- Feltre, A., Charlot, S., & Gutkin, J. 2016, MNRAS, 456, 3354\n- Ferland, G. J., Chatzikos, M., Guzmán, F., et al. 2017, Rev. Mexicana Astron. Astrofis., 53, 385\n- Feroz, F., Hobson, M. P., & Bridges, M. 2009, MNRAS, 398, 1601\n- Finkelstein, S. L., D'Aloisio, A., Paardekooper, J.-P., et al. 2019, ApJ, 879, 36\n- Fosbury, R. A. E., Villar-Martín, M., Humphrey, A., et al. 2003, ApJ, 596, 797 García-Rojas, J. & Esteban, C. 2007, ApJ, 670, 457\n- García-Rojas, J., Esteban, C., Peimbert, A., et al. 2005, MNRAS, 362, 301\n- García-Rojas, J., Esteban, C., Peimbert, A., et al. 2007, Rev. Mexicana Astron. Astrofis., 43, 3\n- García-Rojas, J., Esteban, C., Peimbert, M., et al. 2004, ApJS, 153, 501 Garnett, D. R. 1992, AJ, 103, 1330\n- Gilli, R., Vignali, C., Mignoli, M., et al. 2010, A&A, 519, A92\n- Gordon, K. D., Clayton, G. C., Misselt, K. A., Landolt, A. U., & Wol ff , M. J. 2003, ApJ, 594, 279  \nGreene, J. E., Labbe, I., Goulding, A. D., et al. 2024, ApJ, 964, 39  \n- Gronke, M. 2017, A&A, 608, A139\n- Groves, B. A., Dopita, M. A., & Sutherland, R. S. 2004, ApJS, 153, 75  \nGunn, J. E. & Peterson, B. A. 1965, ApJ, 142, 1633  \nPeimbert, A., Peimbert, M., & Ruiz, M. T. 2005, ApJ, 634, 1056  \n- Gustafsson, B., Karlsson, T., Olsson, E., Edvardsson, B., & Ryde, N. 1999, A&A, 342, 426\n- Gutkin, J., Charlot, S., & Bruzual, G. 2016, MNRAS, 462, 1757\n- Hägele, G. F., Pérez-Montero, E., Díaz, Á. I., Terlevich, E., & Terlevich, R. 2006, MNRAS, 372, 293  \nHainline, K. N., D'Eugenio, F., Jakobsen, P., et al. 2024a, arXiv e-prints, arXiv:2404.04325  \n- Hainline, K. N., Johnson, B. D., Robertson, B., et al. 2024b, ApJ, 964, 71  \nHarikane, Y., Nakajima, K., Ouchi, M., et al. 2024, ApJ, 960, 56  \n- Harikane, Y., Zhang, Y., Nakajima, K., et al. 2023, ApJ, 959, 39\n- Heintz, K. E., Brammer, G. B., Watson, D., et al. 2024, arXiv e-prints, arXiv:2404.02211\n- Heintz, K. E., Watson, D., Brammer, G., et al. 2023, arXiv e-prints, arXiv:2306.00647  \nHirschmann, M., Charlot, S., Feltre, A., et al. 2019, MNRAS, 487, 333  \nHolwerda, B. W., Bouwens, R., Oesch, P., et al. 2015, ApJ, 808, 6  \nHutchison, T. A., Papovich, C., Finkelstein, S. L., et al. 2019, ApJ, 879, 70  \nHutter, A., Trebitsch, M., Dayal, P., et al. 2023, MNRAS, 524, 6124  \nIani, E., Zanella, A., Vernet, J., et al. 2023, MNRAS, 518, 5018  \nInoue, A. K., Shimizu, I., Iwata, I., & Tanaka, M. 2014, MNRAS, 442, 1805  \nIsobe, Y., Ouchi, M., Nakajima, K., et al. 2023a, ApJ, 956, 139  \n- Isobe, Y., Ouchi, M., Tominaga, N., et al. 2023b, ApJ, 959, 100  \nIzotov, Y. I. 2006, in Astronomical Society of the Pacific Conference Series, Vol.  \n353, Stellar Evolution at Low Metallicity: Mass Loss, Explosions, Cosmol-  \nogy, ed. H. J. G. L. M. Lamers, N. Langer, T. Nugis, & K. Annuk, 349  \nIzotov, Y. I., Schaerer, D., Guseva, N. G., Thuan, T. X., & Worseck, G. 2024,  \nMNRAS, 528, L10  \n- Izotov, Y. I., Stasi'nska, G., Meynet, G., Guseva, N. G., & Thuan, T. X. 2006, A&A, 448, 955\n- Izotov, Y. I., Thuan, T. X., & Privon, G. 2012a, MNRAS, 427, 1229  \nIzotov, Y. I., Thuan, T. X., & Privon, G. 2012b, MNRAS, 427, 1229  \nJames, B. L., Pettini, M., Christensen, L., et al. 2014, MNRAS, 440, 1794  \nJi, X., Maiolino, R., Ferland, G., et al. 2024a, arXiv e-prints, arXiv:2405.05772  \nJi, X., Übler, H., Maiolino, R., et al. 2024b, arXiv e-prints, arXiv:2404.04148  \nJones, G. C., Bunker, A. J., Saxena, A., et al. 2024, A&A, 683, A238  \nJones, T., Sanders, R., Chen, Y., et al. 2023, ApJ, 951, L17  \nKobayashi, C. & Ferrara, A. 2024, ApJ, 962, L6  \n- Kobayashi, C., Karakas, A. I., & Lugaro, M. 2020, ApJ, 900, 179\n- Kobayashi, C., Karakas, A. I., & Umeda, H. 2011, MNRAS, 414, 3231  \nKobayashi, C. & Taylor, P. 2023, arXiv e-prints, arXiv:2302.07255  \n- Kokorev, V., Caputi, K. I., Greene, J. E., et al. 2024, arXiv e-prints, arXiv:2401.09981  \nKroupa, P., Tout, C. A., & Gilmore, G. 1993, MNRAS, 262, 545  \nLangeroodi, D. & Hjorth, J. 2023, arXiv e-prints, arXiv:2307.06336  \n- Laseter, I. H., Maseda, M. V., Curti, M., et al. 2023, arXiv e-prints, arXiv:2306.03120  \nLecroq, M., Charlot, S., Bressan, A., et al. 2024, MNRAS, 527, 9480 Leitherer, C., Tremonti, C. A., Heckman, T. M., & Calzetti, D. 2011, AJ, 141, 37 Leja, J., Carnall, A. C., Johnson, B. D., Conroy, C., & Speagle, J. S. 2019, ApJ, 876, 3  \n- Looser, T. J., D'Eugenio, F., Maiolino, R., et al. 2023, arXiv e-prints, arXiv:2306.02470  \nLópez-Sánchez, Á. R., Esteban, C., García-Rojas, J., Peimbert, M., & Rodríguez, M. 2007, ApJ, 656, 168  \n- Mainali, R., Kollmeier, J. A., Stark, D. P., et al. 2017, ApJ, 836, L14\n- Mainali, R., Stark, D. P., Tang, M., et al. 2020, MNRAS, 494, 719\n- Maiolino, R. & Mannucci, F. 2019, A&A Rev., 27, 3\n- Maiolino, R., Scholtz, J., Curtis-Lake, E., et al. 2023a, arXiv e-prints, arXiv:2308.01230  \nMaiolino, R., Scholtz, J., Witstok, J., et al. 2023b, arXiv e-prints, arXiv:2305.12492  \n- Marques-Chaves, R., Schaerer, D., Kuruvanthodi, A., et al. 2024, A&A, 681, A30  \nMaseda, M. V., Lewis, Z., Matthee, J., et al. 2023, ApJ, 956, 11  \nMason, C. A. & Gronke, M. 2020, MNRAS, 499, 1395  \n- Matteucci, F. 1986, MNRAS, 221, 911\n- Matthee, J., Naidu, R. P., Brammer, G., et al. 2024, ApJ, 963, 129\n- Matthee, J., Sobral, D., Hayes, M., et al. 2021, MNRAS, 505, 1382  \nMcLeod, D. J., Donnan, C. T., McLure, R. J., et al. 2024, MNRAS, 527, 5004  \nMéndez-Delgado, J. E., Esteban, C., García-Rojas, J., Kreckel, K., & Peimbert,  \nM. 2023, Nature, 618, 249  \n- Mignoli, M., Vignali, C., Gilli, R., et al. 2013, A&A, 556, A29  \nMingozzi, M., James, B. L., Berg, D. A., et al. 2024, ApJ, 962, 95  \n- Nagele, C. & Umeda, H. 2023, ApJ, 949, L16\n- Nakajima, K., Ouchi, M., Isobe, Y., et al. 2023, ApJS, 269, 33\n- Nissen, P. E., Chen, Y. Q., Carigi, L., Schuster, W. J., & Zhao, G. 2014, A&A, 568, A25\n- Oesch, P. A., Bouwens, R. J., Illingworth, G. D., et al. 2014, ApJ, 786, 108 Oesch, P. A., Brammer, G., Naidu, R. P., et al. 2023, MNRAS, 525, 2864  \nArticle number, page 22 of 23  \n- Peimbert, M. & Costero, R. 1969, Boletin de los Observatorios Tonantzintla y Tacubaya, 5, 3\n- Pilyugin, L. S., Mattsson, L., Vílchez, J. M., & Cedrés, B. 2009, MNRAS, 398, 485  \nPlanck Collaboration, Aghanim, N., Akrami, Y., et al. 2020, A&A, 641, A6  \nPlat, A., Charlot, S., Bruzual, G., et al. 2019, MNRAS, 490, 978  \nPérez-Montero, E. & Amorín, R. 2017, MNRAS, 467, 1287  \nPérez-Montero, E. & Contini, T. 2009, MNRAS, 398, 949  \nReddy, N. A., Topping, M. W., Sanders, R. L., Shapley, A. E., & Brammer, G. 2023, ApJ, 952, 167  \nReddy, N. A., Topping, M. W., Shapley, A. E., et al. 2022, ApJ, 926, 31 Revalski, M., Rafelski, M., Henry, A., et al. 2024, arXiv e-prints, arXiv:2403.17047  \nRigby, J. R., Florian, M., Acharyya, A., et al. 2021, ApJ, 908, 154  \n- Robertson, B., Johnson, B. D., Tacchella, S., et al. 2023a, arXiv e-prints, arXiv:2312.10033  \nRobertson, B. E., Tacchella, S., Johnson, B. D., et al. 2023b, Nature Astronomy, 7, 611  \nRogers, N. S. J., Skillman, E. D., Pogge, R. W., et al. 2021, ApJ, 915, 21  \nSaccardi, A., Salvadori, S., D'Odorico, V., et al. 2023, ApJ, 948, 35  \n- Sanders, R. L., Shapley, A. E., Topping, M. W., Reddy, N. A., & Brammer, G. B. 2023, ApJ, 955, 54  \nSaxena, A., Robertson, B. E., Bunker, A. J., et al. 2023, A&A, 678, A68  \nSchaerer, D., Marques-Chaves, R., Xiao, M., & Korber, D. 2024, arXiv e-prints,  \narXiv:2406.08408  \n- Scholtz, J., Maiolino, R., D'Eugenio, F., et al. 2023, arXiv e-prints, arXiv:2311.18731\n- Senchyna, P., Plat, A., Stark, D. P., & Rudie, G. C. 2023, arXiv e-prints, arXiv:2303.04179  \nSenchyna, P., Stark, D. P., Charlot, S., et al. 2022, ApJ, 930, 105  \n- Senchyna, P., Stark, D. P., Vidal-García, A., et al. 2017, MNRAS, 472, 2608\n- Shapley, A. E., Reddy, N. A., Sanders, R. L., Topping, M. W., & Brammer, G. B. 2023a, ApJ, 950, L1\n- Shapley, A. E., Sanders, R. L., Reddy, N. A., Topping, M. W., & Brammer, G. B. 2023b, ApJ, 954, 157  \nSimmonds, C., Tacchella, S., Maseda, M., et al. 2023, MNRAS, 523, 5468  \nSmith, A., Kannan, R., Garaldi, E., et al. 2022, MNRAS, 512, 3243  \nStanway, E. R. & Eldridge, J. J. 2018, MNRAS, 479, 75  \n- Stark, D. P., Richard, J., Charlot, S., et al. 2015a, MNRAS, 450, 1846\n- Stark, D. P., Richard, J., Siana, B., et al. 2014, MNRAS, 445, 3200\n- Stark, D. P., Walth, G., Charlot, S., et al. 2015b, MNRAS, 454, 1393\n- Steidel, C. C., Strom, A. L., Pettini, M., et al. 2016, ApJ, 826, 159\n- Tacchella, S., Eisenstein, D. J., Hainline, K., et al. 2023a, ApJ, 952, 74  \nTacchella, S., Johnson, B. D., Robertson, B. E., et al. 2023b, MNRAS, 522, 6236  \n- Tacchella, S., McClymont, W., Scholtz, J., et al. 2024, arXiv e-prints, arXiv:2404.02194  \nTasitsiomi, A. 2006, ApJ, 645, 792  \n- Tominaga, N., Umeda, H., & Nomoto, K. 2007, ApJ, 660, 516\n- Topping, M. W., Stark, D. P., Senchyna, P., et al. 2024, MNRAS, 529, 3301  \nToribio San Cipriano, L., Domínguez-Guzmán, G., Esteban, C., et al. 2017, MN-  \nRAS, 467, 3759  \nToribio San Cipriano, L., García-Rojas, J., Esteban, C., Bresolin, F., & Peimbert, M. 2016, MNRAS, 458, 1866  \n- Torres-Peimbert, S. & Peimbert, M. 1977, Rev. Mexicana Astron. Astrofis., 2, 181\n- Tsamis, Y. G., Barlow, M. J., Liu, X. W., Danziger, I. J., & Storey, P. J. 2003, MNRAS, 338, 687\n- Übler, H., Maiolino, R., Curtis-Lake, E., et al. 2023, A&A, 677, A145\n- Vale Asari, N., Stasi'nska, G., Morisset, C., & Cid Fernandes, R. 2016, MNRAS, 460, 1739\n- Valerdi, M., Peimbert, A., & Peimbert, M. 2021, MNRAS, 505, 3624  \nVanzella, E., De Barros, S., Cupani, G., et al. 2016, ApJ, 821, L27  \n- Vincenzo, F. & Kobayashi, C. 2018, MNRAS, 478, 155  \nVincenzo, F. & Kobayashi, C. 2018, A&A, 610, L16  \nWatanabe, K., Ouchi, M., Nakajima, K., et al. 2024, ApJ, 962, 50  \nWitstok, J., Maiolino, R., Smit, R., et al. 2024a, arXiv e-prints, arXiv:2404.05724  \nWitstok, J., Smit, R., Saxena, A., et al. 2024b, A&A, 682, A40  \nYin, J., Matteucci, F., & Vladilo, G. 2011, A&A, 531, A136  \nZeimann, G. R., Ciardullo, R., Gebhardt, H., et al. 2015, ApJ, 798, 29  \n- 1 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany\n- 2 Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK\n- 3 Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK\n- 4 Cosmic Dawn Center (DAWN), Copenhagen, Denmark\n- 5 Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK2200, Copenhagen, Denmark\n- 6 Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK\n- 7 Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA\n- 8 INAF - Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy\n- 9 Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK\n- 10 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK\n- 11 Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 OHA, UK.\n- 12 Cavendish Laboratory - Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 OHE, UK.\n- 13 Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy\n- 14 Centro de Astrobiología (CAB), CSIC-INTA, Cra. de Ajalvir Km. 4, 28850- Torrejón de Ardoz, Madrid, Spain\n- 15 European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s / n, 28692 Villanueva de la Cañada, Madrid, Spain\n- 16 School of Physics, University of Melbourne, Parkville 3010, VIC, Australia\n- 17 ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia\n- 18 Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge MA 02138 USA\n- 19 Sorbonne Université, CNRS, UMR 7095, Institut d'Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France\n- 20 AURA for European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive. Baltimore, MD, 21210\n- 21 Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706 USA\n- 22 Department of Astronomy and Astrophysics University of California, Santa Cruz, 1156 High Street, Santa Cruz CA 96054, USA\n- 23 NSF's National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA\n- 24 NRC Herzberg, 5071 West Saanich Rd, Victoria, BC V9E 2E7, Canada"}