# Metal-Enriched Neutral Gas Reservoir around a Strongly-lensed, Low-mass
Galaxy at $z=4$ Identified by JWST/NIRISS and VLT/MUSE

Xiaojing Lin<EMAIL_ADDRESS><EMAIL_ADDRESS>Department of Astronomy, Tsinghua University, Beijing 100084, China  Zheng Cai
<EMAIL_ADDRESS><EMAIL_ADDRESS>Department of
Astronomy, Tsinghua University, Beijing 100084, China  Siwei Zou
<EMAIL_ADDRESS><EMAIL_ADDRESS>Department of
Astronomy, Tsinghua University, Beijing 100084, China  Zihao Li
<EMAIL_ADDRESS><EMAIL_ADDRESS>Department of
Astronomy, Tsinghua University, Beijing 100084, China  Zuyi Chen Steward
Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA
Fuyan Bian<EMAIL_ADDRESS>European Southern Observatory, Alonso de Córdova
3107, Casilla 19001, Vitacura, Santiago 19, Chile  Fengwu Sun Steward
Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA
Yiping Shu  Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuan
Hua Road, Nanjing, Jiangsu 210023, China  Yunjing Wu
<EMAIL_ADDRESS><EMAIL_ADDRESS>Department of
Astronomy, Tsinghua University, Beijing 100084, China  Steward Observatory,
University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA  Mingyu Li
<EMAIL_ADDRESS><EMAIL_ADDRESS>Department of
Astronomy, Tsinghua University, Beijing 100084, China  Jianan Li
<EMAIL_ADDRESS><EMAIL_ADDRESS>Department of
Astronomy, Tsinghua University, Beijing 100084, China  Xiaohui Fan Steward
Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA
J. Xavier Prochaska Department of Astronomy and Astrophysics, UCO, Lick
Observatory, University of California 1156 High Street, Santa Cruz, CA 95064,
USA  Kavli IPMU, the University of Tokyo (WPI), Kashiwa 277-8583, Japan
Daniel Schaerer Observatoire de Geneve, Universite de Geneve, Chemin Pegasi
51, CH-1290 Versoix, Switzerland  Stephane Charlot Sorbonne Université, CNRS,
UMR7095, Institut d’Astrophysique de Paris, F-75014, Paris, France  Daniel
Espada Departamento de Física Teórica y del Cosmos, Campus de Fuentenueva,
Edificio Mecenas, Facultad de Ciencias, Universidad de Granada, E-18071,
Granada, Spain  Instituto Carlos I de Física Teórica y Computacional, Facultad
de Ciencias, E-18071, Granada, Spain  Miroslava Dessauges-Zavadsky  Department
of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix,
Switzerland  Eiichi Egami Steward Observatory, University of Arizona, 933 N
Cherry Ave, Tucson, AZ 85721, USA  Daniel Stark  Steward Observatory,
University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA  Kirsten K.
Knudsen  Institutionen för rymd-, geo- och miljövetenskap, Department of
Space, Earth and Environment Chalmers tekniska högskola, Chalmers University
of Technology  Gustavo Bruzual  Institute of Radio Astronomy and Astrophysics,
National Autonomous University of Mexico, San José de la Huerta 58089 Morelia,
Michoacán, México  Jacopo Chevallard Scientific Support Office, Directorate of
Science and Robotic Exploration, ESA/ESTEC, Keplerlaan 1, NL-2201 AZ
Noordwijk, the Netherlands

(Received xxx; Revised xxx; Accepted xxx)

###### Abstract

Direct observations of low-mass, low-metallicity galaxies at $z\gtrsim 4$
provide an indispensable opportunity for detailed inspection of the ionization
radiation, gas flow, and metal enrichment in sources similar to those that
reionized the Universe. Combining the James Webb Space Telescope (JWST),
VLT/MUSE, and ALMA, we present detailed observations of a strongly lensed,
low-mass ($\approx 10^{7.6}$ ${\rm M}_{\odot}$) galaxy at $z=3.98$ (also see
Vanzella et al. 2022). We identify strong narrow nebular emission, including C
IV $\lambda\lambda 1548,1550$, He II $\lambda 1640$, O III] $\lambda\lambda
1661,1666$, [Ne III] $\lambda 3868$, [O II] $\lambda 3727$, and Balmer series
of Hydrogen from this galaxy, indicating a metal-poor H II region ($\lesssim
0.12\ {\rm Z}_{\odot}$) powered by massive stars. Further, we detect a metal-
enriched damped Ly$\alpha$ system (DLA) associated with the galaxy with the H
I column density of $N_{\rm{HI}}\approx 10^{21.8}$ cm-2. The metallicity of
the associated DLA may reach the super solar metallicity (${\gtrsim
Z}_{\odot}$). Moreover, thanks to JWST and gravitational lensing, we present
the resolved UV slope ($\beta$) map at the spatial resolution of $\approx 100$
pc at $z=4$, with steep UV slopes reaching $\beta\approx-2.5$ around three
star-forming clumps. Combining with low-redshift analogs, our observations
suggest that low-mass, low-metallicity galaxies, which dominate reionization,
could be surrounded by a high covering fraction of the metal-enriched,
neutral-gaseous clouds. This implies that the metal enrichment of low-mass
galaxies is highly efficient, and further support that in low-mass galaxies,
only a small fraction of ionizing radiation can escape through the
interstellar or circumgalactic channels with low column-density neutral gas.

DLA — quasar — host galaxy — lensing — high-redshift

††journal: ApJL

##  1 Introduction

It is now believed that low-mass galaxies significantly contribute the major
fraction of the ionization photons in the epoch of reionization (e.g., Atek et
al., 2015; Bouwens et al., 2016). Observations of faint galaxies at high-
redshift, though very difficult, are required, especially in the era of James
Webb Space Telescope (JWST), to directly constrain the ionization radiation,
gas environment, galaxy growth, and metal enrichment of the surrounding
intergalactic medium (IGM).

Extensive efforts have been conducted to study the intrinsic ultra-violet (UV)
continuum and the ionization of low-mass galaxies. These studies aim to
understand how ionization radiation escapes into the IGM (e.g., Ramambason et
al., 2020). Before UV photons reach the IGM, how much radiation can escape
from galaxies depends on: (1) the intrinsic intensity and the slope of UV
spectra; and (2) the distribution of optically-thick neutral hydrogen (H I)
gas in the interstellar and circumgalactic medium (CGM) (e.g., Chisholm et
al., 2018). It is therefore crucial to study both UV continua and gas
properties around low-mass galaxies to quantify the H I (Ly$\alpha$) optical
depth in the CGM for better understanding the reionization process.

The existence of the neutral gas in and around low-mass star forming galaxies
has been suggested mainly through observations of Ly$\alpha$ emission (e.g.,
Stark et al., 2017; Gronke et al., 2017). Low-mass galaxies often have strong
Ly$\alpha$ and other ionized nebular emission (e.g. Erb et al., 2010; Stark et
al., 2014). The presence of optically-thick, H I “shell” makes the Ly$\alpha$
emission showing the double-peak structure (e.g., Yang et al., 2017), with the
expanding outflow further suppressing blue peaks of Ly$\alpha$ emission (Yang
et al., 2014).

Nevertheless, only through emission, the nature of the interplay between the
neutral gas and galaxy cannot be completely understood. Detailed gas
properties, including the gas metallicity, geometry and density need to be
constrained. The strong lensing, plus the deep integration of JWST and
VLT/MUSE provide us with an excellent opportunity to conduct direct and
detailed observations of a low-mass galaxy at $z=4$, through both emission and
absorption. Here, we present a detailed analysis of a strongly magnified
arclet galaxy in the Abell2744 field, combined with optical, infrared and
millimeter observations. The system is lensed into three multiple images, and
the arclet is composed of two of them lying on both sides of the critical
line, with three bright compact clumps (knots in the arc) mirrored on each
side (Mahler et al., 2018; Bergamini et al., 2022). We name this galaxy
“A2744-arc3”. It was also reported in Vanzella et al. (2022). Through
spectral-energy-distribution (SED) fitting, they found that these clumps are
gravitationally-bound young massive star clusters with bursty star formation.

In this paper, we probe detailed properties of the interstellar and
circumgalactic medium (CGM) of A2744-arc3. With VLT/MUSE and JWST, we provide
detailed analysis of both nebular emission and absorption lines. Copious
emission lines, including the detection of C IV $\lambda\lambda 1548,1550$, He
II $\lambda 1640$, O III] $\lambda\lambda 1661,1666$, [Ne III] $\lambda 3868$,
[O II] $\lambda 3727$, and Balmer series of Hydrogen, allow us to quantify the
low metallicity of the ionized medium. Further, the high signal-to-noise (S/N)
observations of absorption enable us, for the first time, to examine the
detailed properties of the neutral gas reservoir surrounding a low-mass galaxy
at $z\approx 4$. The excellent de-lensed spatial resolution down to $\approx
100$ pc in rest-frame UV observations allow the examination of the spatial
variations of the UV radiation. All these information yield unparalleled
understanding of low-mass, young galaxies at high-redshift using A2744-arc3 as
a textbook example.

This paper is organized as follows. In Section 2, we describe the observations
and data reduction, including JWST/NIRISS imaging and spectroscopy, HST
imaging, VLT/MUSE and ALMA continuum observations; In Section 3, we provide
details of photometry, SED fitting, and the lensing reconstruction in source-
plane. We also show the spatially resolved UV slope map. In Section 4, we
present further analysis on the rest-frame UV and optical spectrum of
A2744-arc3, including the analysis of both emission and absorption features.
Discussions are conducted in Section 5. We assume a flat cosmological model
with $\rm{\Omega_{M}=0.3,\Omega_{\Lambda}=0.7}$ and $\rm{H_{0}=70\ km\ s^{-1}\
Mpc^{-1}}$. The Glafic lensing model (Oguri, 2010; Kawamata et al., 2016) is
adopted.

##  2 Data

###  2.1 JWST/NIRISS imaging & grism observations

NIRISS observations of A2744 were taken by the GLASS JWST Early Release
Science Program (Treu et al., 2022), devoting about
$2834.504\,\mathrm{s}\simeq 0.79\,\mathrm{hours}$ for F115W, F150W, and F200W
direct imaging, respectively. Especially the NIRISS/F200W data, spanning from
1.7$\mu{\rm m}$ to 2.2 $\mu{\rm m}$, allows us to better improve constraints
on its stellar mass. The data were reduced in the same way as shown in Wu et
al. (2022). Here we summarize the procedure as follows: the data were reduced
using the standard JWST pipeline111https://github.com/spacetelescope/jwst
v1.6.2 with calibration reference files “jwst_0944.pmap”; 1/f noise (see
Schlawin et al. 2020) was modeled and removed using the code
tshirt/roeba222https://github.com/eas342/tshirt, and the “snowball” artifacts
from cosmic rays (Rigby et al., 2022) were identified and masked; the world
coordinate system of mosaicked images were registered using the Pan-STARRS1
catalog (Flewelling et al., 2020); the pixel scale of final mosaicked images
was resampled to 0.03″ with pixfrac $=$ 0.8. We reduced NIRISS grism data
using Grizli 333https://github.com/gbrammer/grizli/, and the 2D grism spectra
were drizzled with a pixel scale 0.065″.

To optimally extract 1D spectra of A2744-arc3, we first split the entire
arclet into six segments, each with one clump. We extracted their 1D spectra
using 5-th Chebyshev polynomials. We summed over five 1D spectra of the six
segments to get the final spectra. One of them was excluded since it is
severely contaminated by zero-th and first order images from other sources.

###  2.2 Hubble Frontier Fields images

The A2744 cluster was observed as part of the Hubble Space Telescope Frontier
Fields program444https://archive.stsci.edu/prepds/frontier/ (HFF; ID: 13495,
PI: J. Lotz) between 2013 October 25 and 2014 July 1 for 164.5 orbits in
total, in three filters with the Advanced Camera for Surveys (ACS; F435W,
F606W, F814W) and four with the Wide Field Camera 3 (WFC3; F105W, F125W, F140W
and F160W). The high-level science product released by Space Telescope Science
Institute (STScI) with a pixel size of 30 mas is used in this study.

###  2.3 VLT/MUSE observation

Multi Unit Spectroscopic Explorer (MUSE) observations toward A2744 were taken
by the GTO Program 094.A-0115 (PI: Richard) between 2014 September and 2015
October, with a 2 arcmin$\times$2 arcmin designed mosaic of pointings centred
at $\alpha$= 00h14m20.952s and $\delta$= -30∘23′53.88′′ (Mahler et al., 2018).
Each pointing was observed for a total of 3.5, 4, 4 and 5h in addition to 2h
at the cluster center. The publicly available data cube555http://muse-
vlt.eu/science/a2744/ is adopted in this work.

###  2.4 ALMA continuum observations

The millimeter continuum observations are obtained from the ALMA Lensing
Cluster Survey (ALCS). A2744 is observed by ALMA band 6 (Program
2018.1.00035.L, PI: Kohno; 2013.1.00999.S, PI: Bauer) which covers a frequency
range with two tunings of $250.0-257.5$ GHz and $265.0-272.5$ GHz,
corresponding to wavelengths of $\sim 1.15$ mm (Sun et al., 2022). The
$1\sigma$ continuum sensitivity or say rms is 45 $\mu$Jy beam-1. The data were
reduced using the standard pipeline with the CASA (McMullin et al., 2007) with
natural weighting, and will be described in details in S. Fujimoto et al., in
prep. This leads to a synthesized beam size (FWHM) of $\sim 1$″, corresponding
to $\sim 6.9$ kpc at $z\sim 4$.

##  3 Result and Analysis

###  3.1 Photometry and SED fitting

We use a box aperture along A2744-arc3 to carry out the photometry on seven
broad-band HST observations (ACS/F435W, F606W, F814W and WFC3/F105W, F125W,
F140W, and F160W) and three JWST NIRISS observations (NIRISS/F115W, F150W, and
F200W). For each band, the length and width of the box aperture are designed
to match the 2$\sigma$ contours of the arc, where $\sigma$ is determined by
the local sky background. A sersic modelling is performed to the nearest
source in the north to exclude potential light contamination. Magnification
and multi-image effects are corrected to determine the delensed flux. The
median magnification along the arc is about 45 according to the Glafic model.
We repeat the photometry and SED analysis using Richard et al. (2021) model,
as described in Appendix A, and yield consistent results.

We perform SED modeling with the Bayesian code BEAGLE (Chevallard & Charlot,
2016). We assume a constant star formation history, the dust extinction curve
of the Small Magellanic Cloud with the optical depth in the V band varying in
the range 0 – 0.5, and the Chabrier (2003) initial mass function (IMF) with an
upper limit of 300${\rm M}_{\odot}$. The metallicity of the interstellar
medium is kept the same as that of the stellar populations. A flat prior is
applied for metallicity, $0.01<{Z}/{\rm Z}_{\odot}<0.2$, and ionization
parameter (the dimensionless ratio of the number density of H-ionizing photons
to that of hydrogen) in log-space, $-3<\log{U}<-1$, for nebular emission
(Gutkin et al., 2016). These priors are chosen on the basis of detections of
the UV emission lines (see Section 4.2 for more details). A JWST NIRISS/F150W
view of A2744-arc3, the SED fitting results, and the NIRISS grism 2D and 1D
spectra are shown in Figure 1. It implies that A2744-arc3 is a young, low-mass
galaxy with massive star formation, accompanied by blue UV slopes, strong
nebular emission lines and a faint intrinsic UV luminosity. The absence of a
Balmer break in the NIRISS spectrum further confirms a young stellar age. The
continuum of ALMA band 6 at rest-frame 0.23 mm shows no clear detection, which
indicates a 3$\sigma$ upper limit of far-infrared luminosity surface density
$<2.2\times 10^{11}{\rm L}_{\odot}/{\rm arcsec^{2}}$ assuming a dust
temperature $T=35{\rm K}$ and dust emissivity spectral index $b=1.6$. This
corresponds to an upper limit of star formation rate surface density $\approx
9\times 10^{-7}{\rm M}_{\odot}{\rm yr^{-1}pc^{-2}}$ (Murphy et al., 2011). and
an upper limit of dust mass surface density $\approx 1.6{\rm M}_{\odot}{\rm
pc}^{-2}$ (da Cunha et al., 2013).

In Vanzella et al. (2022), SED analysis was performed on each mirrored clump
using images from NIRISS/F115W, F150W, F200W, and HST/WFC3 F105W, F125W,
F140W, F160W, along with the ground-based VLT/HAWKI Ks-band. Their derived
star formation rate (SFR) is $1.47^{+0.85}_{-0.25}/{\rm M}_{\odot}{\rm
yr}^{-1}$, total stellar mass of $1.6^{+0.4}_{-1.3}\times 10^{8}{\rm
M}_{\odot}$ (Table 1, 3c), and age of $126^{+33}_{-108}$ Myr. Our derived
properties are marginally consistent with theirs within $1\sigma$. They found
that these nucleated star-forming regions have optical/UV effective radii
spanning 3–20 pc, bursty star formation activities ($\sim 10{\rm Gyr}^{-1}$)
and UV slopes of $\beta\simeq-1.9$, and stellar mass surface densities
resembling those of local globular clusters. Our conclusion is consistent with
theirs. Note that the upper limit of IR-based SFR density is smaller than that
obtained from UV ($\gtrsim 10^{-5}{\rm M}_{\odot}{\rm yr^{-1}pc^{-2}}$ for
clumps in Vanzella et al. (2022)), and this may be due to a low dust-obscured
fraction of SFR in this low-mass galaxy with negligible dust content.

Figure 1:  Top left: JWST NIRISS F150W surface brightness image of A2744-arc3
with overlaid continuum contours of ALMA band 6. The contours levels are -2
(white dashed), 1 and 2$\sigma$ (white solid) with $1\sigma=44.5\mu$Jy/beam,
showing no clear detection at a rest-frame wavelength of 0.23mm. Top right:
Photometry and SED fitting for A2744-arc3, with derived physical parameters.
Blue triangles denote the measurement and red circles the best-fit result.
Bottom: Optimally extracted 1D spectra (top panel) and joint 2D spectra
covered by NIRISS three filters (F115W, F150W and F200W). In the top panel,
shaded blue regions indicate flux uncertainties, and vertical red lines label
common emission lines, among which Mg II is not detected. The 2D spectra of
the bottom panel are obtained by summing 2D spectra of the five segments as
described in Section 2.1

###  3.2 Source Plane Reconstruction and Spatially-resolved UV Slope

To explore the spatially-resolved UV slope variation, we construct UV slope
$\beta$ maps in both the image plane and source plane using HST and JWST
broad-band images. We recover the intrinsic source morphology by ray-tracing
back the image-plane pixels on a regular source plane grid based on the
corresponding deflection field (Patrício et al., 2016). More details about the
selection of lensing models and the reconstruction are presented in Appendix
A.

The image-plane and reconstructed source-plane UV slope maps are shown in
Figure 2. The HST image plane $\beta$ map is constructed using PSF-matched
WFC3/F105W, F125W, F140W and F160W images which trace continua at 2125, 2513,
2805, 3091 Å in the rest frame respectively. The JWST image plane $\beta$ map
is constructed from PSF-matched NIRISS/F115W and F150W, corresponding to rest-
frame continua at 2315 and 3015 Å. NIRISS/F200W is discarded to avoid
potential [O II] $\lambda\lambda 3729,3726$ and [O III] $\lambda 4363$
emission. The $\beta$ map constructed from JWST images exhibits more fine
structures and variations, thanks to its high spatial resolution. On both
image- and source-plane $\beta$ maps, $\beta$ spans from -1.5 to -2.5. The
bluest $\beta$ resides around the compact star clumps, reaching -2.5, caused
by ionizing radiation from massive star formation. On the source plane, we
confirm that $\beta$ varies on the physical scale of $<100$ pc.

Figure 2:  The UV slope $\beta$ map in the image plane and source plane
measured from HST and JWST broadband images. The HST UV slope maps are
measured from WFC3/F105W, F125W, F140W and F160W images (2125-3091 Å in the
rest frame), while the JWST UV slope maps are constructed only using
NIRISS/F115W and F150W images (2315-3015 Å in the rest frame), to avoid
emission lines captured by the NIRISS/F200W filter. The overlaid green
contours in HST image/source plane denote the lensed/de-lensed WFC3/F160W
surface brightness levels of 0.05,0.075,0.1 MJy sr-1, and the contours in JWST
image/source plane show NIRISS/F150W lensed/de-lensed surface brightness
levels of 0.075,0.1,0.135 MJy sr-1 respectively. The ellipses on the upper
right corners correspond to the FWHM of image/source plane point spread
function (PSF). Note that source-plane PSF varies among positions due to
different magnification and distortion, and the shown one is with the largest
FWHM determined by deflection fields at one of the three clumps.

##  4 Associated Damped Ly$\alpha$ System with Super-solar Metallicity

The optical spectrum of A2744-arc3, as shown in Figure 3(a), is extracted from
the MUSE data cube by making a 2$\sigma$ deblended segmentation map on a 5
arcsec$\times$ 5 arcsec white-light image cutout around it. Abundant
absorption and emission features are highlighted, including high-column
density Ly$\alpha$ absorption line and the multiplicity of metal absorption
lines (e.g., O I $\lambda 1302$, Si II $\lambda 1304$, Si IV $\lambda\lambda
1393,1402$), strong high-ionization emission lines such as C IV
$\lambda\lambda 1548,1550$, HeII $\lambda 1640$ and O III] $\lambda\lambda
1661,1666$. We also detect a series of intervening Fe II absorbers at
$z=2.582$ along the sightline. The spectral properties of the arc are reported
in Table 1.

Figure 3: (a) Rest-frame UV spectrum of A2744-arc3 extracted from the VLT/MUSE
3D datacube, with the error spectrum shown in red. Nebular emission lines and
absorption features associated with A2744-arc3 are labelled with dashed red
and blue lines, respectively A series of absorption features from an
intervening Fe II absorber at $z=2.582$ are identified in orange. (b)
Normalized spectrum zoomed into the DLA Ly$\alpha$ and Ly$\beta$ line profiles
with the Voigt profile fits, with flux uncertainties indicated by gray shaded
region. The red solid lines imply the best-fit result yielding
$\log(N_{\mathrm{HI}}/{\rm cm}^{-2})=21.8\pm 0.1$. Tentative residual in the
trough is identified by the red shaded region. (c) Normalized spectrum zoomed
into the metal absorption line profiles. The blue shaded regions are used to
measure the equivalent width (EW) of the absorbers. A tentative velocity
offset of -36 km s-1 relative to $z=3.980$ (blue dashed) is marked in green
dashed lines. (d) The neutral gas metallicity derived from the O I absorber in
compared with metal abundances of DLAs (Rafelski et al. 2012; gray squares)
and Lyman break galaxies (LBGs) (Troncoso et al. 2014; blue triangles) out to
$z=5$. The gray dashed lines indicate the cosmic evolution of DLA
metallicities in Rafelski et al. (2012). The gray shaded region marks
metallicities lower than solar values. The purple shadow ranging from $-1.39$
to $-0.94$ marks the H II region [O/H] abundance of A2744-arc3 derived from
mass-metallicity relations and [Ne III]/[O II] empirical metallicity
calibrations, as described in Section 4.2.  Table 1: Physical Properties of
the arc derived from the photometry, absorption and emission features. In
addition, column densities and abundances of neutral gas metallicity tracers
are provided. Basic Physical Parameters
---
R.A. (deg) | Dec (deg) |  $z_{\rm spec}$aaRedshift determined by O I absorption. |  $\log M_{*}/M_{\odot}$ bbPhysical parameters are derived from BEAGLE SED fitting. | Age (Myr) | SFR ($M_{\odot}$/yr) | $\log{\rm U}$ |  $\mu_{\rm med}$ccMagnification from the Glafic model. $\mu_{\rm med}$ is the median value along the arc based on the JWST/NIRISS F115W $2\sigma$ profile. $\mu_{1}$, $\mu_{2}$ and $\mu_{3}$ is the median magnification of the three mirrored clump pairs from inside (closest to the axis of symmetry/critical curve) to outside respectively. |  $\mu_{1}$ccMagnification from the Glafic model. $\mu_{\rm med}$ is the median value along the arc based on the JWST/NIRISS F115W $2\sigma$ profile. $\mu_{1}$, $\mu_{2}$ and $\mu_{3}$ is the median magnification of the three mirrored clump pairs from inside (closest to the axis of symmetry/critical curve) to outside respectively. |  $\mu_{2}$ccMagnification from the Glafic model. $\mu_{\rm med}$ is the median value along the arc based on the JWST/NIRISS F115W $2\sigma$ profile. $\mu_{1}$, $\mu_{2}$ and $\mu_{3}$ is the median magnification of the three mirrored clump pairs from inside (closest to the axis of symmetry/critical curve) to outside respectively. |  $\mu_{3}$ccMagnification from the Glafic model. $\mu_{\rm med}$ is the median value along the arc based on the JWST/NIRISS F115W $2\sigma$ profile. $\mu_{1}$, $\mu_{2}$ and $\mu_{3}$ is the median magnification of the three mirrored clump pairs from inside (closest to the axis of symmetry/critical curve) to outside respectively.
3.589089 | -30.393844 | 3.980 | 7.59${}^{+0.12}_{-0.10}$ | 23.95${}^{+5.70}_{-8.44}$ | 1.80${}^{+0.24}_{-0.21}$ | -1.34${}^{+0.32}_{-0.26}$ | 45 | 142 | 46 | 30
Absorption Features
Ion | $\lambda$ (Å) |  | CoG Method |  | Voigt Profile Fitting
| EW (Å)ddRest-frame equivalent width of the absorbers. The uncertainties are estimated by varying the continuum level by 10%, combined with the flux uncertainties. |  $\log$(N/cm2)eeColumn densities are derived assuming $b$=10km s-1. Uncertainties are estimated by varying the doppler parameters $b$ from 5-15km s-1, combined with the uncertainties from EW measurements. | [X/H]ff[X/H] = $\log$(X/H) - $\log$(X/H)⊙. Solar (photosphere) abundances from the compilation by Asplund et al. (2009). |  |  $\log$(N/cm2)ggColumn densities are determined by two-component Voigt profile fits to the absorption features simultaneously, assuming $b=10$km s-1. We also perform Voigt profile fits using $b$=5 and 15km s-1, and the uncertainties presented here include systematics induced by different $b$ values. | [X/H]hhMetallicities are calculated by summing over column densities of the two Voigt profiles.
H I | 1215.670 |  | - | - | - |  | 21.8$\pm 0.1$11The uncertainty is determined by varying the continuum level by 10% and the doppler parameter $b$ from 50 to 200km/s. | -
O I | 1302.168 |  | 0.77$\pm$0.10 | 18.9${}^{+0.2}_{-0.3}$ | 0.41${}^{+0.20}_{-0.30}$ |  | 18.7$\pm$0.4 | 0.21$\pm$0.40
Si II22The Si II $\lambda$1260 and Fe II $\lambda$1608 absorption might be blended with other lines. | 1260.422 |  | 1.12$\pm$0.11 | - |  |  | - |  |  |
1304.370 |  | 0.47$\pm$0.10 | 17.8${}_{-0.9}^{+0.4}$ | 0.74${}^{+0.36}_{-0.53}$ |  | 18.0${}^{+0.4}_{-0.6}$ | 0.81${}^{+0.36}_{-0.71}$
Si IV | 1402.770 |  | 0.67$\pm$ 0.11 | 17.7$\pm$0.3 |  | 17.5${}^{+0.2}_{-1.6}$
C II33The C II $\lambda$1334 absorption is saturated.  | 1334.532 |  | 1.31$\pm$0.09 | - | - |  | - | -
Fe II22The Si II $\lambda$1260 and Fe II $\lambda$1608 absorption might be blended with other lines. | 1608.451 |  | 0.80$\pm$0.08 | 18.7$\pm 0.2$ | 1.40$\pm$0.20 |  | 19.1${}^{+0.2}_{-0.4}$ | 1.80${}^{+0.20}_{-0.40}$
Al II | 1670.787 |  | 0.72$\pm$0.10 | 16.4$\pm$0.4 | 0.15$\pm$0.40 |  | 16.4${}^{+0.3}_{-1.3}$ | 0.15${}^{+0.30}_{-1.30}$
Emission Lines
Ion |  $\lambda$ (Å) |  | EW (Å) | FWHM (km s-1)iiFWHMs are measured in the rest-frame relative to $z=3.980$, using Gaussian fits and eliminating the effect of MUSE line spread function (LSF) (Bacon et al., 2017). | Flux (10-18 erg s-1 cm-2)
C IV | 1548.187 |  | 3.14$\pm$0.23 | 118$\pm$9 | 7.8$\pm$0.655There might be potential C IV absorption from neutral gas blended with its emission.
1550.772 |  |  $<$46144The C IV $\lambda$1550 emission line is likely interfered by noise.
He II | 1640.420 |  | 1.62$\pm$0.09 | 143$\pm$8 | 3.8$\pm$0.2
O III] | 1660.809 |  | 6.25$\pm$0.27 | 144$\pm$9 | 3.7$\pm$0.2
1666.150 |  | 139$\pm$6 | 7.9$\pm$0.6
[O II]66Measured from the coadd of JWST/NIRISS slitless spectra as described in Section 2.1. We do not provide FWHMs due to blending effect in grism data and the low resolving power of NIRISS (R$\approx$150). Flux is measured by fitting single Gaussian profiles to each emission line. | 372777The doublet of [O II] at 3726.032 and 3728.815Å cannot be distinguished. |  | 80.2$\pm$12.9 | - | 56.0$\pm$6.4
[Ne III] | 3868.760 |  | 105.6$\pm$16.7 | - | 66.1$\pm$7.3

###  4.1 Metal-enriched H I gas with high column density

In Figures 3(b) and 3(c), we display the DLA H I and its associated metal
absorption-line profiles, respectively. The metal lines exhibit tentative
P-Cygni features. We use the Voigt profile to fit absorption lines. The
Ly$\alpha$ and Ly$\beta$ absorption associated with the galaxy at $z=3.98$,
depicted in Figure 3(b), indicates a H I column density of
$N_{\rm{HI}}=10^{21.8\pm 0.1}$cm-2, i.e. an extremely strong damped Ly$\alpha$
systems (ESDLA, $N_{\rm{HI}}>5\times 10^{21}{\rm cm}^{-2}$; Noterdaeme et al.,
2014). The measurement is performed using VoigtFit (Krogager, 2018), varying
the continuum level by 10% and the doppler parameters in the range 50-200 km
s-1. Tentative detection of residual Ly$\alpha$ emission in the absorption
trough might imply partial DLA coverage. Nevertheless, the possibilities of
contamination from foreground sources or imperfect sky subtraction cannot be
excluded. The source-reconstruction in Figure 2 illustrates that the ESDLA
extends over at least sub-kpc scales.

We measure the gas-phase metallicity from the accurate indicator O I. Because
O I and H I have similar ionization potentials and are coupled by resonant
charge exchange reactions, we then have [O I/H I] $\approx$ [O/H]. Considering
the O I $\lambda$ 1302 line may be saturated, we estimate the metallicity from
both the Voigt profile fitting and curves of growth (CoG) analysis. By varying
the Doppler parameter $b$ between 5–15 km s-1, we obtain the metallicity [O/H]
varying from $-0.11$ to 0.61. We list both the Voigt fitting results ($b=10$
km s-1) and the estimates from the curves of growth of metallicity in Table 1.
More details about the Voigt fitting results are presented in Appendix C. The
estimates from O I and other metal lines such as Si II all yield a super-solar
metallicity, with [O/H]=0.41${}^{+0.20}_{-0.30}$ and
[Si/H]=0.74${}^{+0.36}_{-0.53}$ (Si II + Si IV) respectively, corresponding to
2.57${}^{+1.50}_{-1.28}$ ${\rm Z}_{\odot}$ and 5.50${}^{+7.09}_{-3.87}$ ${\rm
Z}_{\odot}$. This metallicity, as highlighted in Figure 3(d), is much higher
than the prediction of the cosmic metallicity evolution of DLAs ([X/H]
$\approx-1.5$ at $z=3.98$, Rafelski et al. 2012), and the metal abundances of
typical star-forming galaxies at $z=3-5$, measured using nebular emission
lines originated from their H II regions (e.g, [X/H] $\approx-0.5$ in Troncoso
et al. 2014). However, we are cautious to conclude the high metallicity given
the low resolution spectra (R $=$ 2000-3000) and the non-detection of weak
lines such as Ni II and Zn II. We need high resolution spectra to confirm our
measurements.

Additionally, we also obtain super-solar metallicity from the Fe II absorption
lines. Since Fe II can be depleted onto the dust, the high abundance of Fe II
indicates not much dust in both the absorbing gas and the galaxy, consistent
with the ALMA millimeter continuum non-detection.

###  4.2 Strong narrow nebular emission – an indication of metal-poor H II
region

A2744-arc3 exhibits significant high-ionization nebular emission in both
optical and infrared spectra, e.g., C IV $\lambda 1548,1550$, He II $\lambda
1640$, O III] $\lambda 1661,1666$ and [Ne III] $\lambda 3638$ (see Figure 1,
3(a) and Table 1), which have been detected in nearby dwarf galaxies and
$z\approx 1-2$ metal-poor or strong LyC emitting galaxies (e.g., Erb et al.,
2010; Schaerer et al., 2022b), but not typically detected in Lyman break
galaxies (LBGs) (e.g., Reddy et al., 2008). These strong narrow nebular
emission lines, especially He II $\lambda 1640$, are believed to be tightly
linked to low-metallicity galaxies with a possible contribution from Pop III-
like stars (Schaerer, 2003; Patrício et al., 2016).

The line ratio between [O II] and [Ne III], two of the principal coolants in H
II regions, serves as a metallicity indicator for star-forming galaxies. In
A2744-arc3, we obtain $\log$([Ne III]/[O II])$=0.07^{+0.06}_{-0.07}$, i.e.
12+$\log{({\rm O/H})}=7.75\pm 0.05$, based on the empirical metallicity
calibrations in Bian et al. (2018), corresponding to [O/H] = $-0.94\pm 0.05$
or ${Z}/{\rm Z}_{\odot}=0.12\pm 0.01$666Possible systematics are discussed in
Appendx B, e.g., line blending and the scatters of the calibration relation.
More observations are expected to confirm its metal abundance.. The mass-
metallicity relation (MZR) in Sanders et al. (2021) predicts
12+$\log$(O/H)=7.7$\pm$0.1 for a $M_{*}\approx 10^{7.6\pm 0.1}{\rm M}_{\odot}$
galaxy at $z\approx 3.3$; Ma et al. (2016) predicts 12+$\log{\rm(O/H)}=7.27\pm
0.04$ at $z=3.98$. These values, including our SED fitting result, are well
consistent with models of He II emitters, which favour sub-solar metallicities
($\sim 0.1~{}{\rm Z}_{\odot}$) and young stellar ages ($10^{7}-10^{8}$ years;
Saxena et al. 2020). The line ratio of O III] $\lambda$1666/He II
$\lambda$1640 is $2.08\pm 0.19$, inconsistent with any photoionization models
powered by AGN or shocks which expect more flux in He II $\lambda$1640 than in
O III] $\lambda$1666 ($\log$(O III] $\lambda$1666/He II $\lambda$1640)
$\approx-3$ to $-0.5$) (Allen et al., 2008; Feltre et al., 2016; Mainali et
al., 2017; Senchyna et al., 2017). This disjoint favours the explanation that
the nebular emission in A2744-arc3 should primarily be powered by
photoionization due to massive stars (Gutkin et al., 2016).

##  5 Discussion

###  5.1 A2744-arc3 has properties similar to those local analogues

SL2S J0217 (James et al., 2014) and SBS 0335-052 (Brammer et al., 2012; Berg
et al., 2018; Erb et al., 2019) at lower redshifts share similar features to
A2744-arc3. Strong nebular emission lines such as C IV $\lambda\lambda
1548,1550$, He II $\lambda 1640$ and O III] $\lambda\lambda 1661,1666$ are
notable in their UV spectrum, suggesting a very high ionizing field from the
low-metallicity starbursting region and young stellar populations. SBS
0335-052 is a nearby blue compact dwarf galaxy ($z=0.0009$) which shows a
damped Ly$\alpha$ profile of $\log{N_{\rm{HI}}/{\rm cm^{-2}}}=21.70\pm 0.05$,
while SL2S J0217, a low-mass ($M<10^{9}\ {\rm M}_{\odot}$), low-metallicity
(${Z}<1/20\ {\rm Z}_{\odot}$) galaxy at $z=1.8$ magnified by a foreground
massive galaxy, may be characterized by a superposition of strong Ly$\alpha$
emission and damped absorption. SL2S J0217 also shows a blue UV slope of
$\beta=-1.7\pm 0.2$ over rest-frame 2100-2800 Å, and an ionization parameter
of $\log{U}\approx-1.5$.

In addition, Green Pea galaxies at $z<1$ with extremely high degree of
ionization in H II regions often present a blend of strong emission and
underlying absorption (McKinney et al., 2019), with column densities ranging
from $\log{N_{\rm HI}/{\rm cm^{-2}}}=19-21$, well above the LyC thick limit
(e.g., Steidel et al., 2018). The presence of the DLA in the spectra of
A2744-arc3 at $z=4$ further suggests that low-mass, low-metallicity galaxies,
even at high redshift of $z\gtrsim 4$, may commonly possess abundant neutral
hydrogen gas, and the ultimate Ly$\alpha$ and ionizing photon escape may
strongly depend on geometrical effects.

###  5.2 The metal-enriched neutral gas reservoir around young, low-mass and
metal-poor Galaxies

As discussed above, A2744-arc3 is a low-mass ($M_{*}\approx 10^{7.6}\ {\rm
M}_{\odot}$) galaxy with low-metallicity (${Z}\approx 0.1\ {\rm Z}_{\odot}$) H
II regions, estimated from the nebular emission lines. In contrast, metal
absorbers associated with the DLA shows that the HI gas around the galaxy may
have the super-solar metallicity. Super-solar metallicity of the neutral gas
around a low-Z, high-U and low-mass galaxy is also reported in Berg et al.
(2018). The metal-enriched gas can be generated by a high efficiency in the
gas cooling and enriched by the recycled inflow from the star formation (e.g.,
Frye et al., 2019). Due to the metal-enrichment process is rapid, the metal-
enriched environment may be further enhanced by the continuous star formation
in the high-redshift Universe.

We propose a simple schematic model to explain the metallicity variances in
the ISM/CGM and star-forming region at $z\approx 4$, as shown in Figure 4.
Massive stars produce a hard radiation field, forming a highly ionized H II
region. As mentioned above, the compact star-forming regions could be
surrounded by H I clouds with varying column densities (Erb et al., 2019). DLA
can be observed when the sightline is completely blocked by high column
density neutral clouds, while residual Ly$\alpha$ emission in the absorption
trough can be detected in partially obscured cases. These outskirt
environments could be metal-polluted by the efficient feedback from massive
stars due to the high-velocity stellar wind. The metal-enriched gaseous
environment could result in the presence of metal absorbers detected in Figure
3(c), sometimes with the detectable P-Cygni profiles (Erb et al., 2012). This
scenario explains the tentative detection of Ly$\alpha$ emission shown in
Figure 3(b) and asymmetry profiles of metal absorbers shown in Figure 3(c).

Figure 4: The schematic toy model of A2744-arc3 as a low-mass, low-metallicty
galaxy with high metallicity neutral gas reservoir. The blue stellar clumps
inside mark the compact and bursty star-forming regions in A2744-arc3, on
scales of sub-kpc. Their hard ionization field forms hot, thin and highly-
ionized H II regions around them, where strong narrow nebular emission lines
are produced. The gas outer layer denotes the neutral gas reservoir with
varying column densities in the CGM. Its geometry determines the shape of
Ly$\alpha$ emission/absorption we observed. When sightlines are blocked by
high-column density gas, damped Ly$\alpha$ absorption is observed; in
partially obscured case, Ly$\alpha$ emission with underlying absorption is
observed. Three arrows mark possible gas cycles within the galaxy. Feedback
from central star-formation, such as stellar winds, could metal-enrich the
neutral gas, leading to strong metal absorbers with P-Cygni profiles. The
recycled neutral gas can further efficiently fuel the next-generation star
formation.

###  5.3 Implications of the spatially resolved UV slope map

The ultraviolet (UV) continuum slope $\beta$
($f_{\lambda}\propto\lambda^{\beta}$) is a prominent metric for properties of
high-redshift galaxies. Both simulations and observations suggest that, bluer
galaxies observed at $z=3--4$ have $\beta=-2$, while values as blue as
$\beta=-3$ can in principle be produced by a young (age $\lesssim 10^{8}$
years), low-metallicity (${Z}\lesssim 10^{-3}{\rm Z}_{\odot}$) stellar
population (e.g., Schaerer, 2003; Jiang et al., 2020; Topping et al., 2022).
Bouwens et al. (2010) predicted that the bluest $\beta$ for low metallicity
($0.02-0.2\ {\rm Z}_{\odot}$) starbursts is about $-2.7$, consistent with
$\beta$ around the three clumps in Figure 2. This further indicate the low-
metallicity nature of these early star clusters. Aside from the blue radiation
from young stars, recombination continuum radiation from the ionized gas can
make the UV spectrum redder. When metallicity is greater than 0.02 ${\rm
Z}_{\odot}$, the UV slope generated by the recombination continuum become
greater (redder) than $-2.5$. With a high spatial resolution, we are able to
separate the relatively red nebular continuum emission from the ionized gas
from the massive star continuum (e.g., Bouwens et al., 2009) which may be
consistent with what we have observed here in A2744-arc3 with JWST.

###  5.4 Implication of such a Low-mass, but dominating population of galaxies
during Reionization

The UV luminosity of A2744-arc3 is $M_{\rm UV}=-17.8\pm 0.1$, corresponding to
0.049 $L_{\rm{UV}}^{*}$ galaxies in the faint-end of the galaxy luminosity
function ($M^{*}=-20.88\pm 0.08$, Bouwens et al., 2015). Through the
spatially-resolved emission and absorption lines, A2744-arc3 at $z=4$ provides
us with a text-book example, allowing an unparalleled view of low-mass
galaxies at the epoch of Reionizaiton (EoR).

Low-mass, metal-poor galaxies with hard ionizing radiation show much promise
to dominate the ionizing photon budget during EoR (Wise et al., 2014), with a
predicted escape fraction of $\sim 20\%$ (Duffy et al., 2014). Similar to
lower-redshift analogs (e.g., James et al., 2014; Berg et al., 2018; Erb et
al., 2019), A2744-arc3 at $z=4$ has shown the prevalence of a neutral gas
reservoir. Given these observations, we argue that, when modelling the escape
of ionizing radiation into the intergalactic medium during EoR, the neutral
gas reservoir surrounding galaxies themselves (see Figure 4) should be
seriously considered. Our identification of the metal-enriched, neutral-
gaseous clouds might support the picture that only a small fraction of
ionizing radiation of low-mass galaxies escape through the circumgalactic
channels with low column density of neutral gas. With more observations of
such galaxies, the statistics of the spatial variations of ionization,
metallicity, HI covering fraction can be better constrained. In particular,
the distribution of the optically thick HI clouds could affect the photon
escape by geometrical effects. Then, these HI clouds could regulate the star
formation and ionization fields through feedback. On the one hand, the outflow
from stellar winds could metal enrich the clouds, and then, the clouds could
further power the star formation through the recycled inflow (e.g, Frye et
al., 2019). In the future, with telescopes such as the Large Ultraviolet
Optical Infrared Surveyor (LUVOIR; Roberge & Moustakas 2018), we will be able
to better constrain the spatial distribution of optically-thick HI absorbers
at $z=2-5$, and improve our understanding towards the UV photon escape of low-
mass galaxies in the high-redshift Universe.

##  6 Conclusion

In this letter, we presented a detailed analysis on a strongly magnified
arclet, A2744-arc3, in the Abell 2744 field with the combination of
JWST/NIRISS (both imaging and grism spectroscopy), HST/WFC3, HST/ACS, VLT/MUSE
and ALMA observations. The photometry and SED fitting imply that A2744-arc3
has a young age of $24^{+6}_{-8}$ Myr, high-ionization of $\log{U}$ of
$-1.3\pm 0.3$, low-mass of $\approx 10^{7.6\pm 0.1}{\rm M}_{\odot}$, and the
low-metallicity H II regions of 12+$\log{\rm(O/H)}\approx 7.75$, corresponding
to 0.12 ${\rm Z}_{\odot}$) at $z=3.98$.

The reconstructed source-plane images based on JWST observations provide the
resolved UV slope at the spatial resolution of $\approx$100 pc. Three compact
clumps show blue UV slopes, ranging approximately from $-1.5$ to $-2.5$,
consistent with Vanzella et al. (2022), implying young massive star clusters
with bursty star formation activities.

Through absorption studies, we have identified an associated extremely strong
DLA system, with $\log N_{\rm HI}=21.8\pm 0.1$. Multiple metal absorbers
indicate that this DLA may be super-solar metallicity, with
[O/H]=0.41${}^{+0.20}_{-0.30}$, higher than typical DLAs and LBGs at similar
redshifts, indicative of strong metal-enrichment from low-mass galaxies.
Additionally, the tentative P-Cygni profiles of the metal lines strengthen
evidences for the young stellar feedback. To confirm our conclusion, we need
higher resolution spectra with further larger wavelength coverage to measure
the absorbing gas-phase metallicity. A2744-arc3 also shows strong narrow
nebular emission lines of C IV $\lambda\lambda 1548,1550$, He II $\lambda
1640$ and O III] $\lambda\lambda 1661,1666$, indicative of a metal-poor H II
region with a hard ionization field.

Combing the lower-redshift analogs SL2S J0217 at $z=1.8$ and SBS 0335-052 at
$z=0.0009$, our observations of A2744-arc3 at $z=4$ suggest that neutral gas
reservoir may be prevalent around low-mass galaxies. In our schematic model
(Figure 4), massive star formation reside within a metal poor H II region. The
galaxy is surrounded by a large reservoir of neutral gas with a metallicity
that could reach solar metallicity. The ionizing photons could only escape
through low column density channels. A2744-arc3 at $z=4$ provide us an
excellent picture in understanding faint-end galaxies that may have played a
vital role during the EoR.

## Acknowledgments

Xiaojing Lin and Zheng Cai thank Eros Vanzella, Lilan Yang, Dandan Xu and
Shiwu Zhang for very helpful discussions. X.L. thanks Zechang Sun for the
useful tool SpecViewer (Sun, 2021). Zheng Cai, X.L., Y.W., Z.L., M.L.,& S.Z.
are supported by the National Key R&D Program of China (grant no.
2018YFA0404503), the National Science Foundation of China (grant no.
12073014), and the science research grants from the China Manned Space Project
with No. CMS-CSST2021-A05. F.S. acknowledges support from the NRAO Student
Observing Support (SOS) award SOSPA7-022. F.S. and E.E. acknowledge funding
from JWST/NIRCam contract to the University of Arizona, NAS5-02105. J.X.P.
acknowledges partial support from NSF AST-2107991. D.E. acknowledges support
from the Beatriz Galindo senior fellowship (BG20/00224) from the Spanish
Ministry of Science and Innovation, projects PID2020-114414GB-100 and
PID2020-113689GB-I00 financed by MCIN/AEI/10.13039/501100011033, project
P20_00334 financed by the Junta de Andalucía, and project A-FQM-510-UGR20 of
the FEDER/Junta de Andalucía-Consejería de Transformación Económica,
Industria, Conocimiento y Universidades.

This work is based on observations made with the VLT/MUSE, ALMA, NASA/ESA
Hubble Space Telescope and NASA/ESA/CSA James Webb Space Telescope. MUSE
observations collected at the European Organisation for Astronomical Research
in the Southern Hemisphere under ESO program 094.A-0115. HST and JWST data
were obtained from the Mikulski Archive for Space Telescopes at the Space
Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127
for JWST and NAS 5–26555 for HST. The JWST observations are associated with
program ERS-1324. The HST observations are associated with the Hubble Space
Telescope Frontier Fields program. The authors acknowledge the GLASS team for
developing their observing program with a zero-exclusive-access period. The
following ALMA data is also adopted: ADS/JAO.ALMA#2018.1.00035.L and
#2013.1.00999.S. ALMA is a partnership of ESO (representing its member
states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and
ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic
of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.
The National Radio Astronomy Observatory is a facility of the National Science
Foundation operated under cooperative agreement by Associated Universities,
Inc.

##  Appendix A Systematics of lensing models

###  A.1 Systematics on photometry

In Section 3.1 we measure the delensed-flux of A2744-arc3 using Glafic lensing
model. Here we repeat the measurement using the lensing model in Richard et
al. (2021, hereafter R21), and the physical properties of A2744-arc3 keeps
consistent with those shown in Figure 5, with the difference of the stellar
mass $\approx 1\%$ and age $\approx 2\%$.

Figure 5: Photometry and SED analysis of A2744-arc3, with delensed fluxes
measured using Richard et al. (2021) lensing model. The blue triangles denote
the measurement and the red circles the best-fit result. Although the adopted
lensing model is different, this R21 results are well consistent with that
shown in Figure 1.

###  A.2 Systematics on source reconstruction

In Section 3.2 we adopt Glafic lensing model to reconstruct the intrinsic UV
slope distribution of A2744-arc3. Limited by the lensing model, we can hardly
reconstruct the three clump pairs onto the source plane simultaneously, where
each pair should have perfectly overlapped. Among all publicly released
lensing models, the Glafic model presents the smallest offsets between source-
plane positions of these clumps pairs, yielding 0$\farcs$010, 0$\farcs$017,
and 0$\farcs$033 from inside to outside respectively. To avoid model-dependent
systematics introduced by the non-overlap, the reconstruction is only
performed on the west side of the arclet that is predicted to have larger
separation for the three clumps.

Different lensing model yields different magnification and deflection fields,
which would lead to different reconstructed morphology. As discussed before,
lensing models can not perfectly recover the symmetric clumps in A2744-arc3 in
the source plane, and we choose Glafic because it provides smallest offsets
for the reconstructed clump pairs. In Figure 6 we show the reconstructed UV
slope using the R21 lensing model, which yields offsets of 0$\farcs$12,
0$\farcs$05 and 0$\farcs$19 for the three clump pairs, as well as the Glafic
reconstruction for the arc on the east side. Though the source-plane
morphology and layout of the three clumps varies with different lensing
models, the UV slope keeps within in the range $-2.5$ to $-1.5$ and the
spatial variation keeps on scales of $<$100 pc.

Figure 6: Reconstructed UV slope distribution on the source plane using
JWST/NIRISS images. The left panel shows the reconstruction for the east side
of A2744-arc3 using the Glafic model. The middle presents the reconstruction
for the west side of A2744-arc3 with the R21 model and the right for its east
part. See more details in Figure 2.

##  Appendix B Sysematics on metallicity

In Section 4.2 we estimate the H II region metallicty of A2744-arc3 using the
[Ne III]/[O II]-metallicity callibration relation in Bian et al. (2018):
$12+\log(\mathrm{O}/\mathrm{H})=7.80-0.63\times\log([\mathrm{NeIII}]/[\mathrm{OII}])$.
Due to the low resolving power of JWST/NIRISS ($R\approx 150$), [Ne III] at
3869Å is blended with He I+ H8 at 3889Å (e.g., Schaerer et al., 2022a), so
that the true [Ne III]/[O II] ratio would be lower and hence the metallicty a
bit higher than the [Ne III]/[O II]-derived one. But this won’t change the
metal-poor nature of A2744-arc3. Despite large scatters in the [Ne III]/[O
II]-metalicity relation (e.g., Schaerer et al., 2022a) concluded from various
galaxies at $z=0-8$, recent observations show that the metalicity of high-
redshift galaxies (Sanders et al., 2020; Curti et al., 2022) tends to be
consistent with or smaller than the prediction from the empirical calibration
in Bian et al. (2018). The latter puts A2744-arc3 to the lower metallicty end.
All these facts, along with the strong UV emission lines, support that
A2744-arc3 should possess a metal-poor H II region. A more precise measurement
of its metallicity is expected in the future with high resolution
spectroscopy.

##  Appendix C Two-component Voigt profile fit

In Figure 7 we present the results of simultaneous Voigt profile fits to Al II
$\lambda$1670, Fe II $\lambda$1608, O I $\lambda$1302, Si II $\lambda$1526 and
Si IV $\lambda$1402 absorbers, assuming each absorber has two components. We
perform the fits by fixing the doppler parameter $b$ to 5, 10, 15 as well as
30km s-1. The column densities and metallicities listed in Table 1 are
concluded from estimates with $b$=5, 10 and 15km s-1 , which is the typical
$b$ values for metal absorbers seen in high-resolution spectra. However, as
discussed in Section 4, a possible large $b$, e.g., 30km s-1, due to possible
unresolved sub-components of the absorption profile in low-resolution spectra,
would yield a lower column density and thus lower metallicity.

Figure 7: Voigt profile fits to the Al II $\lambda$1670, Fe II $\lambda$1608,
O I $\lambda$1302, Si II $\lambda$1526 and Si IV $\lambda$1402 absorbers,
assuming two components for each absorbers and fixing the doppler parameters
$b$ to 5, 10, 15 and 30 km s-1. Red lines are the best-fit profiles and the
blue vertical lines mark the positions of the two components for each
absorber. The top row of each panel shows residuals of the best-fit profiles
(black) and 3-$\sigma$ uncertainties (red dashed). The total column density of
each absorber, obtained by summing over their two components, is labelled, in
units of logN/cm-2.

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