We present an analysis of the JWST NIRCam and MIRI morphological and structural properties of 80 massive logSUB10SUBMSUBSUBMSUBSUB  11.2  0.1 dusty starforming galaxies at z  2.7SUB0.7SUBSUP1.2SUP identified as submillimetre galaxies SMGs by ALMA which have been observed as part of the JWST PRIMER project. To compare the structure of these massive active galaxies to more typical less actively starforming galaxies we defined two comparison samples. The first of 850 field galaxies matched in specific star formation rate and redshift and the second of 80 field galaxies matched in stellar mass. From the visual classification of the SMGs we have identified 20  5 as candidate latestage major mergers a further 40  10 as potential minor mergers and 40  10 that have comparatively undisturbed disclike morphologies with no obvious massive neighbours on 2030 kpc projected scales. These rates are comparable to those for the field samples and indicate that the majority of the submillimetredetected galaxies are not latestage major mergers but have interaction rates similar to the general field population at z  23. Through a multiwavelength morphological analysis using parametric and nonparametric techniques we establish that SMGs have comparable nearinfrared massnormalised sizes to the less active population RSUB50SUBSUPF444WSUP  2.7  0.2 kpc versus RSUPF444WSUPSUB50SUB ltinlineformula idFI3gt ltalternativesgt mmlmath xmlnsmmlhttpwww.w3.org1998MathMathML displayinline idmmleq3 mmlmsubsup mmlmiRmmlmi mmlmrow mmlmn50mmlmn mmlmrow mmlmrow mmlmi mathvariantnormalFmmlmi mmlmn444mmlmn mmlmi mathvariantnormalWmmlmi mmlmrow mmlmsubsup mmlmath texmath idtexeq3ltCDATA R50mathrmF444W gttexmath ltinlinegraphic xmlnsxlinkhttpwww.w3.org1999xlink idimgeq3 mimesubtypegif mimetypeimage xlinkhrefaa5100624eq3.gifgt ltalternativesgt inlineformula  3.1  0.1 kpc but exhibit lower Srsic indices consistent with bulgeless discs nSUBF444WSUB  1.1  0.1 compared to nSUBF444WSUB  1.9  0.1 for the less active field galaxies and nSUBF444WSUB  2.8  0.2 for the most massive field galaxies. The SMGs exhibit greater singleSrsic fit residuals and their morphologies are more structured at 2 m relative to 4 m when compared to the field galaxies. This appears to be caused by significant structured dust content in the SMGs and we find evidence for dust reddening as the origin of the morphological differences by identifying a strong correlation between the F200WF444W pixel colour and the 870 m surface brightness using highresolution ALMA observations. We conclude that SMGs and both massive and less massive starforming galaxies at the same epochs share a common disclike structure but the weaker bulge components and potentially lower black hole masses of the SMGs result in their gas discs being less stable. Consequently the combination of high gas masses and instabilities triggered either secularly or by minor external perturbations results in higher levels of activity and dust content in SMGs compared to typical starforming galaxies.
['galaxies: evolution', 'galaxies: high-redshift', 'galaxies: structure', 'submillimeter: galaxies', 'Astrophysics - Astrophysics of Galaxies']
The structure of massive starforming galaxies from JWST and ALMA Dusty highredshift disc galaxies
{'The Structure of Massive Star-Forming Galaxies from JWST and ALMA: Dusty, High Redshift Disk Galaxies': 'Steven Gillman, 1 , 2 ⋆ Ian Smail, 3 Bitten Gullberg, 1 , 2 A. M. Swinbank, 3 Aswin P. Vijayan, 1 , 2 , 4 Minju Lee, 1 , 2 Gabe Brammer, 1 , 5 U. Dudzeviˇci¯ut˙e, 6 Thomas R. Greve, 1 , 2 , 7 Omar Almaini, 8 Malte Brinch, 1 , 2 Scott C. Chapman, 9 Chian-Chou Chen, 10 Soh Ikarashi, 11 , 12 , 13 Yuichi Matsuda, 11 , 14 , 15 Wei-Hao Wang, 10 Fabian Walter, 6 , 16 and Paul P. van 17  \nder Werf  \n- 1 Cosmic Dawn Center (DAWN), Denmark e-mail: srigi@space.dtu.dk\n- 2 DTU-Space, Elektrovej, Building 328 , 2800, Kgs. Lyngby, Denmark\n- 3 Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK\n- 4 Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK\n- 5 Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen N, Denmark\n- 6 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany\n- 7 Dept. of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom\n- 8 School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK\n- 9 Dept of physics, Dalhousie University, Halifax NS, Canada\n- 10 Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Section 4, Roosevelt Road, Taipei 10617\n- 11 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan\n- 12 Junior College, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku, Fukuoka, 811-0295 Japan\n- 13 Department of Physics, General Studies, College of Engineering, Nihon University, 1 Nakagawara, Tokusada, Tamuramachi, Koriyama, Fukushima, 963-8642, Japan\n- 14 Graduate University for Advanced Studies (SOKENDAI), Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan\n- 15 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, MS 249-17, Pasadena, CA 91125, USA\n- 16 National Radio Astronomy Observatory, Pete V. Domenici Array Science Center, P.O. Box O, Socorro, NM 87801, USA\n- 17 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands  \nAccepted September 13, 2024', 'ABSTRACT': 'We present an analysis of the JWST NIRCam and MIRI morphological and structural properties of 80 massive (log 10 ( M ∗ [ M ⊙ ]) = 11.2 ± 0.1) dusty star-forming galaxies at z = 2.7 + 1 . 2 -0 . 7 , identified as sub-millimetre galaxies (SMGs) by ALMA, that have been observed as part of the JWST PRIMER project. To compare the structure of these massive, active galaxies to more typical less actively star-forming galaxies, we define two comparison samples. The first of 850 field galaxies matched in specific starformation rate and redshift and the second of 80 field galaxies matched in stellar mass. From visual classification of the SMGs, we identify 20 ± 5%as candidate late-stage major mergers, a further 40 ± 10% as potential minor mergers and 40 ± 10% which have comparatively undisturbed disk-like morphologies, with no obvious massive neighbours on ≲ 20 - 30 kpc (projected) scales. These rates are comparable to those for the field samples and indicate that the majority of the sub-millimetre-detected galaxies are not late-stage major mergers, but have interaction rates similar to the general field population at z ∼ 2-3. Through a multi-wavelength morphological analysis, using parametric and non-parametric techniques, we establish that SMGs have comparable near-infrared, mass normalised, sizes to the less active population, R F444W 50 = 2.7 ± 0.2 kpc versus R F444W 50 = 3.1 ± 0.1 kpc, but exhibit lower Sérsic indices, consistent with bulge-less disks: n F444W = 1.1 ± 0.1, compared to n F444W = 1.9 ± 0.1 for the less active field and n F444W = 2.8 ± 0.2 for the most massive field galaxies. The SMGs exhibit greater single-Sérsic fit residuals and their morphologies are more structured at 2 µ m relative to 4 µ m when compared to the field galaxies. This appears to be caused by significant structured dust content in the SMGs and we find evidence for dust reddening as the origin of the morphological di ff erences by identifying a strong correlation between the F200W -F444W pixel colour and the 870 µ m surface brightness using high-resolution ALMA observations. We conclude that SMGs and both massive and less massive star-forming galaxies at the same epochs share a common disk-like structure, but the weaker bulge components (and potentially lower black hole masses) of the SMGs result in their gas disks being less stable. Consequently, the combination of high gas masses and instabilities triggered either secularly or by minor external perturbations results in higher levels of activity (and dust content) in SMGs compared to typical star-forming galaxies.  \nKey words. Galaxies: high-redshift - Galaxies: structure - Galaxies: evolution - Sub-millimetre: galaxies', '1. Introduction': "The relative proportions of highly-dust-obscured and lessobscured star formation appears to vary over the history of the Universe, with the dust-obscured component dominating in  \ngalaxy populations at z ∼ 4-5 down to the present day (e.g., Dunlop et al. 2017; Bouwens et al. 2020; Long et al. 2023). The cause of this transition in star-formation mode may reflect the growing metallicity of the interstellar medium (ISM) in galaxies, less e ffi -cient removal of dust from deeper potential wells or structural or geometrical changes in the star-forming regions within galaxies.  \nThe most extreme examples at high redshifts of systems dominated by dust-obscured star formation are the submillimetre galaxies (SMGs) with dust masses of M d ∼ 10 8 -9 M ⊙ and far-infrared luminosities of L IR ∼ 10 12 -13 L ⊙ (e.g., Magnelli et al. 2012; Rowlands et al. 2014; Miettinen et al. 2017b; Dudzeviˇci¯ut˙e et al. 2020), placing them into the Ultra or Hyperluminous Infrared Galaxies (U / HyLIRGs) classes, see Hodge & da Cunha (2020) for a full review. Most of these galaxies show high star-formation rates, SFR ∼ 10 2 -3 M ⊙ yr -1 (e.g., Swinbank et al. 2014), and correspondingly short gas consumption timescales (e.g., Greve et al. 2005; Miettinen et al. 2017a; Tacconi et al. 2018; Birkin et al. 2021), suggesting that they represent relatively short-lived starbursts, ≲ 100 Myrs, which will result in massive systems with stellar masses of M ∗ ∼ 10 11 M ⊙ (e.g., Wardlow et al. 2011; Simpson et al. 2014; Miettinen et al. 2017a; Dudzeviˇci¯ut˙e et al. 2020).  \nThe high star-formation rates and large stellar masses of SMGs have proved challenging to reproduce in theoretical galaxy formation models (e.g., Baugh et al. 2005; Swinbank et al. 2008; Hayward et al. 2013; McAlpine et al. 2019), although more recent attempts have been more successful (e.g., Lovell et al. 2022; Lower et al. 2023; Cochrane et al. 2024). The models suggest that the high star-formation rates in this population are driven by a mix of secular instabilities in gas-rich disks and dynamical triggers due to minor and major mergers (e.g., McAlpine et al. 2019). However, attempts to observationally test these claims using the available rest-frame ultra-violet (UV) Hubble Space Telescope ( HST ) imaging of z ∼ 1-3 SMGs were challenging (e.g., Chapman et al. 2003). While the galaxies frequently exhibited irregular morphologies with apparently multiple components (e.g., Swinbank et al. 2010; Chen et al. 2015; Gómez-Guijarro et al. 2018; Zavala et al. 2018; Cowie et al. 2018; Lang et al. 2019; Ling & Yan 2022), their significant dust attenuation ( AV ≳ 2-6, e.g., Dudzeviˇci¯ut˙e et al. 2020) means that robustly assessing the intrinsic stellar mass morphology of the galaxies from these data was incredibly di ffi cult. This is especially the case for the subset of the population that is undetectable in the near-infrared ('NIR-faint' SMGs, Simpson et al. 2014; Dudzeviˇci¯ut˙e et al. 2020; Smail et al. 2021; Ikarashi et al. 2022; Kokorev et al. 2023). Detecting and resolving the restframe near-infrared emission of SMGs, which is much less affected by dust attenuation and more closely traces the bulk of the stellar population, is essential to robustly constrain the morphology of their stellar mass. However, this has been beyond the reach of previous instrumentation such as the Infrared Array Camera (IRAC; Fazio et al. 2004) on the Spitzer Space Telescope, which, whilst providing the required near-infrared wavelength coverage from 3.6 - 8 µ m, lacked the sub-arcsecond spatial resolution required to constrain the underlying morphologies of the stellar emission in SMGs.  \nHowever, some progress has been made in understanding the structure of SMGs using high-resolution observations of the dust continuum and gas kinematics in SMGs (e.g., Simpson et al. 2015; Ikarashi et al. 2015; Fujimoto et al. 2017). Thus FWHM ∼ 0 '' . 1-0 '' . 2 sub-millimetre mapping with ALMA has revealed bright dust continuum emission arising from compact disk-like structures (i.e., R e ∼ 1 - 2 kpc, Sérsic n ≈ 1) as well as evidence for a fainter more extended component with R e ≃ 4 kpc  \n(e.g., Gullberg et al. 2019; Ivison et al. 2020). The bright compact component is roughly half the size of the galaxy extent in HST H -band imaging (Hodge et al. 2016; Gullberg et al. 2019; Chen et al. 2020; Cochrane et al. 2021). At the highest resolutions, FWHM ∼ 0 '' . 05, Hodge et al. (2019) identified potential arms and bar-like structures in the bright dust continuum from a small sample of high-redshift SMGs.  \nResolved kinematic studies using molecular and atomic fine structure emission lines in the rest-frame far-infrared and sub-/ millimetre have uncovered disk-like kinematics for at least a significant fraction of the population (e.g., Hodge et al. 2012; Chen et al. 2017; Lelli et al. 2021; Rizzo et al. 2021; Amvrosiadis et al. 2023). With the advent of the James Webb Space Telescope (JWST, Gardner et al. 2023), high-resolution observations from the near (1 µ m) to the mid-infrared (25 µ m) are now feasible, enabling the infrared emission from counterparts of sub-millimetre bright galaxies to be resolved and the issue of their stellar morphology and structures to be finally quantified.  \nInitial morphological studies with JWST have found size variations with wavelength in sub-millimetre galaxies and less active colour-selected populations at z ∼ 2 (Chen et al. 2022; Cheng et al. 2023; Suess et al. 2022), with a reduction in halflight radius when observed in the rest-frame near-infrared compared to the rest-frame optical / UV as previously studied with HST . A number of studies have subsequently expanded the samples with JWST and ALMA coverage, confirming that SMGs become more compact (higher concentration, smaller size) at longer wavelengths (e.g. Cheng et al. 2023; Gillman et al. 2023; Price et al. 2023) with the potential impact of dust on the observed morphology becomingly less evident at longer wavelengths (e.g., Kokorev et al. 2022; Wu et al. 2023; Kamieneski et al. 2023; Sun et al. 2024). There are also claims of frequent stellar bars in many high-redshift star-forming galaxies (Guo et al. 2023; McKinney et al. 2024), including examples in SMGs (Smail et al. 2023). However, all of these studies suffer from modest sample statistics, limiting the interpretations of their findings.  \nIn this paper, we present an analysis of the structural properties of a statistically robust sample of ALMA-detected, submillimetre detected galaxies that have been observed with JWST NIRCam and MIRI. In Section 2 we define the sample of SMGs that we use in our analysis whilst in Section 3 we present the observations, data reduction and analysis undertaken on these systems (and matched control samples of less active galaxies selected from the field). In Section 4 we give our results and in Section 5 we discuss their implications before summarising our main conclusions in Section 6. Throughout the paper, we assume a Λ CDM cosmology with Ω m = 0 . 3, ΩΛ = 0 . 7, and H 0 = 70 km s -1 Mpc -1 . All quoted magnitudes are on the AB system and stellar masses are calculated assuming a Chabrier initial mass function (IMF) (Chabrier 2003).", '2. Sample selection': "To build a sample of ALMA-detected SMGs with near-infrared JWST coverage, we utilise the AS2UDS (Stach et al. 2019) and AS2COSMOS (Simpson et al. 2020) surveys as our parent sample. The AS2UDS and AS2COSMOS surveys are ALMA 870 µ m follow-up programs of 716 and 180 850 µ m SCUBA-2 sources that are detected at > 4 σ in the S2CLS (Geach et al. 2017) map of the UKIDSS Ultra-Deep Survey (UDS; Lawrence et al. 2007) and the S2COSMOS (Simpson et al. 2019) map of the Cosmic Evolution Survey (COSMOS; Scoville et al. 2007) respectively. These two surveys provide an initial candidate sam-  \nFig. 1: The combined AS2UDS and AS2COSMOS SMG sample in the PRIMER survey ranked in descending ALMA 870 µ mflux. For each SMG we show the 3 '' . 0 × 3 '' . 0 false colour image (F444W / F356W / F277W as R / G / B), labelled with the SMG ID, redshift ( magphys derived, z p, or spectroscopic, z s) and ALMA 870 µ m flux density. We further label the SMGs visually classified as major mergers (M) minor mergers or interactions (I), regular disks (D) or compact (C) as well as potentially those a ff ected by gravitational lensing (Lens). The red cross indicates the ALMA 870 µ mposition.  \n<!-- image -->  \nof 896 SMGs with precise ALMA identifications, selected at S / N ≳ 4, for which we can use to characterise their near-infrared JWST counterparts.  \nWe cross-match the ALMA positions for the SMGs with the footprint of the JWST / NIRCam and JWST / MIRI observations from The Public Release IMaging for Extragalactic Research (PRIMER; Dunlop et al. 2021) survey. PRIMER is a multi-band, multi-instrument survey of UDS and COSMOS covering 234 and 144 sq. arcmin respectively. The observations consist of the seven wide-band and one medium-band NIRCam filter (F090W, F115W, F150W, F200W, F277W, F356W, F410M, F444W) as well as two wide-band MIRI filters (F770W, F1800W). Parts of the UDS and COSMOS fields also benefit from HST ACS and  \nWFC3 coverage from CANDELS (Grogin et al. 2011) sampling the observed frame optical (0.4 µ m) to near-infrared (1.6 µ m).  \nFor our analysis, we require the SMGs to be covered by the JWST / NIRCam long-wavelength filter F444W ensuring that the observed-frame near-infrared is sampled. This results in a sample of 66 AS2UDS and 22 AS2COSMOS SMGs. A summary of the ALMA properties and HST and JWST coverage is presented in Appendix A. Of the 88 SMGs, 12 have no HST coverage. Two SMGs have JWST imaging in only three bands (F444W, F277W, F200W) while the other 86 SMGs are covered by three or more JWST bands, with 37 SMGs having the maximum ten available JWST filters covering from 0.9 to 18 µ m (Appendix A).  \nThe 88 SMGs have a median ALMA 870 µ m flux of S 870 µ m = 3.8 ± 0.4 mJy, with a 16 th to 84 th percentile range of  \nS 870 µ m = 1.8 - 6.1 mJy with the brightest, AS2COS0002.1 having S 870 µ m = 13.2 mJy (see Appendix A). The SMGs sample the bulk of the AS2COSMOS and AS2UDS S 870 µ m distribution which ranges from S 870 µ m = 0.6 - 19.2 mJy. In addition the majority of the AS2UDS SMGs (51 / 66) were observed with ALMA at resolutions of ≃ 0 '' . 3 FWHM as presented in Stach et al. (2019) with 15 SMGs observed at a higher resolution of ≃ 0 '' . 2 FWHM (see Gullberg et al. 2019). The ALMA maps of the 22 SMGs in the COSMOS field were tapered to a resolution of 0 '' . 8 FWHM (Simpson et al. 2020).", '3. Reduction and analysis': 'In this section, we present our data reduction and compilation of the photometric data that we use to characterise the multiwavelength properties of the SMGs and their analysis.', '3.1. JWST and HST': "We homogeneously process the JWST NIRCam and MIRI observations, retrieving the Level-2 data products (version = jwst\\_1069.pmap) from the STScI website 1 and processing them with the grizli pipeline (Brammer & Matharu 2021; Brammer et al. 2022) 2 . For the NIRCam data, additional steps were employed to deal with diagonal striping seen in some exposures, cosmic rays and stray light. Whilst for the MIRI exposures time-dependent sky flats are applied in an approach similar to recent JWST studies (e.g., Yang et al. 2023). We further incorporate the available optical and near-infrared data available in the Complete Hubble Archive for Galaxy Evolution (CHArGE, Kokorev et al. 2022), providing multi-band imaging from 0.4 to 1.6 µ mfor a subset of the sources (see Appendix A). We align all the imaging to Gaia DR3 (Gaia Collaboration et al. 2021), coadd, and drizzle the final mosaics to a 0 '' . 04 pixel scale (Fruchter &Hook 2002) for all JWST and HST filters.", '3.2. Photometry': "For JWST and HST bands we extract sources using sep (Barbary et al. 2016), a python version of source extractor (Bertin & Arnouts 1996), with a noise-weighted combined longwavelength (LW) stacked (F277W + F356W + F444W) NIRCam image as the detection image. For each source, we make a cutout in each of the HST and JWST bands, centred on this NIRCam long-wavelength detected source. Colour images made from the F444W / F356W / F277WNIRCambands for the SMGs are shown in Figure 1. Aperture photometry is performed in elliptical apertures with a minimum diameter of 1 '' . 0 and corrected to the 'total' values following Kron (1980). The aperture corrections are computed on the NIRCam LW stacked image and applied to all bands. The MIRI observations were processed prior to the release of the updated photometric calibrations (August 2023). We thus scale the measured flux in the F770W and F1800W bands by 0.85 and 1.03 respectively following the JWST documentation. We determine a median F444W AB magnitude for the SMG sample of m F444W = 21.7 ± 0.3 with a 16 th - 84 th percentile range of m F444W = 20.3 - 23.1.  \nWe compile the mid-infrared to radio (5.8 µ m to 1.4GHz) photometry for our sources from Dudzeviˇci¯ut˙e et al. (2020) and Simpson et al. (2020). For the SMGs with no MIRI F770W  \nand F1800W imaging, we adopt the IRAC channel 3 (5.8 µ m) and channel 4 (8 µ m) photometry, if IRAC channel 1 (3.6 µ m) flux is within 10 percent of the NIRCam F356W and IRAC channel 2 (4.5 µ m) flux is within 10 percent of the NIRCam F444W. We employ the IRAC photometry for two out of seven AS2COSMOS SMGs without MIRI coverage and 11 out of 38 AS2UDS SMGs.", '3.3. Final sample': "Having collated a sample of 88 AS2UDS and AS2COSMOS SMGs lying within the JWST PRIMER fields, we visually inspect each of the galaxies' JWST imaging to verify the association of the near-infrared counterpart to the ALMA source.  \nWe remove three UDS sources (AS2UDS0106.0, AS2UDS0175.0 and, AS2UDS0069.0) and three COSMOS sources (AS2COS0017.1, AS2COS0032.1 and, AS2COS0035.1) due to partial coverage in the JWST NIRCam imaging. Of the remaining 82 SMGs, we identify one SMG, AS2UDS0490.0, with no clear NIRCam counterpart, with a F444W magnitude of m F444W = 26.7 ± 0.6 detected at 1.4 σ . There is no MIRI coverage or any detection of this S870 = 2.7 mJy (S / N = 4.5) source between UV and radio, thus we conclude it is likely spurious, with the ALMA catalogue of 707 sources from Stach et al. (2019) expected to have a two percent spurious fraction, and so exclude the source from our sample. We also remove two SMGs, AS2COS0005.1 and AS2COS0005.2, which appear strongly gravitational lensed and thus the derivation of their intrinsic properties including morphology would require detailed and uncertain lens modelling (e.g. Amvrosiadis et al. 2018; Bendo et al. 2023; Pearson et al. 2024). We also flag AS2UDS0281.0 and AS2UDS0382.0 as possibly weakly lensed, opting to keep them in the sample.  \nFinally, one ALMA source from Simpson et al. (2020), AS2UDS00071.0, is deblended in the NIRCam F444W imaging, and high-resolution ALMA observations (Gullberg et al. 2019), into two sources which we label AS2UDS00071.0a and AS2UDS00071.0b. We model the morphology of AS2UDS0071.0a and AS2UDS0071.0b independently and scale the blended photometry by the flux ratio of the two sources. Thus in the final sample, we have 80 SMGs, for which the HST and JWST coverage for each SMG is detailed in Appendix A.", '3.4. SED fitting': "Having defined a final sample of 80 SMGs with robust nearinfrared counterparts, we use the SED fitting code Multiwavelength Analysis of Galaxy Physical Properties and Photometric Redshift ( magphys+photo -z , hereafter referred to as mag -phys ; Cunha et al. 2015; Battisti et al. 2019), to derive the physical properties of each SMG 3 . For 22 of the SMGs spectroscopic redshifts are available from the literature and ongoing millimetre and near-infrared spectroscopic surveys (e.g., McLure et al. 2018; Mitsuhashi et al. 2021). For these sources we use the highredshift version of magphys , ( magphys+high z ( v 2); Cunha et al. 2015; Battisti et al. 2019) with the redshift fixed to the spectroscopic redshift of each source, as detailed in Appendix A.  \nmagphys is a physically motivated SED fitting code that utilises the energy balance technique to fit the multi-wavelength  \nFig. 2: The multi-wavelength SEDs for four example SMGs that demonstrate the range of observed-frame infrared colours displayed by the SMG sample. Appendix B shows the SEDS for the full sample. For each source, we indicate the observed flux and error in each band with a red circle and errorbar. Arrows show upper limits at 3 σ . We over-plot the best-fit magphys derived SED (orange line) and model fluxes (black circles). Below each SED we display the probability distributions of the magphys derived properties: redshift (unless spectroscopic), stellar mass and A V , and report the best-fit redshift and reduced χ 2 from the fit in the top right. Above the SED we display the multi-wavelength JWST and ALMA 870 µ m imaging used as part of the analysis, with the SMG centroid indicated by the red tick markers.  \n<!-- image -->  \nphotometry from UV to radio. This approach, as tested on SMGs by Cunha et al. (2015) and Dudzeviˇci¯ut˙e et al. (2020), and also on simulated galaxies (e.g., Dudzeviˇci¯ut˙e et al. 2020; Haskell et al. 2023), models the sub-millimetre and optical emission as originating from the same region of the galaxy. For a discussion of using the magphys SED code to model high redshift SMGs, we refer the reader to Dudzeviˇci¯ut˙e et al. (2020). For consistencies with Dudzeviˇci¯ut˙e et al. (2020), for non-detections at wavelengths shorter than 8 µ m, we adopt a 3 σ upper limit while for those beyond 10 µ m, we use 1.5 ± 1.0 σ .  \n.  \nIn Figure 2, we show examples of the multi-wavelength photometry and magphys SED fits, with the fits for the whole sample presented in Appendix B. We summarise some of the key derived physical properties of the SMG sample in Figure 3. We estimate a median redshift of z = 2.70 ± 0.15 with a (16 th to 84 th percentile range of z = 1.9 - 3.9), which is comparable to that derived by Dudzeviˇci¯ut˙e et al. (2020) for AS2UDS of z = 2.6 ± 0.8 and the median redshift of the AS2COSMOS survey of z = 2.7 ± 0.9 as derived by Simpson et al. (2020), as well as other studies of SMGs at similar 870 µ m flux densities(e.g., Cunha et al. 2015; Ling & Yan 2022). From the magphys SED fitting, we derive a median stellar mass for the SMGs of log 10 (M ∗ [M ⊙ ]) = 11.20 ± 0.10 with a 16 th - 84 th percentile range of log 10 (M ∗ [M ⊙ ]) = 10.6 - 11.6, and a median specific star-formation rate of log 10 (sSFR[yr -1 ]) = -8.7 ± 0.1 with a 16 th - 84 th percentile range of log 10 (sSFR[yr -1 ]) = -9.2 to -8.0. We derive a median dust mass of log 10 (Md[M ⊙ ]) = 8.8 ± 0.1 with a 16 th - 84 th percentile range of log 10 (Md[M ⊙ ]) = 8.4 - 9.1 and a median infrared luminosity of log 10 (LIR[L ⊙ ]) = 12.6 ± 0.1 with a 16 th - 84 th percentile range of log 10 (LIR[L ⊙ ]) = 12.3 12.8. These are consistent with the sample of 707 SMGs in AS2UDS, for which Dudzeviˇci¯ut˙e et al. (2020) derived a median stellar and dust mass of log 10 (M ∗ [M ⊙ ]) = 11 . 1 + 0 . 2 -0 . 3 and log 10 (Md[M ⊙ ]) = 8 . 8 + 0 . 4 -0 4 and a median specific star-formation  \nrate and infrared luminosity of log 10 (sSFR[yr -1 ]) = -8.73 ± 0.04 and log 10 (LIR[L ⊙ ]) = 12.45 ± 0.02  \nAs expected given that the JWST photometry agrees with earlier IRAC and HST observations, while typically being more sensitive, the basic derived properties of the SMGs don't alter significantly when JWST photometry is included. The main benefit of deeper observations at 1 - 3 µ mis improved constraints on the presence of Balmer breaks at z ∼ 1 - 6.", '3.5. Field sample': "To investigate the physical mechanism that drives and di ff erentiates our SMG sample from the typical star-forming population at the same epochs ( z = 2 - 5), we compare their multi-wavelength properties to two samples of less active field galaxies. We construct the field samples from the K -band selected sample analysed by Dudzeviˇci¯ut˙e et al. (2020) in the UKIDSS UDS field for which deep 22-band photometry is available covering the UV / optical through to the far-infrared / submillimeter and radio. Dudzeviˇci¯ut˙e et al. (2020) used the magphys code to derive photometric redshifts, stellar masses and other physical properties for the ∼ 300,000 K AB ≤ 25.7 galaxies in this ∼ 0.8 degree 2 field from the UKIDSS survey (Almaini in prep.).  \nStarting from this field catalogue we first select galaxies with no photometric flags that lie within the footprint of the JWST PRIMER survey and have estimated dust masses and far-infrared luminosities that ensure they are less actively star-forming than our SMG sample: M dust = 10 7 -10 8 M ⊙ and L IR = 10 11 -10 12 L ⊙ . We then bin the redshift distribution of the SMGs and field galaxies in ∆ z = 0.5 bins and in each bin we rank the field galaxies based on their stellar masses. We then determine the number of SMGs for each bin and select ten times as many field galaxies from the corresponding redshift bin, starting with the most massive and going down the stellar-mass ranked list. In this way, we construct a field control sample that accu-  \nFig. 3: Distributions of the magphys derived SED properties for the SMG sample ( red ) and field (hatched grey, scaled down by a factor 10) and massive field (M. Field) samples ( open black histograms). We show the F444W AB magnitude (a) , photometric redshift (b) , stellar mass (c) , star formation rate (d) , specific star formation rate (e) , infrared luminosity (f) , dust mass (g) and V -band attenuation (AV) (h) . For each distribution, we indicate the bootstrapped median and uncertainty for SMGs ( red marker and errorbar ) and the matched field ( grey marker and errorbar ) and massive field sample ( black marker and errorbar ). By construction, the less massive field sample mirrors the SMG sample in redshift and specific star-formation rate distributions resulting in lower stellar masses, whilst the massive field sample more closely matches the SMGs distribution in stellar mass but with a lower median redshift. Both field samples have significantly lower dust masses, infrared luminosity, star-formation rates and A V .  \n<!-- image -->  \nmatches the redshift distribution of our SMG sample and comprises the most massive galaxies at each epoch that have dust masses and far-infrared luminosities below those determined for the SMG population. We adopt the photometry and magphys SED fitting results from Dudzeviˇci¯ut˙e et al. (2020).  \nIn Figure 3, we show the distribution of physical properties for the UDS field sample in comparison to the SMGs. By construction, the field sample has similar redshift and specific starformation rates to the SMG sample, whilst having significantly lower dust masses and AV. The field sample has a median redshift and median absolute deviation of z = 2.5 ± 0.1, whilst their specific star-formation rates are sSFR = -8.88 ± 0.04 yr -1 and A V = 0.90 ± 0.03. The field sample is also fainter in F444W magnitude with a median value of m field F444W = 22.74 ± 0.10 compared to m SMGs F444W = 21.70 ± 0.30, which is reflected in the lower median stellar mass of log 10 (M ∗ [M ⊙ ]) = 10.4 ± 0.1. Unfortunately, it is not possible to select a large sample of less active star-forming galaxies that are both matched to the SMG's stellar mass and redshift distributions, because most massive galaxies at z ≥ 2 - 3 are also SMGs.  \nTo address the mass o ff set of the field sample we define a second 'massive' field sample. We again select galaxies with no photometric flags lying within the PRIMER survey, but now require that they have estimated dust masses below M dust ≤ 10 8 M ⊙ (to ensure they are distinct from the SMGs) and stellar masses of M ∗ ≥ 3 . 2 × 10 10 M ⊙ (to more closely match the stellar masses of the SMGs). Finally, the binned redshift distribution was used to attempt to maximise both the size of the sample and its median redshifts, while also having a comparable breadth to the  \nSMGredshift distribution. This resulted in a final 'massive' field sample comprising 80 galaxies with a median stellar mass of log 10 ( M ∗ [ M ⊙ ]) = 11.0 ± 0.1, a median far-infrared luminosity of log 10 ( L IR[ L ⊙ ]) = 11.3 ± 0.1 and a median redshift of z = 2.3 ± 0.1 (16 - 84 th percentile range of z = 1.7 - 2.9). Hence this second field sample is comparable in mass to the SMGs and comprises much less actively star-forming galaxies, but lies at somewhat lower redshifts than the SMGs. The magphys derived SED properties of the SMGs, field and 'massive' field samples are shown in Figure 3.", '3.6. Morphological analysis': "In Figure 1 we show NIRCam F277W / F356W / F444W colour images of the SMGs (ordered by decreasing 870 µ m flux), highlighting the diverse range of rest-frame optical - near-infrared morphologies from faint and red galaxies, interacting and merging systems to disks and regular spirals. These near-infrared observations sample the galaxy population at ≤ 1 µ m in the rest frame at ≈ 1 kpc resolution and thus provide insights into the structure and morphology of the stellar continuum emission which are much less a ff ected by dust than previously possible.  \nWe visually assess as well as quantify the rest-frame nearinfrared morphology of the SMGs and field galaxies using nonparametric and parametric analyses of the JWST observations. We exclude the HST observations from our morphological analysis because at the median redshift of our sample ( z ∼ 2.7), HST observations only sample the rest-frame UV / optical emission of the galaxies where SMGs are inherently faint. The longer wave-  \nngth HST observations sampling the observed frame ≃ 1.6 µ m emission is also well covered by higher signal to noise JWST observations.  \nThe parametric and non-parametric analysis of the JWST imaging employs 4 '' . 0 × 4 '' . 0 cutouts ( ≃ 30 kpc square) of each source with a pixel scale of 0 '' . 04 per pixel. The cutout in each filter is centred on the source detection in the NIRCam longwavelength (F277W + F356W + F444W) stacked image, as detailed in Section 3.2. We first smooth the source segmentation map generated from sep (see Section 3.2) using the binary dilation routine in photutils (Bradley et al. 2022). Then, excluding this segmented region, we mask the remaining sources in the cutout, down to a 1 σ isophote. This ensures full masking of any contaminants (spurious or otherwise) 4 . We further use photutils to model (and remove) the background level in each cutout as well as to quantify the root-mean-square (rms) noise. In the following sections 'cutout' refers to these 4 '' . 0 × 4 '' . 0, background subtracted and masked image that is used in the morphological analysis that follows.  \nPrior to measuring the morphology of the galaxies we first derive the point spread function (PSF) for each of the JWST bands. We use webbpsf (version 1.2.1; Perrin et al. 2014) to generate PSF models for the MIRI and NIRCam detectors (for both short- and long-wavelength channels) which are calibrated with wavefront models at the epoch of the observations. The PSFs for each NIRCam and MIRI filter are then inserted into individual exposures of the final mosaic and drizzled to the final world coordinate system solution, producing a field of view averaged PSF model.", '3.6.1. Visual morphology': "We first undertake a crude initial visual assessment of the morphologies of the SMGs and field samples. This involved inspection of both the colour images and galfit residual maps in the F444Wband (Figure 1 & 4) of the ALMA counterpart to identify the overall structure and any distorted morphologies, asymmetric structures or potential tidal features. In addition, we assessed the presence of companions (either within ∼ 1 mag in brightness of the target galaxy, or fainter) within a wider 10 '' . 0 × 10 '' . 0 region (out to a radius of ≃ 40 kpc at z ∼ 2 . 7). Strongly disturbed galaxies, those with tidal features or disturbed galaxies with bright companions were classed as 'major' mergers (M), while less disturbed galaxies or those with asymmetries and fainter companions were classed as potential 'minor' mergers or interacting (I) in addition to undisturbed disks (D) and compact (C) sources. These classifications are marked in Figure 1.", '3.6.2. Non-parametric morphology': "To measure the half-light radii of the SMGs, we employ a curve of growth approach that makes no assumptions about the underlying structure of the galaxy's light distribution. We adopt this approach because SMGs have long been assumed to originate from merger-driven events with clumpy unstable gas-rich disks, (e.g., Smail et al. 1998; Greve et al. 2005; Tacconi et al. 2008; Engel et al. 2010) and therefore the morphologies of some sources may deviate from simple parametric profiles.  \nWe first perform a curve of growth analysis in each of the JWST bands from 0.9 µ m to 18 µ m. This is achieved by fitting a Gaussian profile to the cutout of each galaxy to determine its  \nimage parameters, allowing the centroid ( x , y ), axis ratio ( b / a ) and position angle (PA) to vary. We note the original centroid of the cutout is derived from the sep source detection on the stacked F277W + F356W + F444W NIRCam bands (Section 3.2), and thus may not be the apparent centre of the galaxy at shorter wavelengths. A curve of growth is then derived in each band using ellipses which align to the galaxy's axis ratio and position angle. From the curve of growth, we measure the convolved radii containing 20, 50 and 80 per cent of the flux of each galaxy. The intrinsic radii of the galaxies are derived by de-convolving the sizes with the PSF in each band, measured through a similar curve of growth analysis.  \nTo provide more quantitative, non-parametric, morphological indicators, we use the statmorph 5 code (Rodriguez-Gomez et al. 2019) which measures the Concentration, Asymmetry and Clumpiness ( C , A , S ; Abraham et al. 2003; Lotz et al. 2008) parameters that quantify how concentrated, asymmetrical and clumpy the galaxies' surface brightness profiles are, with higher values indicating more concentrated, asymmetric, or clumpier light profiles. We run statmorph on both SMG and field samples in all JWST bands using the same segmentation maps and PSFs as for the growth curve analysis described above.  \nIn addition, the Gini and M20 parameters are also derived (for full definitions see Lotz et al. 2004; Snyder et al. 2015). Briefly, the Gini parameter defines the pixel distribution of the galaxy's light, where G = 1 corresponds to all of the light concentrated in one pixel whilst G = 0 indicates each pixel contributes equally. The M20 parameter measures the second moment of the brightest 20 percent of pixels in the galaxy. This is normalised by the total moment for all pixels. Highly negative values indicate a high concentration of light, not necessarily at the centre of the galaxy. To validate the robustness of the statmorph measurements we compare the half-light radius to that derived from our curve of growth analysis. We derive a median curve of growth to statmorph median half-light radius ratio of R h , CoG / R h , statmorph = 1.06 ± 0.01 with a 16 th - 84 th percentile range of R h , CoG / R h , statmorph = 0.93 - 1.45 for the field sample in the F444W band. This indicates good agreement between the two independent morphological measurements 6 .", '3.6.3. Parametric morphology': 'To quantify both the parametric morphological profiles of the galaxies and the deviations from these, we use the galfitm code (Häußler et al. 2013). galfitm is a Python-wrapper for galfit (Peng et al. 2010), that allows multi-component parametric models to be fit to a galaxy\'s multi-wavelength light distribution. For our analysis we use a single Sérsic model, convolved with the PSF of the relevant JWST band. We fit each band independently thus allowing us to constrain the intrinsic wavelength dependence of the galaxy\'s morphology.  \nTo constrain the accuracy of the parametric analysis, we compare the galfitm Sérsic index ( n ) to that derived by stat -morph for the field galaxies in the NIRCam F444W band. We establish a median ratio of n GalfitM F444W / n statmorph F444W = 1.00 ± 0.01 and a 16 th - 84 th percentile range of n GalfitM F444W / n statmorph F444W = 0.81 - 1.26, indicating good agreement between the two independent codes.  \nAlthough a parametric Sérsic fit can model the overall distribution of a galaxy\'s light profile, it is the deviations (residuals)  \nFig. 4: For both SMGs ( left ) and field galaxies ( right ) we show the examples of the F356W / F277W / F115W RGB-colour image. For the SMGs we overlay the ALMA 870 µ m 11 σ contour ( purple ) from Stach et al. (2019) or Simpson et al. (2020). In the lower panel of each source, we show the residuals of the galfitm single Sérsic model fit. The blue ellipse marks the region within which we calculate the residual flux fraction (RFF) of the Sérsic fit, as quantified in the lower-left corner of each panel. The galaxies are selected to have a m F444W = 20 - 22 and are ranked from low-to-high RFF. No clear distinction is identified in the F444W RFF values between the SMGs and field galaxies.  \n<!-- image -->  \nfrom this simple profile, as shown in Figure 4, that encode important information about the detailed structure of the galaxies. To quantify the residuals we use the residual flux fraction (RFF), as defined in Hoyos et al. (2011, 2012),  \nRFF = P i , j ∈ A | Ii , j -I model i , j | -0 . 8 × P i , j ∈ A σ Bkgi , j P i , j ∈ A Ii , j (1)  \nArticle number, page 8 of 23  \nwhere the sum is performed over all pixels within 2.5 times the Kron radius, as derived in Section 3.2. | Ii , j -I model i , j | is the absolute value of pixel i , j \'s residuals to model Sérsic model fit, whilst P i , j ∈ A Ii , j indicates the total flux measured in the source as defined in Section 3.2. The 0.8 factor multiplied by the sum over the background rms of the region ( σ Bkgi , j ), ensures that a blank image with constant variance has a RFF = 0.0 (for details see Hoyos et al. 2012).  \n<!-- image -->  \n<!-- image -->  \nFig. 5: The distribution of (a) NIRCam colours, (b) MIRI colours and (c) F444W AB magnitude as a function of redshift , with lines of constant stellar mass overlaid in the latter. A representative error bar is indicated in the lower-right corner of each panel. Panel a) demonstrates the SMGs are much redder than our massive and sSFR-matched field populations, as expected. Panel b) highlights that a fraction of SMGs and field galaxies may host AGN with a very red F770W -F1800W colour and bluer F444W -F770W indicating a strong upturn in the SED beyond rest-frame 3 µ m. We label the \'AGN" and star-forming galaxy (\'SFG") regions in the colour space adapted from Kirkpatrick et al. (2013, 2017). We note the F770W -F1800W > 1 colour can also be driven by the 6.2 µ mPAH feature detected in the F18000W filter at z ∼ 1.9. Panel c) demonstrates that the SMGs are the most massive (and so typically also brightest) galaxies at their epoch where the two field samples have been selected to be the most massive low far-infrared luminosity galaxies.  \n<!-- image -->  \nIt is well known that morphological codes such as gal -fitm and statmorph systemically underestimate the uncertainties on the derived morphological parameters (e.g., Wel et al. 2012, 2024). To overcome this we employ an empirical approach which utilises the unique wavelength coverage of the NIRCam observations in the PRIMER Survey. Specifically, the majority of field galaxies (816 / 850) and SMGs (75 / 80) in our sample have both F410M and F444W observations. The F410M is a medium band filter covering the observed-frame 3.8 - 4.3 µ m emission, whilst the wide-band F444W filter is sensitive to the 3.8 - 4.9 µ m emission. Thus, for each galaxy with observations in both filters, we can make two independent measures of the galaxy\'s morphology at very similar rest-frame wavelengths. Analysing the variance in morphological parameters between the two filters as a function of signal-to-noise allows us to infer a representative uncertainty on the morphological parameters at a given wavelength for each source given its signal-to-noise in that band. We use this conservative approach to quantify the uncertainties in the following analysis.', '4. Results': "From an initial sample of 88 SMGs, we have constructed a sample of 80 not-strongly-lensed SMGs with reliable (S / N > 4.5) ALMA 870 µ mdetections and multi-wavelength HST and JWST coverage from 0.4 - 18 µ m. Full details of the wavelength coverage for the individual SMGs in our sample are given in Appendix A. From the initial sample, we omitted two strongly lensed SMGs due to uncertainties on the photometry and structural properties introduced by the lensing configuration of each source, along with flagging two candidate weakly lensed sources, AS2UDS0281.0 and AS2UDS0382.0 (Figure 1). Identifying 5 ± 3% of ALMA-detected SMGs in our sample are affected by foreground galaxy lensing matches the prediction of Chapman et al. (2002), that between 3 and 5 per cent of submillimetre sources identified in blank-field SCUBA observations would be a ff ected by lensing.  \nFor the 60 SMGs with HST coverage, we detect no F160W counterpart at S / N > 3 in 10 sources (17%). All of these have clear near-infrared counterparts in the NIRCam F444W band at  \nS / N > 5, whilst three also have a S / N > 3 in the NIRCam F150W band. This leaves seven 'NIR-faint' (sometimes imprecisely described as ' HST -dark') SMGs with no detection in the H -band (S / N < 3). These seven SMGs have significant levels of dust attenuation as derived from magphys with A V > 3.9. This indicates that whilst the significant dust content of these galaxies can lead to them being undetected in HST observations, it is also driven by the lack of sensitivity and depth of previous observations.  \nOf the 78 SMGs with NIRCam short-wavelength (SW) coverage, we identify one galaxy, AS2UDS0346.0 at z = 4.1, that is undetected (S / N < 3) blueward of 2.7 µ m and is first detected by JWST in the F277W band at S / N ≥ 5. From our magphys analysis we derive significant dust attenuation with A V = 5.1 + 2 . 0 -0 . 6 for AS2UDS0346.0 with a stellar mass of log 10 ( M ∗ [ M ⊙ ]) = 11.2 + 0 . 1 -0 . 2 that is representative of our sample. For SMGs with suitable NIRCam coverage we identify detections (S / N > 3) in F200W for 96% (70 / 73), in F150W for 87% (68 / 78) and in F090W for 59% (43 / 73) of the sources respectively. All have S / N > 3 detections in the NIRCam-LW and MIRI bands.  \nIn Figure 5 we investigate the photometric properties of the SMGs in the JWST bands by comparing the NIRCam and MIRI colours. Specifically, in Figure 5a we compare the NIRCam F200W -F444W colour with the F090W -F200W colour. At the median redshift of the SMG and field galaxies, these filters sample the rest-frame UVJ regime of the galaxies' spectral energy distributions, highlighting the distinct red nature of the SMGs compared to the general field. Given this, it may be possible by colour selection to identify the JWST counterpart to the sub-millimetre bright SCUBA-2 source without requiring higher resolution ALMA observations (e.g., Alberts et al. 2013; Chen et al. 2016; An et al. 2018; Hwang et al. 2021). To investigate this, for each SMG we compare the NIRCam F200W - F444W colour of all sources detected in the 14 '' . 8 diameter SCUBA-2 beam to that of the ALMA-selected SMG. However only in 51% (41 / 80) of the SCUBA-2 observations, the ALMA source represents the reddest source in the beam. This indicates that whilst colour is a good identifier of SMGs, further information such as proximity to source centroid, predicted 870 µ mfluxes and multiwavelength observations from infrared to radio are required to isolate the exact near-infrared counterpart (e.g., Downes et al.  \n1986; Ivison et al. 2002; Hodge et al. 2013; Casey et al. 2013; An et al. 2018; Gillman et al. 2023).  \nFigure 5b compares the MIRI and NIRCam colours with the F770W -F1800W colour shown as a function of F444W -F770W colour. At the median redshift of the SMG and field galaxies, these filters sample the rest-frame near and midinfrared. The SMGs on average have redder F444W -F770W colours with comparable F770W -F1800W colours to the field population, reflecting the dust attenuation of the shorter wavelength light. The SMGs and field galaxies which exhibit very red F770W -F1800W colours with bluer F444W -F770W colour may indicate the presence of Active Galactic Nuclei (AGN) as noted by Ivison et al. (2004) who used MIPS / 24 µ m and IRAC colours to identify AGN and starburst galaxies. We highlight the regions of the colour diagram that an 'AGN' or star-forming galaxy ('SFG') would likely inhabit, adapted from Kirkpatrick et al. (2013, 2017). Of those with MIRI coverage, we identify 24 ± 5% (69 / 282) of the field galaxies (and 21 ± 3% (7 / 33) of the massive field sample) have near-infrared colours that may indicate the presence of AGN activity whilst for the SMGs we derive a lower fraction of 3 ± 1% (2 / 37). The two SMGs with 'AGN' like colours, AS2UDS0259.0 and AS2UDS0659.0, both exhibit extended morphologies in the MIRI F770W and F1800W observations with no visible point source, indicating any contribution from an AGN is minimal. This is in agreement with previous studies which identify the AGN contribution in SMGs to be small (e.g., Stach et al. 2019). There are four SMGs with a compact, point-source like morphology. AS2UDS0173.0 which is not covered by the MIRI observations as well as AS2UDS287.1, AS2UDS509.0 and AS2UDS516.0. All sources appear pointsource like in all JWST bands, suggesting they may just be compact galaxies as indicated in Figure 1. The SFG and AGN classification in Figure 5b does not fully encapsulate the bi-modality present in the colour space, especially in the field galaxies. Galaxies with F1800W -F770W > 1 and F444W -F770W < 0, represent a bright ( m F444W = 21.5 ± 0.1) lower redshift subset ( z = 1.9 ± 0.04) with significantly brighter F1800W emission ( m F1800W = 20.3 ± 0.1) compared to the 'bluer' population ( z = 2.9 ± 0.1, m F444W = 22.9 ± 0.1, m F1800W = 23.3 ± 0.1). At z = 1.9, the F1800W filter is sensitive to the 6.2 µ mPAH feature, which can strongly enhance the mid-infrared emission in massive galaxies, resulting in red F770W -F1800W colours (e.g., Draine et al. 2007; Aniano et al. 2020; Shivaei et al. 2024).  \nIn Figure 5c we show the correlation between NIRCam F444W magnitude and the redshift of the galaxies. We overlay lines of constant stellar mass, highlighting that the SMGs are the most massive galaxies at their epoch (Dudzeviˇci¯ut˙e et al. 2020), the massive field sample being comparable and the less massive field sample exhibiting lower stellar masses at all epochs.", '4.1. Near-infrared morphology': "While SMGs have long been known to exhibit complex, potentially merger-like, morphologies based on rest frame UV imaging (e.g., Smail et al. 1998; Chapman et al. 2004; Swinbank et al. 2010; Aguirre et al. 2013) the influence of dust has meant their true stellar morphologies are still unknown. The combination of near-infrared colour images (Figure 1) and F444W Sérsic fit residual maps (Figure 4) provides an unprecedented insight into the stellar structures and asymmetries in the SMGs.  \nFrom a visual inspection, in 21 / 80 SMGs' we identify clear spiral arm features in the Sérsic model residuals, as well as clear clumpy structures in 30 ± 4% (24 / 80) of the SMGs. In particular, one galaxy AS2UDS0259.0, shows a bar-like structure in  \nthe F444W image indicating complex stellar structures may be present, similar to those identified in recent JWST and ALMA studies (e.g., Hodge et al. 2019; Smail et al. 2023; Rujopakarn et al. 2023; Liu et al. 2023).  \nWe estimate that 40 ± 10% (32 / 80) of the SMGs are isolated systems, with no strong perturbations or obvious bright neighbours on ≃ 20 - 30 kpc (projected) scales, while 16 SMGs (20 ± 5%) in our sample have potential companions or are interacting with another massive galaxy suggesting major mergers, the remaining 32 SMGs (40 ± 10%) have faint companions and show signs of disturbance (e.g., asymmetries in the galfit F444W residuals) that suggest potential minor interactions and mergers. There are at least two such examples in our sample of SCUBA-2 sources, S2UDS0322 and S2COS0034, and one ALMA source (AS2UDS0071.0) that comprise multiple ALMA-detected, or NIRCam-F444W detected, galaxies that show clear signs of galaxy interactions with S2COS0034 comprising two SMGs that are both detected in [C ii ] emission at z [CII] = 4.62 (Mitsuhashi et al. 2021).  \nThe median far-infrared luminosity of the candidate major mergers is L IR = 10 12 . 53 ± 0 . 05 L ⊙ , compared to L IR = 10 12 . 49 ± 0 . 05 L ⊙ for the remainder of the sample, indicating no strong dependence of far-infrared luminosity on merger state (e.g., Hopkins et al. 2010). Indeed, an equivalent visual classification of the less active field sample returns very similar merger statistics with 25 ± 5% of the sample showing evidence for potential major mergers and the remaining ∼ 75% showing much weaker or no evidence for merger-related disturbance or companions on ≃ 20-30 kpc scales. This similarity in the rate of mergers between SMGs and less active galaxies is consistent with the theoretical results from the eagle simulation from McAlpine et al. (2019) who found that simulated SMGs had similar rates of recent mergers to less active galaxies, in part because most galaxies at high redshifts are undergoing continuous infall and merging.  \nTo summarise, while signs of potential dynamical disturbance are frequently seen in our SMG sample, we conclude that the majority of SMGs do not appear to be the result of late-stage, major mergers. This is in contrast with the situation for similar far-infrared luminous populations at z ∼ 0, where systems with far-infrared luminosities of L IR ≥ 10 12 L ⊙ are frequently associated with late-stage major mergers (e.g., Sanders et al. 1988; Farrah et al. 2001), although there are claims that this fraction declines at z ∼ 1 (e.g., Kartaltepe et al. 2010), with an increasing fraction of starburst galaxies appearing to be isolated systems (e.g., Faisst et al. 2024). It is also in contrast to early studies of SMGs with resolved CO kinematics (e.g., Engel et al. 2010) as well near-infrared spectroscopic observations (e.g., AlaghbandZadeh et al. 2012; Drew et al. 2020) which suggested that the majority of SMGs are mergers, with distinct separated dustdetected components.", '4.1.1. Sizes, sérsic indices and axis Ratios': "We measure the half-light radius of the SMGs and field galaxies in the F444W band, determining median values of R SMG h = 2.70 ± 0.23 kpc and R field h = 2.50 ± 0.10 kpc with 16 th - 84 th percentile ranges of R SMG h = 1.82 - 4.36 kpc and R field h = 1.64 - 3.76 kpc respectively. For the massive field sample, we determine a median of R M . field h = 2.42 ± 0.15 kpc with a 16 th -84 th percentile range of R M . field h = 1.70 - 3.47 kpc. This is comparable to the rest-frame 1.5 µ m size measured by Martorano et al. (2024) for massive log 10 (M ∗ [M ⊙ ]) = 11.0 - 11.5 star-forming  \nFig. 6: The correlation between a) stellar mass and half-light radius, and b) the Clumpiness ( S ) and Asymmetry ( A ) for the SMG and field samples in the NIRCam F444W filter. In each panel we show histograms of each parameter for the field ( hatched grey histogram, scaled down by a factor 10 ), massive field ( black ) and SMG sample ( red ). In panel a) we also overlay the mass size relation for 2 < z < 3 star-forming galaxies from Ward et al. (2024) and in panel b) the relation between Asymmetry and Clumpiness identified by Conselice et al. (2003). In panels c) and d) we show the distributions of Sérsic index ( n ) and axis ratio ( b / a ) in the F444W band. For each distribution, we display the KS probability ( p KS) that each field sample and the SMG distributions are drawn from the same parent population, where p KS < 0.001 suggests a significant di ff erence in the distributions. We plot the bootstrap median and uncertainty for the two distributions at the top of each panel. The SMGs, on average, have comparable sizes and lower Sérsic indexes than both field samples, whilst exhibiting similar axis ratios and clumpiness with higher asymmetry.  \n<!-- image -->  \ngalaxies at 2.0 ≤ z ≤ 2.5 with a median size of R 1 . 5 µ m h = 2.24 kpc with a 16 th - 84 th percentile range of R 1 . 5 µ m h = 1.55 - 4.27 kpc.  \nTo establish whether the SMGs and field galaxies are drawn from the same underlying distribution, we perform a Kolmogorov-Smirnov (KS) test on the mass normalised distributions of half-light radius. To mass normalise the half-light radius distributions, we adopt the rest-frame optical mass size relation from Ward et al. (2024) at 2 < z < 3, noting that this is consistent with the trends seen in our samples. From the median stellar mass of the less activate field galaxies to the median stellar mass of the SMG sample, we derive an increase in half-light radius of 27 ± 6%, which we apply to the less massive field galaxies' half-light radii to account for the o ff set in stellar mass between the two samples. For the massive field sample we make a direct comparison to the SMGs. For the non-Gaussian distributions of SMG and field galaxy morphological properties, we require p KS ≤ 0.003 ( > 3 σ ) to confirm the populations are inherently di ff erent. We conclude the SMGs and both field galaxy  \nsamples have indistinguishable distributions of mass normalised half-light radius with p KS = 0.06 for the less massive field sample and p KS = 0.36 for the massive field sample.  \nIn Figure 6a we show the correlation between stellar mass and F444W half-light radius for both field samples and the SMGs. We overlay the rest-frame optical mass size relation for star-forming galaxies at 2 < z < 3 from Ward et al. (2024). On average, the field galaxies follow the trend identified by Ward et al. (2024), whilst the SMGs, as indicated by the running median in Figure 6a, exhibit marginally smaller F444W sizes for stellar masses above log 10 (M ∗ [M ⊙ ]) > 10.5. We note however that the majority of galaxies used to define the mass size relation in Ward et al. (2024) have a stellar mass of log 10 ( M ∗ [ M ⊙ ]) < 10.5. In panel c) of Figure 6, we compare the distribution Sérsic index for the field sample to those of the SMGs, identifying more disc-like Sérsic indices for the SMGs with a median of n F444W = 1.10 ± 0.10 and a 16 th - 84 th percentile range of n F444W = 0.63 - 1.98 whilst the lower mass field  \nFig. 7: A comparison of the ALMA 870 µ mmorphological properties to the NIRCam F444W morphologies for the subset of SMGs with high-resolution ALMA observations from Gullberg et al. (2019). We compare (a) the half-light radii and (b) the position angle as a function of axis ratio, with a running median (blue line and shaded region) shown in each panel. On average we identify significantly smaller 870 µ m sizes ( R F444W h / R 870 µ m h = 2.7 ± 0.4) but consistent axis ratios ( b / a F444W / b / a 870 µ m = 0.95 ± 0.11) and position angles (PAF444W -PA870 µ m = 17 ± 5 · ), with a larger o ff set in position towards more circular axis ratios where the position angle is more degenerate.  \n<!-- image -->  \ngalaxies have a higher median value of n F444W = 1.85 ± 0.07 with a 16 th - 84 th percentile range of n F444W = 0.88 - 4.35. The massive field sample has an even higher median Sérsic index of n F444W = 2.78 ± 0.20 and a 16 th - 84 th percentile range of n F444W = 1.51 - 4.89. Applying a KS test, we identify that the SMGs are distinct compared to either the field or the massive field distributions with p KS < 0.001 for both 7 .  \nFrom our non-parametric analysis, we derive a median axis ratio of b / a = 0.55 ± 0.03 for the SMGs and a 16 th - 84 th percentile range of b / a = 0.37 - 0.82 in the NIRCam F444W band. This is comparable to the axis ratio distribution for 870 µ m dust continuum in SMGs with b / a = 0.63 ± 0.02 (Gullberg et al. 2019). For the field galaxies, we derive a median F444W axis ratio of b / a = 0.53 ± 0.01 and 16 th - 84 th percentile range b / a = 0.31 - 0.77, whilst for the massive field sample we derive a median b / a = 0.64 ± 0.03 and 16 th - 84 th percentile range b / a = 0.35 - 0.84. The distribution of axis ratios, as shown in Figure 6d, is comparable to that of the SMGs with p KS = 0.23 for the field sample and p KS = 0.72 for the massive field sample.", '4.1.2. Residuals, clumpiness and asymmetry': "Whilst the Sérsic profiles indicate more disc-like surface brightness distributions in F444W for the SMGs, it is the deviations from the parametric light profiles, as quantified by the RFF parameter, that encodes the detailed morphological properties of each galaxy. In Figure 4, we show examples of the NIRCam F444W Sérsic model residuals, and the derived RFF values, for both SMGs and field galaxies. Whilst some galaxies exhibit smooth residuals, and low RFF values, several galaxies (both SMGs and field) display complex morphologies with multiple clumps, spiral arms or bright compact point sources. For the SMGs we identify a median RFF value in the F444W band of RFF F444W = 10.2 ± 0.5 with a 16 th - 84 th percentile range  \nof RFF F444W = 5.9 - 16.3 whilst the field galaxies have a median value of RFF F444W = 8.5 ± 0.2 with a 16 th - 84 th percentile range of RFF F444W = 4.5 - 17.2. The massive field galaxies have a median RFF value in the F444W band of RFF F444W = 7.0 ± 0.63 with a 16 th - 84 th percentile range of RFF F444W = 4.4 - 11.85. We compare the distributions of RFF for SMGs and field samples finding a KS-statistic of p KS = 0.002 for the lower mass field sample, and p KS < 0.001 for the high mass field. This indicates that the larger, less massive, field sample is not distinguishable at > 3 σ level to the SMGs, whilst the massive field sample, with lower RFF values, is distinct from the SMGs.  \nAlthough the 870 µ m ALMA observations of the majority of the SMGs in our sample are not high enough resolution (FWHM ≤ 0 '' . 2) to identify these structural features, highresolution sub-millimetre studies have identified spiral arms, bars and star-forming rings embedded in exponential dust disks (e.g. Hodge et al. 2019; Amvrosiadis et al. 2024). For a subset of the SMGs (15 / 80), however, 0 '' . 2 resolution ALMA 870 µ mobservations are available as discussed by Gullberg et al. (2019). These resolved observations are insu ffi cient to identify complex dust structures, but the resolved 870 µ m emission's morphology provides insight into any underlying structural features. For these 15 SMGs we first compare the ALMA 870 µ m size measured in the uv -plane to the F444W growth curve size, as shown in Figure 7a. We estimate a median ratio of R F444W h / R 870 µ m h = 2.7 ± 0.4 with a 16 th - 84 th range of R F444W h / R 870 µ m h = 1.5 - 3.5. As identified by previous studies the far-infrared emission is much more compact than the stellar emission at shorter wavelengths (e.g. Simpson et al. 2015; Lang et al. 2019; Gullberg et al. 2019; Chen et al. 2022). We perform a Spearman rank test on the correlation between F444W / 870 µ m size ratio and the A V of the galaxy, identifying no significant correlation ( p SR = 0.33). However, a larger sample is required to robustly confirm the lack of correlation.  \nTo investigate the presence of complex dust structures we compare the 870 µ m axis ratio and position angle to that de-  \nFig. 8: a) The growth curve half-light radius ( R h) of the SMGs, normalised by the rest-frame 1 µ mhalf-light radius of the respective galaxy as a function of rest-frame wavelength and colour-coded by the galaxy integrated A V . We show individual measurements for SMGs, and a representative error bar, as well as the running median for the field sample (grey points and line) and massive field sample (black points and line). On the right-hand axis we show the normalised half-light radius scaled by the median rest-frame 1 µ m size ( R 1 µ m h = 3.1 kpc) for the SMGs. b) Sérsic index derived from the galfitm parametric fitting as a function of rest-frame wavelength. We indicate the NIRCam measurements by circles and MIRI measurements by squares. We show a running median, and standard error ( σ/ √ n ) in fixed logarithmic bins of wavelength for the SMGs and field galaxies. In both panels, we plot the parametric fit for the field (solid grey line) and SMGs (solid red line). We find that the SMGs, on average, have a stronger size variation with wavelength, becoming more compact at longer wavelengths quicker than either field sample, whilst exhibiting lower Sérsic index at all wavelengths.  \n<!-- image -->  \nived for the F444W emission. For the 870 µ memission we adopt the free Sérsic fitting parameters from Gullberg et al. (2019). We identify median ratios of b / a F444W / b / a 870 µ m = 0.95 ± 0.11 and, as shown in Figure 7b, a median o ff set in axis ratio of PAF444W -PA870 µ m = 17 ± 5 · , indicating good agreement in the alignment of the near-infrared (rest-frame ≃ 1 µ m) and far-infrared (rest-frame ≃ 250 µ m) emission, suggesting they are tracing the same underlying structure, in contrast to the UV / optical and far-infrared o ff sets identified in previous multiwavelength high-resolution studies (e.g., Chen et al. 2015; Calistro Rivera et al. 2018; Hodge et al. 2019).  \nIn Figure 6b, we show the correlation between asymmetry ( A ) and clumpiness ( S ) in the NIRCam F444W band as derived from statmorph and over plot the relation between A and S derived by Conselice et al. (2003) 8 . We find that the SMGs and field galaxies have similar clumpiness with a median value of S SMGs F444W = 0.02 + 0 . 03 -0 . 02 and S field F444W = 0.03 + 0 . 04 -0 . 04 respectively with a p KS = 0.02. The massive field sample has S M . Field F444W = 0.03 + 0 . 05 -0 . 01 , with a p KS = 0.01. This indicates that both field and massive field samples are broadly consistent with being drawn from similar distributions to the SMGs. The asymmetry in the F444W band for the SMGs is higher with a median value of A SMGs F444W = 0.13 + 0 . 09 -0 . 06 compared to the field galaxies ( A field F444W = 0.08 + 0 . 12 -0 . 06 ) and a p KS < 0.001. The massive field sample has median value of A M . field F444W = 0.07 + 0 . 06 -0 . 03 with a p KS < 0.001 indicating the field galaxies and massive field galaxies asymmetry distributions are distinct from the SMGs.  \nThe correlation between asymmetry and clumpiness highlights two distinct regions. At constant asymmetry, there are two populations, one at low clumpiness ( S < 0.04) and one at high clumpiness ( S > 0.04), where the majority of the SMGs and field galaxies have low clumpiness with smooth, symmetric light distributions whilst those galaxies at high clumpiness show signs of concentrated star-forming regions. The SMGs at S > 0.04 are more compact in the F444W filter with a median size of R h = 1.72 ± 0.14 kpc compared to the SMGs with lower clumpiness ( S < 0.04) with a median size of R h = 2.96 ± 0.25 kpc, whilst exhibiting comparable morphological and magphys derived properties. We note that several studies have highlighted the uncertainty introduced in non-parametric morphological measurements when the Petrosian radius of the galaxy ( R p) is comparable to the HWHM of the observations (e.g., Yu et al. 2023; Ren et al. 2024), especially when the clumpiness parameter is measured between 0.25 R p and 1.5 R p, see Lotz et al. (2004) for details. This suggests that the non-parametric measurements for these compact galaxies maybe more uncertain than for the more extended sources.", '4.2. Wavelength dependent morphology': "The preceding analysis of the SMGs and field galaxies stellar (near-infrared) morphologies has revealed broad similarities between the two populations. Many previous studies suggest the extreme properties of SMGs originate from merger-driven events (e.g., Smail et al. 1998; Swinbank et al. 2010; Aguirre et al. 2013). We do not expect the morphological signatures of these mergers to show wavelength dependence in contrast to the ef-  \nFig. 9: The rest-frame 1 µ mhalf-light radius for SMGs ( red circles ), less massive field galaxies ( grey dots ) and massive field galaxies ( black stars ) as a function of ( a ) redshift and ( b ) stellar mass . In each panel, we show a running median for SMGs (dashed blue line), field galaxies (grey dashed line) and massive field galaxies (black dashed line) as well as a representative uncertainty. We identify a consistent reduction in rest-frame 1 µ m size with increasing redshift in both field samples and the SMG sample, as well as a consistent mass size relation compared to that seen in Figure 6. This indicates that the morphology variation with wavelength identified in both field samples and the SMGs in Figure 8 is not driven by K-correction e ff ects.  \n<!-- image -->  \nfects of dust. Thus, by quantifying the morphological variation with wavelength in the field and SMG sample, we can test the applicability of the merger-driven scenario.  \nFor the SMGs and field galaxies we measure a median F200W half-light radii of R SMG h , F200W = 4.0 ± 0.3 kpc and R field h , F200W = 2.8 ± 0.1 kpc respectively. Compared to F444W, for the SMGs this represents a 48 ± 0.17% reduction in size between 2 µ m and 4 µ m whilst for the field galaxies the reduction in size is only 12 ± 6%, although we note this comparison is not done at fixed stellar mass it highlights the variation of morphology with wavelength for the two samples. The massive field sample has a median F200W half-light radii of R M . field h , F200W = 2.9 ± 0.2 kpc which corresponds to a 20 ± 11% reduction in size. To further investigate the variation in the SMGs and field galaxy morphology as a function of wavelength, in Figure 8 we plot the half-light radius, normalised by the rest-frame 1 µ m half-light radius, and Sérsic index against the rest-frame wavelength probed by the NIRCam and MIRI observations.  \nBelow 1 µ m, for SMGs in Figure 8 there is increased spread in half-light radius with an apparent correlation with dust content as quantified by A V . At rest-frame wavelengths greater than 3 µ m where we use the MIRI F770W and F1800W bands to quantify the morphology, we identify a decrease in the median Sérsic index of both field and SMGs to n ∼ 0.4 and an increase in the half-light radius. We suspect that this trend is driven by the larger beam size of the MIRI observations with FWHM = 0 '' . 27 and 0 '' . 59 respectively for the F770W and F1800W filters, in combination with the shallower depth compared to the NIRCam observations, resulting in a smoother, less-structured, light distribution. In addition, in the rest-frame near-infrared ( > 3 µ m) any AGN component that is present in the galaxy starts to be more prominent over the stellar emission, with point-source ( n ∼ 0.5) morphology.  \nTo quantify the field galaxy and SMGs size evolution with wavelength, we perform fits to the median values shown in Figure 8 up to rest-frame 3 µ musing an orthogonal distance relation  \n(ODR) algorithm that takes into account the uncertainties on the median values in both wavelength and size. We define the function as,  \nR ( λ ) R 1 µ m = d ( R h / R 1 µ m) d λ log 10 ( λ ) , (2)  \nwhere λ is the rest-frame wavelength probed by the observations. For the SMGs we derive a slope of d ( R h / R 1 µ m) d λ = -0.60 ± 0.09 whilst for the field galaxies we estimate d ( R h / R 1 µ m) d λ = -0.15 ± 0.07 and the massive field galaxies d ( R h / R 1 µ m) d λ = -0.04 ± 0.09. This indicates the SMGs have stronger variation in size with wavelength, becoming more compact faster than either field sample, as highlighted by the best-fit solution plotted in Figure 8. However, this trend may be driven by centrally concentrated dust inflating the rest-frame optical sizes of the SMGs resulting in a stronger observed variation with wavelength. To ensure size ( R h) variation with wavelength identified in the SMGs and field galaxies in Figure 8 is not driven by the redshift evolution of the galaxies, in Figure 9, we correlate the rest-frame 1 µ m size of the galaxies with their redshift. On average we identify a consistent redshift evolution in the near-infrared sizes of the SMGs and both field samples, indicating that redshift dependent Kcorrections are not driving the morphological wavelength variation.  \nFurthermore Figure 8 shows that the SMGs, on average, have a lower Sérsic index than the field galaxies at all wavelengths. To quantify the evolution of Sérsic index with wavelength we fit a similar relation to that used to quantify the size evolution of the form,  \nn ( λ ) = n 0 + dn d λ log 10 ( λ ) , (3)  \nup to a rest-frame wavelength of 3 µ m. The median Sérsic index of the SMGs have an dn d λ = -0.12 ± 0.11 and n 0 = 1.00 ± 0.04, whilst the field galaxies exhibit strong variation with wavelength, but at higher average Sérsic index ( dn d λ = -1.10 ± 0.23  \nFig. 10: We show the Gini - M20 relation for the less active field galaxies (grey dots and contours), massive field galaxies (black stars) and SMG sample (red circles), in the a) F200W and b) F444W bands as well as histograms showing the distribution of each parameter on each axis, with the low mass field scaled down by a factor 10. The solid-black line indicates the boundary line between mergers and non-mergers as defined in Lotz et al. (2008). Both SMGs and field galaxies scatter about the line in the F200W band whilst the majority lie in the non-merger region of the equivalent F444W plot. In panels c) and d) we show the distributions of M F444W 20 -M F200W 20 and RFF F444W -RFF F200W . For each distribution, we indicate the KS-statistic showing that the SMGs have a stronger variation in morphology with wavelength with higher RFF and M20 values at 2 µ m than 4 µ m indicating more structure in the residuals and disturbed morphologies.  \n<!-- image -->  \nand n 0 = 1.51 ± 0.10). For the massive field galaxies we measure dn d λ = -2.16 ± 0.81 and n 0 = 2.37 ± 0.32. Thus indicating on average, the SMGs have light distributions closer to pure disk-like exponentials.  \nTo further investigate the variation of morphology with wavelength in the SMG and field samples, in Figures 10a & b, we show the Gini - M20 relation which has been used as a crude late-stage merger indicator (Lotz et al. 2008; Liang et al. 2024; Polletta et al. 2024) in the F200W and F444W bands. We select the F444W band as this most closely traces the stellar morphology of the galaxies, whilst F200W band corresponds to the restframe V -band for the median redshift of the samples, at a similar wavelength to the R -band where the Gini - M20 'merger' and 'non-merger' boundary was calibrated by Lotz et al. (2008). At 2 µ m (rest-frame V -band) both field and SMG samples scatter about the boundary between 'merger' and 'non-merger', with 40 ± 5% of the SMGs lying in the merger region. Although the SMGs are o ff set relative to the field populations with higher M20 and lower Gini values. At 4 µ m (rest-frame z -band) almost all  \nSMGs (92 ± 2%) and field galaxies lie in the 'non-merger' region of the relation, with consistent Gini and M20 values. Between 2 µ mand 4 µ m the relative shift in the Gini - M20 parameters is larger for the SMGs than for the field samples.  \nTo understand the origin of this shift in the Gini-M20 plane, in Figure 10c we show the di ff erence between the M20 parameter in the F444W and F200W bands. This plot highlights that the SMGs have higher M20 in the F200W than F444W band which indicates SMGs have less concentrated F200W morphologies whilst both the field and massive field galaxies have more similar M20 values between the two bands. The significance of the di ff erence between the field samples and SMGs is quantified by the KS-statistic value of p ks < 0.001. To highlight the reduction in structured emission between the F200W and F444W bands, in Figure 10d we show the di ff erence between RFF in the two bands for the SMGs and field galaxies. On average the SMGs exhibit a greater di ff erence in RFF across bands, with more significant F200W residuals, with a median value of RFF F444W -RFF F200W = -10.0 ± 1.0 with  \nFig. 11: A comparison of the 2 µ mand4 µ mmorphological properties of the SMGs and both field samples. We compare the di ff erence in M20 parameter between the F444W and F200W bands (M F444W 20 -M F200W 20 ) with dust attenuation (A V ) in panel a) . In panel b) , we further compare the di ff erence in RFF between the F444W and F200W bands (RFF F444W -RFF F200W ) with the di ff erence in Asymmetry ( A F444W -A F200W ). Finally we plot the di ff erence in M20 between F444W and F200W (M F444W 20 -M F200W 20 ) with the di ff erence in Concentration (C F444W -C F200W ) (panel c) and the di ff erence in RFF (panel d) . All four panels indicate the SMGs are much more structured than either field population, with less uniform light distributions in the F200W band compared to F444W, in contrast, the field galaxies show minimal wavelength variation.  \n<!-- image -->  \na 16 th - 84 th percentile range of RFF F444W -RFF F200W = -2.0 to -17.4 whilst for the field galaxies the median is RFF F444W -RFF F200W = -4.6 ± 0.3 with a 16 th - 84 th percentile range of RFF F444W -RFF F200W = -0.2 to -11.6. The massive field galaxies have RFF F444W -RFF F200W = -3.2 ± 0.8 with a 16 th - 84 th percentile range of RFF F444W -RFF F200W = -0.15 to -9.5. This is additional evidence of the dust obscuration at shorter wavelengths in the SMGs, resulting in deviations from a simple light profile. This comparison suggests the SMGs have more complex morphologies at bluer wavelengths, which is likely to be linked to their intense star formation rates and the presence of highly structured dust in the SMGs, attenuating the shorter wavelength light.  \nTo identify the physical mechanism driving the increase in structured emission identified in the SMGs (as quantified by the RFF and M20) as a function of wavelength, in Figure 11 we compare the di ff erence in morphological parameters measured between the F444W and F200W NIRCam bands for the SMGs and field populations. In Figure 11a we compare the dust content, as quantified by the A V from SED fitting (Section 3.2), with M F444W 20 -M F200W 20 . As shown in Figure 3, both field samples have significantly lower A V and less morphological variation with wavelength, with a median M F444W 20 -M F200W 20 = -0.06 ± 0.01 for the field sample and M F444W 20 -M F200W 20 = 0.09 ± 0.03 for the massive field sample. The SMGs on the other hand, with higher A V indicate a greater disparity between the F200W and F444W light distribution with  \na median value of M F444W 20 -M F200W 20 = -0.34 ± 0.04. To quantify the trend between A V and M F444W 20 -M F200W 20 we perform an ODR fit to the running median shown in Figure 11 of the form,  \nA V = A V , 0 + α (M F444W 20 -M F200W 20 ) , (4)  \nWe determine for the SMGs best-fit parameters of α = -1.0 ± 0.3 and A V , 0 = 3.1 ± 0.1, indicating a strong negative correlation for the SMGs, whereby more dust obscured (higher A V ) SMGs, have a more negative M F444W 20 -M F200W 20 , indicating less concentrated light profiles in the F200W band compared to the F444W.  \nTo analyse the connection to other morphological parameters, in Figures 11b & d we plot RFF F444W -RFF F200W against the di ff erence in Asymmetry (A F444W -A F200W ) and M F444W 20 -M F200W 20 , whilst in Figure 11c we compare the di ff erence in Concentration (C F444W -C F200W ) with M F444W 20 -M F200W 20 . All three relations indicate that on average the SMGs in the F200W band are more asymmetric and less concentrated, with more in-homogeneous light distributions leading to larger residuals to a single Sérsic fit at 2 µ m as compared to their 4 µ m morphologies.", '5.1. Does structured dust drive the differences between SMGs and field galaxies?': "The morphological properties of the SMGs in the NIRCam F444W band are comparable to the field galaxies, with similar sizes at a fixed mass, slightly lower Sérsic indices and higher asymmetry (Figure 6). The F444W filter traces the rest-frame near-infrared ( ≈ 1 µ m) out to z ∼ 3.5, and thus is less a ff ected by dust obscuration and recent star formation than shorter bands in most of our sample. A greater di ff erence between SMGs and field samples is identified at bluer wavelengths.  \nBy investigating the variation M20 parameter and RFF between the NIRCam F444W band ( λ rest ≈ 1 µ m) and F200W band ( λ rest ≈ 0.5 µ m), Figures 10 and 11 highlight that the SMGs have more structured light profiles at shorter wavelengths, that results in larger residuals to single Sérsic fits. This morphological variation with wavelength was also highlighted by Chen et al. (2016) for SMGs and Nedkova et al. (2024) for SFGs, the latter identifying that the most massive (M ∗ ≳ 10 10 M ⊙ ) disc galaxies in their sample at 0.5 < z < 3 have larger half-light radii in the rest-frame ultra-violet than optical, which they attribute to the significant dust attenuation in the central regions of the galaxies compared to the outskirts. A similar connection between with inferred galaxy morphology and dust content was identified in the Illustris - The Next Generation (TNG50; Pillepich et al. 2019) simulation by Popping et al. (2022). They suggested that the observed H -band to 870 µ msize ratio increases towards higher redshift (hence bluer rest-frame sampling) due to dust attenuating the central regions of galaxies, resulting in larger half-light radii in the H -band. The impact of dust on size measurements has also been quantified in the First Light And Reionisation Epoch Simulations (FLARES; Lovell et al. 2021; Vijayan et al. 2021), where intrinsically massive compact galaxies appear significantly more extended when the e ff ects of dust are taken into account (Roper et al. 2022, 2023).  \nTo isolate the contribution of dust to the SMGs morphological evolution as a function of wavelength, we examine the F200W -F444W pixel colours. In particular, given the M F444W 20 -M F200W 20 parameter indicates the greatest morphological di ff erence between field galaxies and SMGs, we calculate  \nthe average colour of the brightest 20 percent of pixels in the F444W image inside the 0 '' . 5 elliptical apertures of each galaxy (Section 3.2). In Figure 12a, for the SMGs and both field samples, we compare this 'Brightest 20%' colour with the average colour of the fainter regions inside the same aperture ('Faintest 80%' F200W -F444W). Galaxies exhibiting no strong colour gradients between bright and faint regions will have consistent 'Brightest 20%' and 'Faintest 80%' colours, lying close to the one-to-one line. The majority of the field galaxies, as well as the massive field galaxies, as shown in Figure 12a, fall into this regime, with on average the brightest regions of the field galaxies being 0.12 ± 0.03 mag, redder than the faintest.  \nFor the SMGs the F200W -F444W pixel colour of the fainter regions that comprise the bulk of the galaxies ('Faintest 80%') indicate on average redder colours, with a median colour of 0.40 ± 0.06 mag compared to -0.74 ± 0.02 mag for the field galaxies and -0.67 ± 0.08 mag for the massive field galaxies. For the 'Brightest 20%' pixel colour, again the SMGs indicate significantly redder colours with a median value of 0.77 ± 0.06 mag compared to -0.66 ± 0.02 mag for the field galaxies and -0.66 ± 0.03 mag for the massive field galaxies.  \nTo understand whether the reddest (and brightest) regions of the SMGs are physically associated with the massive dust reservoirs, and thus 870 µ memission, of the galaxies we analyse highresolution ALMA 870 µ m observations for a sub-sample of 15 SMGs with ALMA maps at ≤ 0 '' . 2 FWHM resolution from Gullberg et al. (2019). We resample the ALMA maps to match the 0 '' . 04 pixel scale of the JWST NIRCam F444W imaging. We then extract the same average sub-millimetre flux density of the regions corresponding to the 'Brightest 20%' and 'Faintest 80%' F444W pixel as before for each SMG. In Figure 12b, we correlate this pixel colour with the ALMA 870 µ m pixel brightness for the 'Brightest 20%' and 'Faintest 80%' pixels. We identify a strong correlation between F200W -F444W pixel colour and 870 µ msurface brightness, demonstrating that the redder colours that originate from the brightest regions of the SMGs are spatially correlated with the brightest regions of the 870 µ m emission and thus high dust column density regions.  \nThe high dust column density of the SMGs, as shown in Figure 12, implies that the optical to near-infrared emission of the SMGs is strongly attenuated. Consequently, physical properties estimated from the SED fitting (e.g. A V ) may be uncertain due to more heavily obscured regions of the galaxies being undetected, a situation that will be even worse when relying solely on UV to near-infrared photometry and no energy-balance assumptions. We can derive the expected V -band extinction (A V ) given the median dust mass (log 10 (Md[M ⊙ ]) = 8.9 ± 0.2) derived from SED fitting (Sec 3.2) and compare this to the A V derived by magphys from fitting the optical to near-infrared SED.  \nAs shown by Güver & Özel (2009), the column density of hydrogen (in cm -2 ) can be approximated from the A V (in mag) as follows,  \nNH = 2 . 21 × 10 21 A V (5)  \nAdopting a dust-to-gas ratio of δ = 63 ± 7 as derived by Birkin et al. (2021), for SMGs with typical metallicity, star-formation rates and stellar masses (e.g., Rémy-Ruyer et al. 2014) and assuming the F444W half-light radius provides an upper-limit on the total extent of the dust region, we obtain a lower-limit on the median V -band dust attenuation of A V = 110 ± 20. This is two orders of magnitude higher than that derived from the SED fitting of SMGs, with the median magphys estimated value of A V = 3.40 ± 0.16. For the A V derived from magphys , which constrains the dust obscuration of the visible stars, an energy bal-  \nFig. 12: a) We plot the average F200W -F444W colour of the faintest pixels (below 80 th percentile of flux) inside 0 '' . 5 elliptical apertures in the F444W image for the SMGs and field samples, as a function of the average colour for the top 20 per cent brightest F444W pixels. Both field samples exhibit almost uniform colours between faint and bright regions, with a small o ff set to the equality line of 0.12 ± 0.03 mag (in the sense that the brighter regions are redder). The SMGs are both generally redder in fainter regions and significantly redder (1.11 ± 0.09 mag) in their brightest regions. In panel b) we show the average F200W -F444W colour of the 'Brightest 20%' and 'Faintest 80%' pixels as a function of ALMA pixel brightness. On the right-hand axis we show the surface dust mass density derived using the 870 µ m flux to dust mass relation from Dudzeviˇci¯ut˙e et al. (2020) at the median redshift of the sample ( z = 2 . 7). The brightest regions in the near-infrared (NIRCam F444W), which have the reddest F200W -F444W colour, correspond to the brightest regions in the rest-frame far-infrared (ALMA 870 µ m).  \n<!-- image -->  \ncalculation is assumed such that far-infrared emission is broadly consistent with the absorbed stellar light of the system (Battisti et al. 2019). This assumption assumes the UV / optical and far-infrared emission are co-spatial and well mixed however, inaccuracies can arise as detectable optical emission does not encode information about the ongoing obscured star formation of the galaxy (e.g., Simpson et al. 2017; Buat et al. 2019; Haskell et al. 2023; Killi et al. 2024), with previous studies identifying a 15% uncertainty in the SFRs derived from magphys for galaxies from the eagle simulation (e.g., Dudzeviˇci¯ut˙e et al. 2020). In the derivation of A V from the dust mass, which defines the dust obscuration of the deepest regions of the clouds, we have estimated a Compton thick HI column where the dust is uniformly distributed in a smooth 'disk-like' component as identified in the high-resolution ALMA studies of SMGs (e.g., Hodge et al. 2016; Gullberg et al. 2019). However, numerical studies suggest that the star dust geometry plays a crucial role in determining the attenuation curve of the galaxies (e.g., Inoue 2005; Sachdeva & Nath 2022; Vijayan et al. 2024) as well as the metallicity of the interstellar medium (e.g., Shivaei et al. 2020).", '5.2. Dust Content and the interstellar medium of SMGs': "As noted earlier, while the properties derived assuming energybalance are uncertain, they would be considerably less reliable if derived solely from the UV to near-infrared photometry and no energy-balance. With such high levels of inferred dust obscuration, and intense star-formation rates, the question remains what are underlying interstellar medium conditions that give rise to these extreme physical properties in the SMGs. To infer the properties of the interstellar medium we compare the surface density of star formation, ( Σ SFR), to the gas surface density ( Σ gas) i.e., the density of fuel for star formation. For the SMGs, we use the gas mass derived above, the star-  \nrmation rate from magphys and the F444W half-light radius as a conservative estimate of the physical extent of the gas content. For the field sample, we use the magphys derived dust masses with a median value of log 10 (Md[M ⊙ ]) = 7.7 ± 0.1 and log 10 (Md[M ⊙ ]) = 7.6 ± 0.1 for the massive field, which are consistent with the dust mass inferred from converting the S 870 µ m limit, derived from stacks in S2CLS UDS 870 µ m map (Geach et al. 2017), to a dust mass using the S 870 µ m to dust mass relation defined in Dudzeviˇci¯ut˙e et al. (2020). Adopting the metallicitydependent dust-to-gas ratio from Tacconi et al. (2018) for massive star-forming galaxies and following the mass metallicity relation defined in Genzel et al. (2015), we derive a median dustto-gas ratio of δ = 163 ± 32 which equates to a median gas mass of log 10 (Mg[M ⊙ ]) = 9.8 ± 0.1 for the field galaxies. Following the same procedure but for the massive field galaxies, we derive a median gas mass of log 10 (Mg[M ⊙ ]) = 9.9 ± 0.1 for the massive field sample.  \nCombining this gas mass estimate for the field samples with the magphys derived star-formation rates and F444W half-light radius, in Figure 13 we show the relation between Σ SFR and Σ gas for both the field populations and SMGs. Given the higher starformation rates and dust masses of the SMGs (Figure 3) and their comparable sizes in the F444W band to the field galaxies (Figure 6), the SMGs have considerably higher star-formation rate surface density and gas surface density. However, both the lower mass field galaxies and SMGs have comparable time scales for star formation, lying close to the 0.1 Gyr star formation time scale. The massive field sample has lower Σ SFR, due to their lower specific star formation rates (Figure 6).  \nBy estimating the Toomre Q parameter (Toomre 1964) we can place constraints on the interstellar medium properties of the galaxies, in particular on the stability of the gas disk. The Toomre Q is defined as,  \nQ = σκ π G Σ gas (6)  \nFig. 13: The relation between star-formation rate surface density and gas surface density for the field sample (grey points and contour) and massive field sample (black stars) and SMGs (red circles). We indicate a constant star-formation time scale of 0.1 Gyr with the black line. Adopting the typical rotation velocity and velocity dispersion from Birkin et al. (2024) derived for a sample of AS2UDS SMGs, we define the boundary between stable ( Q > 1) and unstable ( Q < 1) gas disks. The majority of the SMGs indicate unstable gas disks with a median Toomre Q parameter of Q = 0.47 ± 0.06 while most of the field samples are stable or marginally stable.  \n<!-- image -->  \nwhere σ is the radial velocity dispersion, κ is the epicylic frequency, for which we adopt κ = √ 2V / R appropriate for a galaxy with a flat rotation curve and Σ gas is the gas surface density. For the rotation velocity and velocity dispersion, we adopt the median values from Birkin et al. (2024), for a sample 31 SMGs in the AS2UDS survey, with comparable median redshift and stellar mass to our sample ( z ∼ 1.3 - 2.6, log 10 (M ∗ [M ⊙ ]) = 11.11 ± 0.06). Birkin et al. (2024) derive a median rotation velocity and velocity dispersion of V circ = 230 ± 20 kms -1 and σ = 87 ± 6 kms -1 .  \nAdopting the F444W half-light radius as an estimate of the extent of the gas disk, we can derive the gas-surface density required for a quasi-stable gas disk, identifying a threshold value of log 10 ( Σ gas) = 2.64 ± 0.05 M ⊙ pc -2 . This implies the gas disk is unstable to collapse ( Q < 1) for gas-surface densities higher than this value. We estimate that the majority of the SMGs lie in this region of 'instability' with a median Toomre Q parameter of Q = 0.47 ± 0.06, while the bulk of the field samples are stable ( Q > 1) or marginally stable ( Q ≃ 1).  \nTo test the sensitivity of these conclusions on our assumptions, we note that given the F444W half-light radius provides a lower limit on the extent of the gas disk. Thus to generate a quasi-stable interstellar medium in the SMGs, we require a larger rotation velocity or velocity dispersion. If the gas disk is required to be quasi-stable i.e Q ≥ 1, we derive a lower limit on the radial velocity dispersion needed. Adopting the rotation velocity from Birkin et al. (2024), we derive a median velocity dispersion for stability of σ stable ≥ 200 ± 30 kms -1 which is significantly larger than that derived for ionised gas in previous studies of high redshift SMGs or U / LIRGS (e.g. Hogan et al. 2021; Birkin et al. 2024; Amvrosiadis et al. 2024), suggesting that the gaseous disks of the SMGs in our sample are very likely unstable to gravitational collapse.", '5.3. Implications for the nature of SMGs': "The SMG and field samples employed in our analysis share several characteristics. By construction, they have similar number densities as a function of redshift with the larger field sample matched in specific star formation rate over z ≃ 1 - 5 and the massive field sample matched in stellar mass over z ≃ 1 3. Furthermore, we have demonstrated they have similar rates of mergers and signatures of disturbance, as well as comparable rest-frame near-infrared axis ratios and (mass normalised) stellar continuum (F444W) sizes. This suggests that the major structural components in both populations comprise (randomly orientated) disks that broadly follow the size-mass relation for disk galaxies at z ∼ 2 - 3. The similarity in the merger fractions for the SMGs and field is consistent with the theoretical investigation of SMGs in the EAGLE simulation by McAlpine et al. (2019), who concluded that there was no di ff erence in merger statistics of SMGs and the general population in the simulations, instead, most massive galaxies at these redshifts were undergoing minor or major mergers.  \nHowever, there are several distinctions between the SMGs and the less-active field samples. The dust-mass-selected SMGs have much higher A V and asymmetry, and they show stronger variations in their morphologies at bluer wavelengths, indicating the presence of structured and centrally concentrated dust. Moreover, both field samples have an average Sérsic index of n ≳ 2, indicating the potential presence of a bulge component in these systems. While the SMGs have consistently lower Sérsic index, with a median of n ∼ 1.1, indicating they are nearly pure exponential disks or that any bulge component if present is invisible at rest-frame ∼ 1 µ m. Either of these interpretations suggests that any bulge component in the SMGs is of low mass compared to the disk or is likely to be young and so only recently formed.  \nWealso infer more than an order of magnitude higher molecular gas surface densities for the SMGs than the field. These di ff erences suggest that one possible cause for a distinction between SMGs and less active galaxies arises from the less developed bulge components present in the SMGs (see McAlpine et al. 2019). Consequently, their gas disks are both more massive and also more sensitive to developing low-order disk instabilities, including m = 1 bars (e.g., Marinova & Jogee 2007; Pettitt & Wadsley 2018). These instabilities can either be triggered by external perturbations from major or minor mergers or tidal interactions, or from secular instabilities in the dense gas disks. These structures, as predicted by theoretical models (e.g., Fragkoudi et al. 2021; Bland-Hawthorn et al. 2023) and identified in recent studies of individual high-redshift SMGs (e.g., Hodge et al. 2019; Smail et al. 2023; Amvrosiadis et al. 2024) serve to funnel gas from the dense gas disk into the central regions of the galaxies, driving a central starburst that creates both the compact sub-millimetre emission we see and the corresponding highly obscured central regions of these systems.  \nTo assess the possible subsequent structured evolution of the SMGs we follow the prescription of Brennan et al. (2015), that relates B / T and Sérsic index. Adopting the F444W / 870 µ m = 2.7 ± 0.4 as the ratio of the disc to bulge size, the SMGs at z = 2.7 ± 0.2 with a n F444W = 1.1 ± 0.1 have a bulgeto-total ratio of B / T = 0.16 ± 0.12. If the SMGs converted their gas mass (which we assume to be equivalent to 50% of their stellar mass following Birkin et al. 2021) to a bulge component, it would result in a B / T = 0.49 ± 0.18 which following Brennan et al. (2015) gives a Sérsic index of n = 2.2 ± 0.8. This is consistent with to the Sérsic index measure for the gas-poor bulgestrong massive field sample, which suggests that the field galax-  \nve already experienced their 'SMG' phase at an earlier epoch and the SMGs may evolve to a gas-poor bulge-strong massive but less active population population within the next ≃ 0.1 Gyr.", '6. Conclusions': "Wepresent a multi-wavelength morphological analysis of a complete sample of ALMA-identified submillimetre-selected galaxies from the AS2UDS and AS2COSMOS surveys (Stach et al. 2019; Simpson et al. 2020) with > 4.5 σ ALMA 870 µ m detections and positions. Utilising public JWST / NIRCam and MIRI imaging from the PRIMER survey, we build a sample of 80 SMGs, determining their multi-wavelength properties through an SED and morphological analysis. We further define two matched samples of less active and more typical K -band selected star-forming field galaxies in the UDS field and analyse the differences between the populations. Our main conclusions are:  \n- -From visually inspecting the F277W / F356W / F444W colour images and F444W galfit residual maps of the SMGs we identify 20 ± 5% as candidate major mergers, a further 40 ± 10% as potential minor mergers and the remaining 40% are isolated undisturbed systems. We find no dependence of the visual classification on far-infrared luminosity, with similar statistics identified in the field samples, suggesting that the majority of SMGs are not ongoing major mergers, although mergers (major or minor) and disturbed appearances are common in both the SMGs and the less active field population at these redshifts.\n- -The SMG and field populations are more distinct in their detailed morphologies in terms of Sérsic index ( n ) and Asymmetry ( A ) in the F444W band, with the SMGs having lower Sérsic indices and higher Asymmetry ( n F444W = 1.1 ± 0.1, A F444W = 0.13 ± 0.02) compared to both the less massive field galaxies ( n F444W = 1.9 ± 0.1, A F444W = 0.08 ± 0.01) or the more and massive field galaxies ( n F444W = 2.8 ± 0.2, A F444W = 0.07 ± 0.02).\n- -A similar disparity between the SMGs and typical starforming galaxies is seen in the variation of their morphologies as a function of wavelength. The size of the SMGs declines more rapidly at longer wavelengths than both field populations ( δ ( R h / R 1 µ m) /δλ = -0.60 ± 0.09 versus δ ( R h / R 1 µ m) /δλ = -0.15 ± 0.07 for the low mass field sample or δ ( R h / R 1 µ m) /δλ = -0.04 ± 0.09 for the massive field sample) with the SMGs exhibiting lower Sérsic indices at all wavelengths.\n- -We further determine the brightest and reddest regions of the SMGsin the NIRCam imaging correspond to the highest surface brightness emission at 870 µ m seen by ALMA, indicating a strong connection between the colour and the local dust columns in the galaxies on ≃ kpc scales.\n- -To understand the physical origin of the di ff erence in morphologies between the SMGs and the field galaxies, we assume a representative gas-to-dust ratio to define the relation between star-formation rate surface density and gas surface density, suggesting that the SMGs have significantly higher gas surface densities than field galaxies. The SMGs on average fall in the unstable ( Q < 1) regime with a median Toomre Q parameter of Q = 0.47 ± 0.06, while the bulk of the field populations are stable or marginally stable ( Q ≳ 1).  \nOur analysis indicates that SMGs and typical star-forming galaxies appear morphologically distinct in the rest-frame optical due to the higher dust content of the SMGs, which preferentially influences the central regions of the galaxies. While at  \nlonger wavelengths, sampling the rest-frame near-infrared, the SMGs and less active field galaxies show more similar massnormalised sizes, although the SMGs exhibit lower Sérsic parameters, suggesting they have weaker bulge components. Both the SMGs and field control samples have comparable rates of mergers and undisturbed galaxies, suggesting that mergers are not a unique driver of the activity in SMGs (as indicated by previous theoretical studies, McAlpine et al. 2019). Instead, we suggest that the higher gas surface densities and weaker bulge components (which may be linked through previous evolution), coupled with the perturbations caused by either major or minor mergers, as well as secular processes, lead to correspondingly more intense star-formation activity in the SMGs, compared to the field. Thus the defining characteristic of SMGs maybe their massive and gas-rich nature, coupled with relatively underdeveloped bulge components and correspondingly low black hole masses (see McAlpine et al. 2019).  \nAcknowledgements. We would like to thank the PRIMER team for designing and executing the observations upon which this work is based. The observations analysed in this work are made with the NASA / ESA / CSA James Webb Space Telescope (DOI: 10.17909 / z7p08481). Certain aspects of the analysis in this paper were undertaken on SCUBA-2 data taken as part of Program ID MJLSC02. SG acknowledges financial support from the Villum Young Investigator grants 37440 and 13160 and the Cosmic Dawn Center (DAWN), funded by the Danish National Research Foundation (DNRF) under grant No. 140. IRS and AMS acknowledge STFC grant ST / X001075 / 1. BG acknowledges support from the Carlsberg Foundation Research Grant CF200644 'Physical pRoperties of the InterStellar Medium in Luminous Infrared Galaxies at High redshifT: PRISM- LIGHT'. TRG is grateful for support from the Carlsberg Foundation via grant No. CF200534. YM acknowledges support of JSPS KAKENHI Grant Numbers JP17KK0098, JP22H01273 and JP23K22544. C.-C.C. acknowledges support from the National Science and Technology Council of Taiwan (NSTC 111-2112M-001-045MY3), as well as Academia Sinica through the Career Development Award (AS-CDA-112-M02). 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Greve 0000-0002-2554-1837  \nOmar Almaini 0000-0001-9328-3991  \nMalte Brinch 0000-0002-0245-6365  \nChian-Chou Chen  \n0000-0002-3805-0789  \nWei-Hao Wang 0000-0003-2588-1265  \nYuichi Matsuda  \n0000-0003-1747-2891  \nFabian Walter h0000-0003-4793-7880  \nPaul P. van der Werf 0000-0001-5434-5942', 'Appendix A: SMG properties': 'Table A.1: Cumulative index for filter coverage in HST and JWST observations', 'Table A.2:': 'Table summarising the AS2UDS and AS2COSMOS SMGs identified in the PRIMER survey. The table is only available in electronic form at the CDS webpage 9 or via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5).', 'Appendix B: SEDs': 'A full version of Appendix B is available in electronic format on the ZENODO platform 10 .'}