# Hidden giants in JWST’s PEARLS:
An ultra-massive $z=$ 4.26 sub-millimeter galaxy that is invisible to HST

Ian Smail Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK  Ugnė Dudzevičiūtė Max-Planck-Institut für Astronomy, Königstuhl 17, 69117 Heidelberg, Germany  Mark Gurwell Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  Giovanni G. Fazio Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  S. P. Willner Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA  A. M. Swinbank Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK  Vinodiran Arumugam Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d’Hères, France  Jake Summers School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Seth H. Cohen School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Rolf A. Jansen School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Rogier A. Windhorst School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Ashish Meena Physics Department, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 8410501, Israel  Adi Zitrin Physics Department, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 8410501, Israel  William C. Keel Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35404, USA  Dan Coe Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Association of Universities for Research in Astronomy for the European Space Agency, STScI, Baltimore, MD 21218, USA  Center for Astrophysical Sciences, Department of Physics and Astronomy, The Johns Hopkins University, 3400 N. Charles St. Baltimore, MD 21218, USA  Christopher J. Conselice Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Oxford Road, Manchester M13 9PL, UK  Jordan C. J. D’Silva International Centre for Radio Astronomy Research and International Space Centre, University of Western Australia, M468, 35 Stirling Hwy, Crawley, WA 6009, Australia  ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions, Australia  Simon P. Driver International Centre for Radio Astronomy Research and International Space Centre, University of Western Australia, M468, 35 Stirling Hwy, Crawley, WA 6009, Australia  Brenda Frye Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ, 85721-0009, USA  Norman A. Grogin Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Anton M. Koekemoer Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Madeline A. Marshall National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada  ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions, Australia  Mario Nonino INAF-Osservatorio Astronomico di Trieste, Via Bazzoni 2, 34124 Trieste, Italy  Nor Pirzkal Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Aaron Robotham International Centre for Radio Astronomy Research and International Space Centre, University of Western Australia, M468, 35 Stirling Hwy, Crawley, WA 6009, Australia  Michael J. Rutkowski Minnesota State University-Mankato, Telescope Science Institute, TN141, Mankato MN 56001, USA  Russell E. Ryan, Jr. Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA  Scott Tompkins School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Christopher N. A. Willmer Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ, 85721-0009, USA  Haojing Yan Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA  Thomas J. Broadhurst University of the Basque Country UPV/EHU, Department of Theoretical Physics, Bilbao, E-48080, Spain  DIPC, Basque Country UPV/EHU, San Sebastian, E-48080, Spain  Ikerbasque, Basque Foundation for Science, Bilbao, E-48011, Spain  Cheng Cheng Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing, 100101, China  Jose M. Diego Instituto de Fisica de Cantabria, Avda. Los Castros, Universidad de Cantabria, 39005 Santander, Spain  Patrick Kamieneski School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA  Min Yun University of Massachusetts Department of Astronomy, 710 North Pleasant Street Amherst, MA 01003-9305, USA

###### Abstract

We present a multi-wavelength analysis using SMA, JCMT, NOEMA, JWST, HST, and
SST of two dusty strongly star-forming galaxies, 850.1 and 850.2, seen through
the massive cluster lens A 1489. These SMA-located sources both lie at $z$ $=$
4.26 and have bright dust continuum emission, but 850.2 is a UV-detected
Lyman-break galaxy, while 850.1 is undetected at $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 2 $\mu$m, even with deep JWST/NIRCam
observations. We investigate their stellar, ISM, and dynamical properties,
including a pixel-level SED analysis to derive sub-kpc-resolution stellar-mass
and $A_{V}$ maps. We find that 850.1 is one of the most massive and highly
obscured, $A_{V}$ $\sim$ 5, galaxies known at $z$ $>$ 4 with $M_{\ast}$ $\sim$
1011.8 M⊙ (likely forming at $z$ $>$ 6), and 850.2 is one of the least massive
and least obscured, $A_{V}$ $\sim$ 1, members of the $z$ $>$ 4 dusty star-
forming population. The diversity of these two dust-mass-selected galaxies
illustrates the incompleteness of galaxy surveys at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle>$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 3–4 based on imaging at
$\mathrel{\raise 1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 2 $\mu$m, the longest wavelengths
feasible from HST or the ground. The resolved mass map of 850.1 shows a
compact stellar mass distribution, $R^{\rm mass}_{\rm e}$ $\sim$ 1 kpc, but
its expected evolution to $z$ $\sim$ 1.5 and then $z$ $\sim$ 0 matches both
the properties of massive, quiescent galaxies at $z$ $\sim$ 1.5 and ultra-
massive early-type galaxies at $z$ $\sim$ 0\. We suggest that 850.1 is the
central galaxy of a group in which 850.2 is a satellite that will likely merge
in the near future. The stellar morphology of 850.1 shows arms and a linear
bar feature which we link to the active dynamical environment it resides
within.

cosmology: observations — galaxies: evolution — galaxies: formation — sub-
millimeter: galaxies

††software:  galfit (Peng et al. 2002); gildas
https://www.iram.fr/IRAMFR/GILDAS; IDL Astronomy Library:
https://idlastro.gsfc.nasa.gov (Landsman 1993); magphys (Battisti et al.
2019); profound (Robotham et al. 2018); smurf (Chapin et al. 2013);
sourcextractor https://www.astromatic.net/software/sextractor/ or
https://sextractor.readthedocs.io/en/latest/ (Bertin & Arnouts 1996); TinyTim
(Krist et al. 2011); topcat (Taylor 2005); WebbPSF (Perrin et al. 2014).
††facilities: SMA, JCMT, NOEMA, JWST, HST, SST, CADC archive, Hubble and James
Webb Space Telescope Mikulski Archive https://archive.stsci.edu

##  1 Introduction

Obscuration by dust has been a complicating factor in understanding the
properties of high-redshift galaxies for the last $\sim$ 30 years. The first
studies of $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle>$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 3 star-forming galaxy samples were
selected through their restframe ultraviolet (UV) emission (Steidel & Hamilton
1993). However, the subsequent identification in the sub-millimeter waveband
of highly dust-obscured galaxies at similar redshifts (e.g., Ivison et al.
1998) demonstrated a much wider diversity in the properties of galaxies at
high redshifts than the just the subset detectable in the restframe UV (Ivison
et al. 2000). The potential for entire classes of dusty, massive galaxies
(detectable in the sub-/millimeter or the mid-infrared with Spitzer/IRAC) to
be missed from UV, visible- and near-infrared-selected samples, variously
termed optically/near-infrared faint/dark (or less precisely “HST-dark”), has
been a concern for studies attempting to construct stellar-mass-limited galaxy
surveys at $z$ $>$ 1 (e.g., Dey et al. 1999; Caputi et al. 2012; Simpson et
al. 2014; Wang et al. 2019). With the launch of JWST, there has been a renewed
appreciation of the significant role of dust in defining the observed visible
to near-infrared properties of high-redshift galaxy populations (e.g.,
Barrufet et al. 2023; Bisigello et al. 2023; Kokorev et al. 2023; Magnelli et
al. 2023; Pérez-González et al. 2023).

The potential significance of missing optically-undetected populations was
underlined by the analysis of large samples of dust-obscured galaxies. These
populations are generally optically faint, but the majority of star formation
in the Universe at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 4–5 was estimated to occur in these
obscured systems (e.g., Dudzevičiūtė et al. 2020; Bouwens et al. 2020). It was
the rapid evolution in this dust-obscured activity that created the peak in
the star-formation rate density at $z$ $\sim$ 2 (e.g., Magnelli et al. 2013).
Discovering the physical processes driving this dust-obscured activity is thus
crucial for understanding galaxy evolution. The most massive and obscured
examples of this high redshift, dust-rich population are known as sub-
millimeter galaxies (SMGs). They are found from surveys at $\sim$ 1 mm that
select galaxies based on cool dust mass out to $z$ $\sim$ 6 (e.g., Simpson et
al. 2014; Brisbin et al. 2017). These galaxies are proposed to evolve into the
cores of massive galaxies at $z$ $\sim$ 0 (e.g., Eales et al. 1999;
Dudzevičiūtė et al. 2020; Amvrosiadis et al. 2023).

Current studies of the properties of dust-obscured galaxies are hampered by
both their complex dust obscuration and high redshift. In particular, their
restframe UV-to-visible morphologies (available from HST) may not provide true
indicators of their structures, necessary to determine the role of
interactions and mergers in driving their evolution (e.g., Gillman et al.
2023; Huang et al. 2023). JWST promises to revolutionize this field by
providing high-spatial-resolution imaging in the restframe near-infrared,
necessary to map the internal variations in the dust reddening and star
formation, as well as isolating any contributions from reddened AGN emission,
which have confounded previous attempts to estimate the stellar masses of some
of these systems (e.g., Hainline et al. 2011). The analysis of resolved JWST
restframe near-infrared imaging will thus hopefully deliver much more reliable
stellar masses (Sorba & Sawicki 2018). In particular, $\sim$ 0$\farcs$1 FWHM
NIRCam imaging will yield more robust stellar morphologies on kpc scales that
may indicate what drives the growth of the population of massive, gas- and
metal-rich galaxies responsible for the increasing obscuration of star
formation at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 4–5.

Figure 1: A three-color JWST NIRCam image of a 100${}^{\prime\prime}\times$
100′′ region (North top, East to the left) around the $z$ $=$ 0.35 cluster A
1489 (where F090W+F150W is shown as blue, F200W+F277W is green and F356W+F444W
is red). Signal-to-noise contours are shown, representing the SCUBA-2 850
$\mu$m emission (starting at 5$\sigma$, in 5$\sigma$ increments, 14$\farcs$5
FWHM beam), the SCUBA-2 450 $\mu$m emission (3$\sigma$ and 4$\sigma$,
7$\farcs$5 FWHM beam), and the NOEMA 3-mm continuum emission (10$\sigma$,
$\sim$ 5′′ FWHM beam). 850.1 and 850.2 are the two bright sources visible at
450 $\mu$m, 850 $\mu$m, and 3 mm to the north east of the cluster core, which
is identified by several bright elliptical galaxies, as well as associated
strong lensing features. The counterparts to these sources are unambiguously
identified using the higher resolution SMA 880 $\mu$m observations shown in
Figure 2.

To exploit the unique JWST imaging from the Prime Extragalactic Areas for
Reionization and Lensing Science (PEARLS) Guaranteed Time program (Windhorst
et al. 2023), we collated the sub-millimeter imaging from the SCUBA-2 camera
on JCMT of all the PEARLS fields visible from Mauna Kea, using the JCMT
archive at CADC. The analysis of a 30-minute exposure of the cluster A 1489
($z$ $=$ 0.35) revealed two previously unknown, bright 850-$\mu$m sources, SMM
J121223.0+273351 and SMM J121220.4+273410 (hereafter 850.1 and 850.2,
respectively) with $S_{850\rm\mu m}$ $\sim$ 60 and 20 mJy. This snapshot
observation was so shallow that it would not normally be expected to yield any
significant detections, so the presence of two bright sources was a surprise.
Initial analysis of these sources suggested that they were likely to be behind
the cluster and hence high-redshift, dusty star-forming galaxies
gravitationally magnified by the massive foreground cluster (Zitrin et al.
2020).

Gravitational lensing has been used as a tool to aid the study of dusty
galaxies at high redshifts since their discovery. The first example of a high-
redshift dust-obscured galaxy, FSC 10214 at $z$ $=$ 2.286 (Rowan-Robinson et
al. 1991), was uncovered by virtue of the gravitational magnification caused
by a serendipitously positioned foreground galaxy. Lensing by clusters and
individual galaxies has been exploited ever since to study high-redshift dusty
star-forming galaxies (e.g., Smail et al. 1997; Swinbank et al. 2010; Ciesla
et al. 2020; Fujimoto et al. 2023), as lensing both boosts the apparent fluxes
and magnifies sources across all wavelengths, making it easier to study these
otherwise very faint galaxies.

Follow up of the two A 1489 sub-millimeter sources with SMA, NOEMA, and JCMT
gave precise locations and redshifts for the two bright sources and
constrained their far-infrared emission. These observations showed that the
two sources were indeed very dusty, star-forming galaxies lying $\sim$ 100 kpc
apart in a structure at $z$ $\sim$ 4.26. One counterpart was undetected in the
HST and Spitzer 3.6/4.5-$\mu$m imaging of this field, while the other
corresponds to a UV-bright Lyman-break galaxy. They are thus examples of the
two populations that dominate the star-formation-rate density in the
transition from primarily unobscured at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle>$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 5 to predominantly obscured at $z$
$\mathrel{\raise 1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 4–5, with the benefit that the lens
magnification will help JWST resolve their internal structures.

The structure of this paper is as follows: §2 describes our new and archival
multi-wavelength observations, their reduction and analysis, including
spatially resolved spectral energy distribution (SED) fitting. §3 presents our
results and discusses these, and our conclusions are given in §4. We assume a
cosmology with $\Omega_{\rm M}$ $=$ 0.3, $\Omega_{\Lambda}$ $=$ 0.7, and
$H_{0}$ $=$ 70 km s-1 Mpc-1. In this cosmology the luminosity distance to $z$
$=$ 4.26 is 39.4 Gpc and 1′′ corresponds to 6.9 kpc before correcting for lens
magnification. All quoted magnitudes are on the AB system, and uncertainties
on median values were derived from bootstrap resampling. The quoted stellar
properties assume a Chabrier initial mass function (IMF).

##  2 Observations, Reduction and Analysis

###  2.1 JCMT

The initial 30-minute SCUBA-2 observation of A 1489 came from program M15AI29,
taken in very good weather, $\tau_{225{\rm GHz}}$ $=$ 0.03, on 2015 May 6. The
850-$\mu$m map showed two significantly detected point sources, one of which
was exceptionally bright. The presence of these unusual sources in this JWST
survey field motivated us to obtain higher resolution continuum observations
at 880 $\mu$m with SMA as well as deeper continuum observations at
450/850$\mu$m with SCUBA-2, and spectroscopic observations covering the 3-mm
band from NOEMA.

To improve the sub-millimeter maps, the cluster was observed for program
M23AP006 with SCUBA-2 simultaneously at 450 $\mu$m and 850 $\mu$m (at
resolutions of 7$\farcs$5 and 14$\farcs$5 FWHM, respectively) in very good
weather conditions $\tau_{225{\rm GHz}}$ $=$ 0.03–0.04 on the nights of 2023
February 15, March 21 and May 14. The total exposure time was 3.5 hours using
a standard constant-velocity daisy mapping pattern, and the 30 minutes of
exposure taken in similar conditions from M15AI29 was added to give a total
exposure time of 4 hours in each band.

Figure 2: Three-color images of 9${}^{\prime\prime}\times$ 9′′ regions around
a) 850.1 and b) 850.2 constructed from JWST NIRCam imaging, with F090W+F150W
as blue, F200W+F277W as green, and F356W+F444W as red. Panels c) and e) show
5${}^{\prime\prime}\times$ 5′′ regions using F277W, F356W and F444W as blue,
green, and red and the same filter combination is used in panels d) and f),
which show views of 850.1 and 850.2 “de-sheared” to correct for the lensing
magnification (the scale bar indicates 1 kpc). Panels a) and b) also show the
SMA 880-$\mu$m contours (starting from 2$\sigma$ in 7-$\sigma$ increments),
these unambiguously identifying the two sub-millimeter sources. 850.1
corresponds to an extremely red, highly structured source lying close to a
bright, foreground disk galaxy with several fainter sources in close
proximity. In contrast, 850.2 is more isolated and appears to be a slightly
bluer, disk-like system. The various sources and features discussed in the
text are identified on each panel: distinct sources have labels starting with
“A” , “B” for potential subcomponents, and “C” for likely emission-line
features. The colors and photometric redshifts suggest that “knot” B1 in 850.1
is likely to lie at $z$ $<$ 4.26, while B2 is probably a close/interacting
companion seen in projection to 850.1 or a less obscured component within the
galaxy. The photometric redshift analysis also suggests that A1, A4, and A5
are likely to be cluster members at $z$ $\sim$ 0.35, while A2 and A3 are
probably behind the cluster, but foreground to 850.1 and 850.2.

Individual maps for each 30 minute observation were reduced using the Dynamic
Interactive Map-Maker (dimm) tool of the Sub-Millimetre User Reduction
Facility (smurf; Chapin et al. 2013) with the blank field configuration in
order to detect point sources within the maps. The bright source 850.1 was
well-detected in each 30 minute exposure, so a shift-and-add method (Ivison
2006) was applied to the data, co-aligning the positions at 850 $\mu$m to the
median position from the M23AP006 observations. The same offsets were applied
to the 450 $\mu$m maps and this modestly improved the signal-to-noise for
sources in both bands.

The individual maps were calibrated using a flux conversion factor of FCF450μm
= 491 Jy beam-1 pW-1 and FCF850μm = 537 Jy beam-1 pW-1 (Dempsey et al. 2013)
and then combined using inverse-variance weighting to create a final map at
each wavelength. To improve point-source detection, the resulting 450 $\mu$m
and 850 $\mu$m maps were match-filtered with their respective beams:
7$\farcs$5 and 14$\farcs$5 FWHM Gaussians. The maps were cropped to radii of
4$\farcm$0, where the noise was low and more uniform, providing coverage of
all of the primary JWST field. The 1-$\sigma$ sensitivity in the region of
850.1/850.2 was 1.5 mJy at 850 $\mu$m and 21 mJy at 450 $\mu$m after matched
filtering. The resulting 450 $\mu$m and 850 $\mu$m maps are shown in Figure 1.

The new maps identified a third bright sub-millimeter source to the north west
of the cluster, SMM J121213.4+273517, with $S_{\rm 850\mu m}$ = 16.5 mJy, but
undetected at 450 $\mu$m (Table 1). Unfortunately this source is $\sim$ 2′
from 850.2 and lies outside the coverage of the existing HST, JWST, SMA, and
NOEMA observations, and there was no obvious counterpart in the Spitzer
imaging. This source is not discussed further, beyond noting that it is likely
to be intrinsically luminous as the lensing magnification from the cluster
will be modest ($\mathrel{\raise
1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 1.5 $\times$) and the 450/850-$\mu$m
flux ratio suggests this source may lie at $z$ $>$ 3, thus it could be another
member of the $z$ $=$ 4.26 structure.

Positions and flux densities for all three 850-$\mu$m sources were derived by
fitting Gaussian beams to the 850-$\mu$m and 450-$\mu$m maps. The resulting
fluxes are given in Table 1, these were deboosted at 450 $\mu$m following
Geach et al. (2017) to reflect the lower signal to noise.

Figure 3: The NOEMA 3-mm spectra for 850.1 (left) and 850.2 (right), showing
the CO(4–3), [Ci](1–0), and CO(5–4) emission lines. The panels are centered on
the expected frequencies of the lines at the adopted redshifts for each source
from Table 2 and a model fit comprising a Gaussian and a constant continuum
level is shown for each line. Both sources have relatively weak [Ci] emission
compared to their CO(4–3) and CO(5–4). A single Gaussian provided an adequate
description of the emission for the lines in 850.1, but all three lines in
850.2 display a dip at the systemic velocity, suggesting a double-peaked
profile, with the higher-frequency CO peak close to the redshift derived from
the restframe UV spectroscopic features. The spectra for 850.2 have been
rebinned to 40 MHz per channel, except for the [Ci] line which has been
rebinned to 100 MHz per channel (and as a result is shown over a slightly
wider frequency window to illustrate the off-line noise).

###  2.2 SMA

The Submillimeter Array (SMA) observed the JCMT-determined positions for 850.1
and 850.2 during a single transit between UT 6.1 and 16.3 on 2022 March 14. At
the time, the SMA was operating with all eight antennas in the EXT
configuration (maximum baselines of $\sim$ 220 m). The array receiving system
was tuned to an LO frequency of 346 GHz (giving an effective band center of
$\sim$ 880 $\mu$m), providing LSB coverage from 330–342 GHz and USB coverage
from 350–362 GHz in each of two orthogonally polarized receivers, resulting in
24 GHz of dual-polarization continuum bandwidth. The weather was good, with
the precipitable water vapor column ranging from 1.5 to 2 mm and stable, low
atmospheric phase variation (“seeing”).

Observations were made in a short repeated cycle, measuring the nearby phase
and amplitude gain calibrator J 1159+292 for 1.5 minutes, then the positions
of 850.1 and 850.2 for 4 minutes each. This resulted in high quality gain
calibration transfer to each target. In total, 850.1 was observed for 3.75
hours and 850.2 for 3.85 hours. Passband calibration was determined from
observations of J 0854+201, J 1159+292, and BL Lac, and the absolute flux
scale was derived from observations of Ceres and MWC 349A, taken immediately
before and after the targets.

For each source, the resolution of the synthesized beam was
0$\farcs$85$\times$ 0$\farcs$63 – around 10 $\times$ better than from SCUBA-2
– and for sources with lensing magnification of $\mu$ $\sim$ 4–6, the
circularised beam corresponds to 2.0–2.5 kpc FWHM in the source plane. The rms
of the maps was $\sigma_{\rm 880\mu m}$ $\sim$ 0.8 mJy beam-1 and both 850.1
and 850.2 were well detected in these sub-arcsecond resolution maps, precisely
locating the sub-millimeter sources. Table 1 reports their precise positions.

850.1 was detected with a peak flux density around 19 mJy. However, this
source was clearly resolved in both dimensions (Figure 2). Similarly, 850.2
had a peak flux density of $\sim$ 9 mJy, although it also appeared resolved in
the East$-$West direction. The vector-average amplitude versus $(u,v)$
distance plots for both sources showed increasing amplitudes on short
baselines. This demonstrated that both sources were resolved. Fitting the
visibility data and extrapolating these to zero spacing indicated total flux
densities of $S_{\rm 880\mu m}$ $=$ 42.3 $\pm$ 1.6 mJy for 850.1 and $S_{\rm
880\mu m}$ $=$ 22.5 $\pm$ 1.6 mJy for 850.2. Table 1 reports these values,
along with the other long-wavelength measurements. The estimated flux density
for 850.2 was in good agreement with that measured at 850-$\mu$m by SCUBA-2,
but the SMA observations were $\sim$ 20% lower for 850.1, a $\sim$ 6.5
$\sigma$ difference. Half of this may be accounted for the difference in the
effective band centers of the two instruments, but the remainder suggests that
there may be further extended continuum, or [Cii] emission (Smail et al.
2011), around 850.1 that was resolved out with the sub-arcsecond SMA beam.

Using the aips task uvfit, elliptical Gaussian models were fitted to the $uv$
data for each source. This derived beam-corrected FWHM of 1$\farcs$00 $\pm$
0$\farcs$14 $\times$ 0$\farcs$65 $\pm$ 0$\farcs$14 (PA of 23 $\pm$ 7 degrees)
for 850.1 and 1$\farcs$05 $\pm$ 0$\farcs$16 $\times$ 0$\farcs$43 $\pm$
0$\farcs$20 (PA of $-$22 $\pm$ 8 degrees) for 850.2. These correspond to
circularised equivalent FWHM sizes of 5.6 $\pm$ 1.4 kpc and 4.6 $\pm$ 2.4 kpc
(uncorrected for lensing).

At the redshifts of 850.1 and 850.2 determined from their NOEMA 3-mm spectra
(see next section), the [Cii] 1900.54 GHz line falls in the spectral coverage
of the SMA observations. The 1-$\sigma$ sensitivity of these data after
binning to $\sim$ 120 km s-1 (143 MHz) channels was $\sim$ 25 mJy. However,
this was insufficient to detect [Cii], assuming typical [Cii] to far-infrared
luminosity ratios for the sources. A search for the line in the sources did
not show any significant line emission associated with the continuum sources
at the relevant frequencies.

###  2.3 NOEMA

NOEMA observations were obtained as part of the S22CX program in several
partial tracks during 2022 July. An earlier Gemini-N GMOS program
(GN-2020A-Q-903, PI: Zitrin) had yielded an optical spectroscopic redshift
from template-fitting of $z$ $=$ 4.267 (Nonino, Zitrin priv. comm.) for the
SMA-located HST counterpart to 850.2. This allowed the LO tunings to be chosen
to cover the expected frequencies of the CO(4–3), CO(5–4), and [Ci] lines of
this source, in the expectation that 850.1 was likely to lie at high redshift
and potentially the same redshift as 850.2. Hence, two setups were used in
Band 1 (3 mm) with LO frequencies of 93.5 GHz and 101.244 GHz. Tthe wide
bandwidth of the PolyFiX correlator means that these two setups covered a
contiguous frequency range of $\sim$ 31 GHz from 81.88–112.86 GHz. All tracks
were observed in the compact D-configuration (maximum baseline length $\sim$
175 m) with 10 antennas in the array for five partial tracks, nine antennas
for three partial tracks, and eight antennas for one partial track.

Calibration was performed using the Grenoble Image and Line Data Analysis
Software (gildas) package. Either MWC 349 or LkHa 101 was observed for use as
the flux calibrator; in cases where one of these sources was either not
observed or the data were of poor quality, the gain calibrator 1156+295 was
used as the flux calibrator, bootstrapping its flux from other tracks with
reliable observations of MWC 349 or LkHa101. Bandpass calibration was obtained
from observations of 3C 273, 3C 279, and BL Lac. After flagging for bad
visibilities, the effective on-source time for a 10-antenna array for each of
850.1 and 850.2 was 1.6 hours from the lower frequency setup and 1.8 hours
from the higher frequency setup. Finally, dirty cubes were produced with
mapping (part of the gildas package) using natural weighting from $uv$-tables
resampled to 20 MHz. The synthesised beam sizes for the cubes ranged between
3$\farcs$8 to 6$\farcs$1 FWHM.

The reduction of the NOEMA observations showed 3-mm continuum detections of
both sources with positions that agree within the relative astrometric
uncertainty with those from SMA (Figure 1). Spectra extracted at the positions
of the two galaxies yielded three lines in each spectrum (Figure 3). The
frequencies of the brighter two lines in each source were 87.65 GHz and 109.56
GHz (Figure 3, Table 2) corresponding to CO(4–3) and CO(5–4), at 461.041 GHz
and 576.268 GHz in the restframe, respectively. The third, fainter line in
each source (at $\sim$ 93.6 GHz) was [Ci](1–0) at 492.161 GHz. These low-
resolution observations in NOEMA’s compact configuration showed no significant
evidence for extended emission in either the continuum or lines.

Gaussian fits to all three emission lines in each source yielded redshifts of
$z$ $=$ 4.2599 $\pm$ 0.0001 for 850.1 and $z$ $=$ 4.2593 $\pm$ 0.0003 for
850.2 (Figure 3). The latter was in reasonable agreement with the optical
measurement for 850.2. The Gaussian fits were then used to derive line centers
and widths, which were used to integrate the emission to derive the intensity
(after continuum subtraction) within a window of $\pm$ FWHM around the peak.
For the weak [Ci] line in 850.2 the FWHM of the CO lines was used to determine
the integration window. From these the line intensity, center and width
(expressed as an equivalent Gaussian FWHM), were measured along with their
uncertainties. The second moment of the line yields FWHM-equivalent estimates
that were $\sim$ 20% smaller than the Gaussian fitted values. 850.2 shows
evidence of a double-peak profile in both the CO(4–3) and CO(5–4) lines
(Figure 3), and so for that source the velocity difference between the two
peaks was instead used as the best estimate of the kinematics of the source.
The line and continuum properties of both sources are given in Tables 1, 2, &
3\. The CO line luminosities were used to estimate line luminosity ratios
between transitions from $J_{\rm up}$ $=$ $i$ to $J_{\rm up}$ $=$ $j$,
$r_{ij}$, following Solomon & Vanden Bout (2005).

Figure 4: 6${}^{\prime\prime}\times$ 6′′ thumbnails of 850.1 and 850.2. From
left to right the panels show the HST ACS F435W, F606W, and F814W imaging and
JWST NIRCam F090W, F150W, F200W, F277W, F356W, and F444W images. The striking
red colors of 850.1 are clear as the bulk of the emission from the galaxy,
apart from the knots B1 and B2 (the former of which we suggest is foreground),
is undetectable shortward of 2 $\mu$m (the F200W band). In contrast the bright
central regions of 850.2 are detectable down to F606W (corresponding to $\sim$
1100Å in the restframe at $z$ $=$ 4.26), but are undetected in F435W
(restframe $\sim$ 830Å) showing the presence of a strong Lyman break. Fitted
ellipses at three isophote levels are overlaid on the F356W image of 850.1,
illustrating the “bar”-like feature seen in the central regions of the galaxy.
At $z$ $=$ 4.26 the 912-Å Lyman break falls at the extreme red end of the
F435W filter transmission, while the Ly$\alpha$ emission line falls in F606W,
H$\alpha$ in F356W (strong emission from which likely explains the point-like
feature C1 in 850.2, which probably corresponds to a giant [Hii] region with
an intrinsic size of $\sim$ 50–100 pc), and the nebular emission lines [Oiii]
4959/5007 are in F277W and [Oii] 3727 lies in F200W. The various sources and
features discussed in the text are identified on the F090W thumbnails.

###  2.4 HST, JWST, & SST

The details of the observations, reduction, and analysis of the HST imaging of
A 1489 (PID: 15959) were reported in Zitrin et al. (2020). The observations
were obtained in 2020 March and comprised a total of five orbits, including
one orbit ($\sim$ 1.9 ks) in each of the F435W, F606W, and F814W filters with
ACS and shorter exposures with WFC3 in the F105W, F125W, F140W, and F160W
filters. The WFC3 data were not used in our analysis as they were shallower
than the equivalent JWST imaging and also do not cover both targets. As
described by Zitrin et al. (2020), mosaiced images were produced for all of
the HST filters using the calibrated exposures at a pixel scale of 0.03′′
pixel-1 (matching that used for NIRCam) on an astrometric grid aligned to Gaia
DR2. See Zitrin et al. (2020) for more details of the reduction and analysis.

JWST observed A 1489 as part of GTO program 1176 (PEARLS, Windhorst et al.
2023) targeting moderate-redshift clusters and blank fields to study galaxy
evolution. A 1489 was imaged in 2023 January with NIRCam filters F090W, F150W,
F200W, F277W, F356W, and F444W. Observations of F090W and F444W were taken
with the MEDIUM8 readout with six groups per integration and four integrations
for a total exposure time of 2.5 ks. F150W and F356W were observed with a
SHALLOW4 readout with nine groups per integration and four integrations for a
total exposure of 1.9 ks. While F200W and F277W were observed with a SHALLOW4
readout with ten groups per integration and four integrations for a total
exposure of 2.1 ks.

Images were initially processed using the STScI calwebb pipeline with the
jwst_0995.pmap context. Further processing was then applied to remove
artifacts in the images. These steps were described in detail by Windhorst et
al. (2023). In brief, “snowballs” (resulting from energetic cosmic ray hits)
were identified and masked, then “wisps” caused by straylight from the
secondary-mirror support were corrected using filter-specific templates, next
striping due to amplifier differences was corrected by matching the background
level in adjacent columns using the ProFound code (Robotham et al. 2018).
Finally, detector-level offsets in the short-wavelength camera were corrected
by constructing a “super-sky” with ProFound and scaling each detector to that
sky level.

Figure 5: a) The spectral energy distributions for 850.1 (including B2) and b)
850.2, showing the measured photometry (Tables 1 & 4) and the best-fit magphys
model at the redshifts determined from the NOEMA CO observations, $z$ $=$
4.26. The lower two panels, c) and d), show an expanded view of the restframe
UV–near-infrared region of the SEDs. Two SEDs are shown for each source, one
derived from summing the resolved pixel-level SED fits ($\Sigma_{\rm pix}$,
including B2) and one from fitting to the integrated source photometry. For
the latter in 850.1, the individual fits to B2 and 850.1 were summed, although
the emission longward of $\sim$ 3 $\mu$m is dominated by 850.1. Wavebands used
in the resolved pixel fits are shown as solid points. For the source-
integrated fits these bands were used as well as the unresolved photometry in
the wavebands marked with open symbols. Also shown for comparison are the
composite SED from the magphys analysis of $z$ $\geq$ 3 SMGs from the AS2UDS
survey by Dudzevičiūtė et al. (2020) and the SED of Arp 220, both normalised
to the SMA 880 $\mu$m flux density. These illustrate that the combined SED of
850.1+B2 is similar to the AS2UDS composite between restframe $B$-band
(observed $\sim$ 2$\mu$m) and near-infrared, but due to the contribution from
B2 it becomes much bluer into the restframe UV, better matching Arp 220. 850.1
alone is redder than the AS2UDS composite shortward of 2 $\mu$m. 850.2 has an
SED comparable to Arp 220 longward of $\sim$ 3 $\mu$m (restframe $\sim$ 6000Å)
but bluer shortward of this and considerably bluer than the AS2UDS composite.
In c) the results of fitting to the photometry and limits for just 850.1 are
shown (gray open squares), as well as a single magphys fit to the combined
(“unresolved”) photometry from 850.1 and B2, illustrating that this provides a
poorer description of the observations, especially longward of 3 $\mu$m. In
all panels, the resolved-pixel modelling, $\Sigma_{\rm pix}$, had only a
single constraint beyond 4.4 $\mu$m from the SMA at 880 $\mu$m, yet it does at
least as well out to 8 $\mu$m as the fits to the integrated source photometry
(that included the IRAC 5.8 and 8.0 $\mu$m fluxes in the fit), although the
absence of spatially resolved constraints at 450 $\mu$m resulted in poorer
agreement in the restframe far-infrared region.

Figure 1 shows a JWST/HST view of the wider environment of 850.1/850.2. More
detailed zooms are shown in Figure 2, including the SMA 880$\mu$m maps that
precisely locate the counterparts to the sub-millimeter sources. Figure 4
illustrates the variation in the brightness of the two counterparts with
wavelength across the nine JWST/HST bands. Sources were identified in a
sequence starting with “A” for distinct sources, “B” for potential
subcomponents, and “C” for likely emission-line features. These are labelled
on the F090W images.

To extend the wavelength coverage of these dusty, high-redshift galaxies, the
pipeline reductions of the Spitzer IRAC and MIPS 24 $\mu$m images of A 1489
were retrieved from the SST archive. This was undertaken prior to both the
launch of JWST and the SMA observations, finding potential 8-$\mu$m bright
counterparts within the SCUBA-2 error circles for both 850.1 and 850.2, that
turned out to be the correct identifications. These data were included in our
SED analysis to extend the wavelength coverage in the gap between JWST and
SCUBA-2. Neither source was detected at 24 $\mu$m with MIPS. This was
unsurprising given their high redshifts and the modest depth of the data, and
so a 3-$\sigma$ limit was adopted at that wavelength (Table 1). The emission
in the IRAC bands was blended with the nearby galaxies for both 850.1 and
850.2, and therefore the flux estimates were based on small
2$\farcs$4–3$\farcs$6 diameter apertures. However, the sub-millimeter galaxies
were increasingly dominant at the longer wavelengths and 850.2 was effectively
uncontaminated by 8 $\mu$m. For both sources, excluding the IRAC and MIPS data
points, or adopting limits instead, did not alter the solutions for the best-
fit SEDs obtained below.

To measure integrated photometry for 850.1/850.2 and the other sources in
close proximity to them in the JWST and HST data, large circular apertures
were used (except for 850.1) with sizes listed in Table 4. These measurements
used the photometric zero points from Windhorst et al. (2023) for NIRCam and
from the ACS pipeline (as given in the FITS file headers) for the ACS imaging.
These provided close-to-total fluxes for most of the sources. PSF matching
employed model PSFs created with WebbPSF v1.1.0 or TinyTim for HST/ACS,
utilizing the odt files closest to the observation date, and convolved to
match the F444W PSF. Due to the large aperture sizes, PSF corrections for
850.1/850.2 were modest in all cases. Sky corrections were derived from large
annuli around the targets. An additional correction was applied for the
contribution from the disk galaxy, A4, to those sources around 850.1. This was
modelled in each filter assuming rotational symmetry and rotating the images
by 180 degrees around the center of A4, then smoothing slightly, before
subtracting this image from the original. The resulting photometry is listed
in Table 4 with 3-$\sigma$ limits for non-detections.

To determine the sizes and profiles of the two sources, the galfit code was
applied to the different JWST wavebands assuming single Sérsic profiles and
correcting for the varying resolution between the bands using an empirical PSF
taken from stars in the images. This process was challenging in the crowded
environment of 850.1, especially given the fine structures visible in the
surrounding galaxies as a result of the superlative resolution and depth of
the data. However, aided by the fact that the bulk of 850.1 was undetected in
the F090W image, a model was constructed in this band of the surrounding
galaxies and used as the basis for constrained fitting to the redder
passbands, where a further component was included to represent 850.1. The
structural parameters of both sources from this analysis are reported in
Tables 2 & 5.

###  2.5 Integrated SED modelling

The SED modelling of the two sub-millimeter sources was iterated with the aim
of understanding the nature of the various subcomponents in these systems and
so derive robust physical properties for the two galaxies. The analysis
employed both the photometric-redshift version of the magphys code (Battisti
et al. 2019) as well as the high-redshift version (da Cunha et al. 2015) to
derive parameters at the known CO redshifts, both using the standard priors
(including a star-formation history that rises at early times, before
declining exponentially, with superimposed random bursts). The selected codes
were applied to the integrated photometry of the sources (including
potentially unrelated subcomponents), to the integrated photometry of the
individual subcomponents, and also to the resolved “pixel-level” photometry
(described in the next subsection). In principle, the latter resolved SED
fitting should be most robust, but the modest resolution of some of the
longer-wavelength observations (Spitzer IRAC, SCUBA-2, and NOEMA) means that
there were fewer constraints available on the form of the SEDs. For this
reason the resolved SED fitting is presented as a proof-of-concept, and for
most of the quantitative analysis, the fitting to the integrated photometry
was used to derive the physical properties.

The photometric analysis of 850.2 was straightforward owing to the relative
isolation of the galaxy from other sources in the field, as well as the
relatively good agreement between the redshifts derived from the CO/[Ci]
emission lines and the archival restframe UV spectroscopy. The latter
confirmed that the emission seen in both sub-millimeter and the restframe
UV–optical all arises from the same physical system, although not necessarily
precisely the same components within this system.

In contrast, 850.1 has a complex environment and the analysis of this system
required iteration. Based on the source morphology and its variation with
wavelength shown in Figure 4, as well as the results from the resolved SED
modelling discussed in the next sub-section, the emission around 850.1 was
divided into three components: two blue “knots” B1 and B2, and the bulk of the
remainder of the source, 850.1, which shows much redder colors and corresponds
to the peak of the SMA map (Figure 2). The analysis started by estimating
likely redshifts for these three components as well as the other sources seen
close to 850.1/850.2 that lacked spectroscopic redshifts (those marked in
Figure 2 and listed in Table 4 & 5), with the photometric redshift version of
magphys (Battisti et al. 2019; da Cunha et al. 2015). These photometric
redshifts are reported in Table 5. These were then used to assess the level of
agreement of the spectroscopic redshift measurements for 850.1 and 850.2 with
their photometric properties.

For 850.2, the photometric redshift, $z_{\rm ph}$ $=$
4.28${}^{+0.08}_{-0.10}$, driven by the strong Lyman break in the broadband
SED (Figures 4 & 5) was in excellent agreement with the spectroscopic redshift
$z$ $=$ 4.26 derived from the multiple lines seen in the NOEMA spectrum
(Figure 3), and also the Gemini-N/GMOS spectrum (Nonino, Zitrin, priv. comm.).
The latter spectrum showed a pronounced decline in flux blueward of $\sim$
1215Å, no Lyman-$\alpha$ emission but a series of strong absorption features
including Siii $\lambda$1260, Oi+Siii $\lambda$1303 (blend), Cii
$\lambda$1334, Siii $\lambda$1526 and Civ $\lambda$1548, Feii $\lambda$1608
and Alii $\lambda$1670\. The Siii $\lambda$1265 and Civ $\lambda$1548 lines
displayed P-Cygni-like profiles. These low-ionisation interstellar absorption
features are commonly seen in Lyman-break galaxies (Shapley et al. 2003). In
850.2, their measured line centers displayed a median blueshift of $\sim-$190
km s-1 relative to the CO-derived systemic redshift. This velocity offset may
be a signature of a wind, or it indicates that the UV-bright element of the
system corresponds to the higher frequency peak in the CO spectrum of the
source (Figure 3), with the redshifted CO component then potentially arising
in the redder part of the system.

A similar photometric analysis for 850.1, used a photometric aperture that
includes the emission from the two knots B1 and B2 (Figures 2 & 4, Table 4).
This resulted in a photometric redshift that was significantly lower than the
CO redshift: $z_{\rm ph}$ $=$ 2.63${}^{+0.46}_{-0.23}$ (Table 5). Figure 4
shows clearly that B1 and B2 are considerably bluer than the bulk of 850.1,
both dominating the emission shortward of $\sim$ 2 $\mu$m, and B1 is visible
all the way down to F435W, where the wavelength coverage of the filter falls
almost completely blueward of 912Å in the restframe at $z$ $=$ 4.26.
Photometric redshifts measured for the three components individually yielded
estimates of $z_{\rm ph}$ $=$ 1.12${}^{+0.11}_{-0.12}$ and $z_{\rm ph}$ $=$
4.17${}^{+0.16}_{-3.28}$ for B1 and B2, respectively, while the very red 850.1
gave $z_{\rm ph}$ $=$ 4.41${}^{+0.77}_{-1.95}$ in reasonable agreement with
the CO redshift (Table 5).

The photometric redshift estimate for B1 indicated that the source was
unlikely to be associated with either 850.1 or the foreground cluster. Thus,
while the alignment of B1 with features in 850.1 is very suggestive that they
are related, B1’s detection in the F435W filter is hard to reconcile with it
lying at $z$ $=$ 4.26, and it is more likely to be a superimposed lower-
redshift source. The situation is more complicated for B2, where the estimated
photometric redshift suggests it could be associated with 850.1 at $z$ $=$
4.26, but the uncertainty allows much lower redshift solutions as well. We
therefore retain B2 as a potential UV-bright component within 850.1 or a
close/interacting companion and discuss how this influences the inferred
properties of the source.

Having confirmed that the measured photometry for 850.1, B2, and 850.2 were
all consistent with the CO-derived redshifts, more representative estimates of
their physical properties were derived by running the high-redshift version of
magphys (da Cunha et al. 2015) with the redshifts fixed at $z$ $=$ 4.26 for
all three components. The main parameters from these fits are reported in
Table 5 and the best-fit SEDs plotted in Figure 5. For 850.1 and B2, the two
best-fit SEDs were summed and labelled “850.1+B2 fits”, and this is plotted
compared to the integrated photometry of the system, although 850.1 dominates
the emission longward of $\sim$ 3 $\mu$m. In addition, Figure 5c show the
results of fitting solely to 850.1, illustrating the very red colors of this
source.

The fit to the combined photometry of 850.1 and B2 using a single magphys
model and labelled “850.1+B2 unresolved” is also shown in Figure 5c. This was
much less successful at reproducing the optical/near-infrared SED of the
combined source. This resulted in a statistically much poorer fit to the
observed photometry that predicted a much lower mass, star-formation rate,
$A_{V}$ and age (see Table 5) but a comparably high dust mass to the preferred
sum of the individual fits to 850.1 and B2. These parameters would imply a
galaxy more similar to 850.2 in its physical properties, but we believe that
this poor fit was erroneous due to the distinct low and high extinction
regions that were being modelled with a single dust extinction term. Similar
model biases may also be responsible for some apparently low-mass sub-
millimeter galaxies identified from SED fitting of systems with a wide range
in $A_{V}$ (e.g., Pope et al. 2023). This erroneous fit also produces an
apparent power-law-like excess in the observed fluxes longward of 3.6 $\mu$m
compared to the model. This is reminiscent of similar features seen in the
SEDs of $\sim$ 10% of sub-millimeter sources analysed by Hainline et al.
(2011), who attributed it to hot dust emission from dust-obscured AGN. If the
origin of this excess is similar in those sources to 850.1, then an obscured
AGN may not be the cause as there is no such bright compact, red point-source
seen in 850.1. Instead the red mid-infrared excess may be a result of fitting
a single SED model to a source with a mixture of internal dust obscuration,
including both weakly and highly reddened regions.

Figure 6: 6${}^{\prime\prime}\times$ 6′′ thumbnails of 850.1 and 850.2 showing
the JWST F200W and F444W images along with the pixel-based magphys estimates
of the log-scaled stellar mass and $A_{V}$. The stellar-mass surface density
is per 0$\farcs$18$\times$ 0$\farcs$18 pixel, corresponding to $\sim$ 0.6
$\times$ 0.6 kpc in the source plane for 850.1 and $\sim$ 0.5 $\times$ 0.5 kpc
for 850.2. B1 was excluded from the map for 850.1 due to the poor quality of
the SED fits at $z$ $=$ 4.26, supporting its likely lower redshift. B2 has
much lower $A_{V}$ and stellar-mass surface density compared to the bulk of
the massive and dusty remainder of 850.1. These very distinct properties
motivate the decision to fit B2 and the bulk of 850.1 separately and then sum
the results. In contrast, 850.2 exhibits a smaller variation in $A_{V}$ and
stellar-mass surface density across the source.

###  2.6 Resolved SED modelling

The high resolution and depth of the JWST/HST imaging of these sources can be
used to go beyond the analysis just described to undertake resolved SED
modelling of the sources to investigate their internal structures.
Specifically to map the distribution of stellar mass and dust reddening within
the two systems (see also, Duncan et al. 2023). To perform this analysis
9${}^{\prime\prime}\times$ 9′′ thumbnails were extracted from the JWST and HST
imaging centered on 850.1 and 850.2. The JWST thumbnails were weighted and
coadded, the combined image smoothed with a Gaussian filter with a FWHM of
0$\farcs$07, and then SExtractor Bertin & Arnouts (1996) used to construct a
segmentation map. The segmentation maps was employed to identify regions of
blank sky that were used to estimate the pixel-to-pixel noise and also to
define areas associated with the various sources. To aid this for 850.1 the
large-scale emission from the bright galaxy, A4, was first removed by rotating
the images by 180 degrees centered on A4 and subtracting these from the
original frames.

To ensure that the final spatially resolved maps were not oversampled, the
JWST and HST images were PSF-matched to the resolution of the F444W
observations, $\sim$ 0$\farcs$15\. These images (and the associated
segmentation masks) were then rebinned from the default 0.03′′ pixel-1
sampling to 0.18′′ pixel-1, to provide nearly independent pixels. From this
binned data, photometry was extracted in all the JWST/HST bands for each
0$\farcs$18 pixel, including the uncertainties measured from the pixel-to-
pixel variations in the background regions.

To include information about the longer-wavelength emission, the SMA
880-$\mu$m map was used to provide a constraint on the long wavelength pixel-
to-pixel photometry. For each source a model was constructed from the
deconvolved 880-$\mu$m source properties measured by SMA and sampled at
0$\farcs$18\. For the 880-$\mu$m photometry, liberal uncertainties of 2 mJy
per pixel were adopted to avoid over-constraining the magphys fits and also to
reflect the fact that the resolution of the SMA map was coarser than the
JWST/HST imaging.

For the magphys analysis of the pixel-based photometry, at least three
detections were required at $>$3 $\sigma$ in the visible or near-infrared
wavebands to include the fitted parameters in the analysis. Fits to pixels
with too few photometric constraints resulted in poorly constrained
properties. However, the results did not change significantly if 2-$\sigma$
limits in three bands or 3-$\sigma$ in two bands were used instead, although
the uncertainties became more significant. To derive the physical properties
of each source the high-redshift version of magphys was applied to the
photometry for each pixel within the segmentation mask that defines each of
the sources. A fixed redshift of $z$ $=$ 4.26 was used, but all other
parameters were allowed to vary according to the standard priors used in the
previous section, and also adopted in previous studies (e.g., Dudzevičiūtė et
al. 2020). For the analysis of 850.1, the pixels that were associated with B1
were included, but then removed as this appears to be an unrelated foreground
source. This conclusion was also supported by the $\chi^{2}$ of the magphys
fits for B1 at $z$ $=$ 4.26 being significantly worse than those in the
remainder of the source ($\gg$ 20, compared to $\sim$ 1).

Two further requirements were applied in the resolved analysis for a fit to a
pixel to be included in the final maps. Firstly, the magphys fit had to
provide a reasonable description of the photometric observations, yielding a
$\chi^{2}<$ 10\. This removed seven pixels in 850.2 and none in 850.1.
Secondly, the predicted 850-$\mu$m flux density from the best-fit magphys SED
for the pixel must not exceed 10 mJy. This caught situations where the
predicted restframe far-infrared SED for the pixel, which was only weakly
constrained by the SMA observations, was strongly at odds with the expected
flux distribution for the source. This affected only four pixels from 116 in
850.1 and twelve from 415 in 850.2. Typically these were in the lowest surface
brightness regions on the periphery of the sources and these pixels were
replaced by the local median fit. With these cuts applied, the total sub-
millimeter flux densities for the sources from summing the predicted magphys
850-$\mu$m fluxes were 52 mJy for 850.1, compared to 42.3$\pm$1.6 mJy and
54.1$\pm$1.5 mJy observed by SMA and SCUBA-2 respectively, and 14 mJy for
850.2, compared to 22.5$\pm$1.6 mJy and 21.2$\pm$1.5 mJy for SMA and SCUBA-2
respectively. Figure 5 shows the predicted full SEDs for both sources,
constructed by summing the individual best-fit pixel SEDs. These provide very
good fits to the visible to near-infrared (including the longer wavelength
Spitzer bands that were not included in the resolved analysis), and while the
agreement at longer wavelengths in the restframe far-infrared to sub-
millimeter was poorer, given the limited information available at these
wavelengths for a spatially resolved analysis at JWST resolution, we view the
agreement as reasonably successful.

This resolved analysis allowed the internal variation in some of the key
physical properties of the sources to be mapped on $\sim$ 0$\farcs$18 scales,
equivalent to 0.5–0.6 kpc in the source plane after accounting for lensing
magnification. The discussion below focuses on the distribution of stellar
mass and dust reddening, $A_{V}$, as these two quantities are most closely
tied to the photometric properties in the JWST and HST wavebands, where the
observations have the highest resolution and signal-to-noise. The absolute
normalisation of these maps will depend on the SED modelling code used (e.g.,
Carnall et al. 2019; Pacifici et al. 2023), but for comparison with the
earlier results and published samples we use magphys. The resolved 2-D maps of
the estimated stellar-mass density and dust reddening for 850.1 and 850.2 are
shown in Figure 6.

##  3 Results and Discussion

Figure 2 illustrates the JWST/HST view of the surroundings of the two sources
as well as their morphologies after correcting for the image shear due to
lensing, while the variation in their brightness across the nine JWST and HST
bands is shown in Figure 4. Strikingly, 850.1 is undetected in all the HST ACS
bands and the bluer JWST bands, the bulk of the galaxy only being
significantly detected at 2 $\mu$m or above, while 850.2 displays emission
down to F606W but then a strong drop between F606W and F435W (see Figures 4 &
5).

There are a number of sources detected with JWST/HST lying close to 850.1 and
850.2, with photometric redshifts reported in Table 5. These redshifts
indicated that two of the galaxies near 850.1, including the bright disk
galaxy A4, are probably members of the cluster A 1489 at $z$ $\sim$ 0.35 and
that similarly, the disk galaxy A5, near 850.2, is also likely to be a cluster
member. This is unsurprising given that the two sub-millimeter sources are
seen through the dense central regions of the foreground cluster. This
analysis also suggested that none of the other galaxies visible in close
proximity to 850.1 and 850.2 is likely to be physically associated with the
sub-millimeter sources.

###  3.1 Gravitational lens model

A first strong-lens model for A 1489 was presented by Zitrin et al. (2020)
based on multiple images identified in their HST observations. The analysis
here uses a new lens model based on the same set of multiple images but
employing a revised and updated version of the Zitrin et al. (2015) parametric
code (Pascale et al. 2022; Furtak et al. 2022). In this model, cluster
galaxies were modeled as double pseudo-isothermal ellipticals (dPIE) and dark
matter haloes were modelled using pseudo-isothermal elliptical mass
distributions (PIEMDs). Two such halos were used for A 1489, centered on the
two main mass clumps identified by Zitrin et al. (2020) and mass components
were included to represent the cluster galaxies near to 850.1 and 850.2,
including A4 and A5.

The updated lens model predicts similar lensing magnification for 850.1 and
850.2 (Table 5) of $\mu$ $=$ 4.0${}_{-2.2}^{+1.0}$ and $\mu$ $=$
5.6${}_{-3.3}^{+1.0}$, respectively, where the uncertainties were judged from
the range of acceptable models and are expected to be conservative. These
values were employed when comparing the properties of these sources to other
populations. Table 6 lists intrinsic, lensing-corrected, properties for both
sources. The NIRCam morphologies of the sources after correcting for the
lensing magnification are shown in Figure 2d and 2f.

Accounting for lensing magnification, 850.1 has an intrinsic 850-$\mu$m flux
density of $S_{\rm 850\mu m}$ $=$ 13.5 $\pm$ 0.4 mJy, while 850.2 has $S_{\rm
850\mu m}$ $=$ 3.8 $\pm$ 0.3 mJy. This shows that 850.1 is an example of a
rare population of very luminous sub-millimeter sources with a surface density
of $\sim$ 10 $\pm$ 5 degree-2 (Geach et al. 2017; Simpson et al. 2019; Garratt
et al. 2023), while 850.2 comes from a much more numerous population around
the knee in the 850-$\mu$m counts (Stach et al. 2018) with a surface density
$\sim$ 50 $\times$ higher ($\sim$ 500 degree-2).

Corrected for lens magnification, $\Delta$ $\sim$ 1.5 mag, 850.1 would have
$H$ $\sim$ 29 and $K$ $\sim$ 27 for an extended source (not a point source as
frequently adopted to define survey limits). This demonstrates that neither
previous HST nor ground-based $K$-band surveys would have been able to detect
this class of source, let alone if there are less-massive or higher-redshift
examples.

After correcting their sky positions for lensing, the relative separation of
850.1 and 850.2 was estimated to be $\sim$ 100 kpc ($\sim$ 15′′) in the source
plane. Thus, without the lensing magnification of the foreground cluster,
these two galaxies would appear as a single blended bright ($S_{\rm 850\mu
m}\sim$ 17 mJy) source in a single-dish sub-millimeter survey, similar to the
associated multiple $z$ $\sim$ 4.62 components found in two SCUBA-2 sources by
Mitsuhashi et al. (2021). This small spatial separation, along with the line-
of-sight velocity difference between 850.1 and 850.2 of only $\Delta v\sim$ 30
$\pm$ 20 km s-1, indicates that these two galaxies are part of a small group
at $z$ $\sim$ 4.26 lying behind the core of the foreground cluster. Indeed,
given the stellar mass for 850.1 estimated below, unless 850.2 has a very high
transverse velocity, then it is very likely on a bound orbit and will merge at
some point in the future.

This group behind A 1489 adds to a growing list of similar systems comprising
one or more sub-millimeter galaxies and other companion galaxies in groups at
$z$ $\sim$ 2–4, lensed by massive foreground clusters (e.g., Ledlow et al.
2002; Borys et al. 2004; Kneib et al. 2005; Caputi et al. 2021; Frye et al.
2023). In view of the large volumes accessible in surveys for sub-millimeter
sources due to the negative $K$-correction, the frequency of identification of
such systems may reflect the similarity between the spatial size of group-
sized halos with mass comparable to those of sub-millimeter galaxies ($\sim$
1013 M⊙, e.g., Stach et al. 2021) and the source-plane extent of the high-
magnification region of the most massive clusters at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 1 (see Frye et al. 2023).

###  3.2 SEDs and stellar properties

The spectral energy distributions of the combined 850.1+B2 and 850.2 shown in
Figure 5 are strikingly different. Shortward of 2 $\mu$m (restframe $U$-band)
the only emission from 850.1+B2 comes from the component/companion B2, with
850.1 itself undetected (see Figure 5c). In contrast, both the disk and the
high surface brightness feature in 850.2 display a blue continuum that is
detected down to the restframe far-UV (Figure 4).

The magphys analysis provided a lensing-corrected stellar mass from the sum of
the fits to 850.1 and B2 $M_{\ast}$ $=$ 5.5${}^{+1.3}_{-0.8}\times$ 1011 M⊙
and a star-formation rate (SFR) of SFR $=$ 900${}^{+400}_{-300}$ M⊙ yr-1
(Tables 5 & 6). These estimates are in reasonable agreement with the sum of
the fits to the individual resolved pixels $M_{\ast}$ $=$
4.0${}^{+1.0}_{-0.3}\times$ 1011 M⊙ and SFR $=$ 1200${}^{+300}_{-100}$ M⊙
yr-1, even though the latter fit lacked several of the mid-infrared and
sub-/millimeter photometric constraints used in the former. If B2 is taken to
be a separate galaxy from 850.1, the estimates for the system do not change
appreciably as the latter contains the majority of the mass and SFR. For 850.2
the estimates were: $M_{\ast}$ $=$ 0.18 $\pm$0.07 $\times$ 1011 M⊙ and a SFR
$=$ 210 $\pm$ 20 M⊙ yr-1, with the resolved pixel-fits again in reasonable
agreement: $M_{\ast}$ $=$ 0.40${}^{+0.4}_{-0.1}\times$ 1011 M⊙ and SFR $=$
200${}^{+160}_{-70}$ M⊙ yr-1. AGN contamination is not biasing the high
stellar mass derived for 850.1 (or indeed 850.2) as the F444W morphology
indicates only a small fraction of the light in the restframe near-infrared
could be from a central point source (see Figures 2d and 2f). Similarly,
removing the IRAC 5.8- and 8.0-$\mu$m photometry from the magphys analysis did
not change the best-fitting SEDs.

After correcting for lensing magnification, 850.1 is fainter than 850.2 at all
wavelengths shortward of 8 $\mu$m (restframe $H$-band), yet the estimated
stellar mass for 850.1 is $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle>$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 30 $\times$ higher, while its star-
formation rate and dust mass are only $\sim$ 3–4 $\times$ higher than those
derived for 850.2. The primary driver of this is the differences in ages for
the best-fit SEDs. 850.1 has a best-fit mass-weighted SED with an age of
$\sim$ 450 Myr, corresponding to a mass-to-light ratio for the stellar
population in the restframe $H$-band (observed $\sim$ 8.5 $\mu$m) $M/L_{H}$
$\sim$ 4.5. In contrast, 850.2 has a much younger best-fit SED with an age of
$\sim$ 50 Myrs and $M/L_{H}$ $\sim$ 0.3. This difference in age accounts for a
factor of $\sim$ 15 in the predicted masses, with the remainder due to the
difference in extinction between the two galaxies, $A_{V}$ $\sim$ 5 mag and
$\sim$ 1 mag, equivalent to $\Delta A_{H}$ $\sim$ 1\. The time needed to form
the stellar masses of 850.1 and 850.2, assuming the current SFRs (averaged
over a 100 Myr timescale), was $\sim$ 600 Myrs and $\sim$ 90 Myrs,
respectively. These are comparable to the magphys mass-weighted ages for both
galaxies, indicating that it is possible that the bulk of the stellar mass in
both systems was formed in the on-going star-formation events, with 850.1
potentially forming at $z_{\rm form}$ $\sim$ 6.

###  3.3 Morphologies and structures

The extended emission of the two galaxies is well detected in the redder JWST
bands and shows that 850.1 is morphologically more complex than 850.2 (Figure
2). The F444W morphology of 850.1 in Figure 4 suggests a nearly face-on system
with two arm-like features extending north and south from the ends of a linear
structure of clumps. This bar-like feature has a source plane radius of $\sim$
2–2.5 kpc, and spans a slightly brighter clump at the center of the galaxy.
The 880-$\mu$m emission detected by SMA in 850.1 peaks on the central clump
(Figure 2), and the resolved SED analysis shown in Figure 6 indicates that the
stellar-mass surface density and the dust reddening also both peak around this
region. The map shows a maximum stellar-mass density of 14 $\times$ 1010 M⊙
kpc-2 and a mean density within $R^{\rm mass}_{\rm e}$ of (6.4$\pm$1.4)
$\times$ 1010 M⊙ kpc-2. As surface densities these quantities are
magnification invariant. The resolved SED maps also indicate that B2, seen in
projection against the northern part of the disk of 850.1, has a lower stellar
mass surface density, $\sim$ 109 M⊙ kpc-2, than the bulk of 850.1 and also a
much lower reddening, $A_{V}$ $\sim$ 0.3–1.

The variation as a function of scale of the orientation of the main structural
features in 850.1 is illustrated in Figure 3. Fitted ellipses are overlaid at
three surface brightness levels onto the F356W frame showing a change in
orientation and ellipticity in the inner regions, compared to the outskirts.
This variation is similar to the characteristic change in orientation and
ellipticity seen from a bar. More qualitatively, the morphology of this source
(seen in the restframe $I$-band) is reminiscent of the bar and arms in NGC
1365, the dominant galaxy the Fornax group, as well as showing features
similar to the arms and bars seen in dust continuum in high-redshift sub-
millimeter galaxies (Hodge et al. 2019).

Although the internal colors of 850.1 appear uniformly red in Figure 2a, this
is misleading as the source is only well detected in the three filters
longward of 2.8 $\mu$m, two of which were combined into the red channel.
Figure 2c (and 2d), uses the three reddest JWST bands and shows variation in
color within the system, with the reddest region coinciding with the central
clump. The map of $A_{V}$ from the resolved pixel-level SED fits in Figure 6
also indicate a reddening gradient in the 850.1 that ranges from $A_{V}$ $=$
6.8 in the center to $A_{V}$ $\sim$ 3 in the outskirts. However, these
reddening estimates are for light detected in F444W, and will be lower than
what would be measured if longer-wavelength resolved photometry was included.
This may explain why the median F444W-weighted $A_{V}$ from the pixel-level
SEDs was $A_{V}$ $=$ 3.6${}^{+1.5}_{-1.2}$ compared to $A_{V}$ $\sim$ 5
measured from the integrated photometry including longer wavelength
constraints.

While the map of $A_{V}$ in 850.1 indicates that the central regions are
highly obscured (probably even more obscured than shown in the map as this
corresponds to light that was still detectable at restframe $I$-band, see
Simpson et al. 2017), the outer parts of the galaxy are also surprisingly
highly and uniformly reddened out to radii of $\sim$ 3–4 kpc. The simplest
explanation may be that this galaxy has a very dusty disk (even if the gas
fraction for the system is low, see §3.5), as well as an even more obscured
nuclear region. Such extended dust disks have been seen in similarly dust-rich
galaxies (e.g., Hodge et al. 2016; Gullberg et al. 2019), in conjunction with
more compact dust continuum structures that have been interpreted as bars and
rings (Hodge et al. 2019). Another, perhaps more speculative explanation for
this wide-spread (and relatively uniform) dust reddening in this (modestly-
inclined, in contrast to Nelson et al. 2023) galaxy is a dusty wind, as is
seen on similar scales in M 82 (e.g., Yamagishi et al. 2012; Beirão et al.
2015), Mgii absorbers at higher redshifts (Ménard & Fukugita 2012), and also
theoretically expected (e.g., Krumholz & Thompson 2013). To illustrate the
plausibility of this explanation, we assume a Milky Way gas-to-dust ratio, for
which an $A_{V}$ $\sim$ 3 corresponds to a hydrogen column density of $\sim$
1022 cm-2 (Bouchet et al. 1985). Integrating over the extent of 850.1 gives a
gas mass of $\sim$ 1010 $M_{\odot}$. If this gas (and associated dust) is
being expelled in a wind with an outflow rate comparable to the current SFR,
then this mass represents just $\sim$ 10 Myrs of activity, during which time
the material would cover the extent of 850.1 if it was travelling at the
escape velocity. Thus it is possible that the relatively uniform red colors of
850.1 may be due to a foreground screen of dust expelled by a starburst-driven
wind.

Turning now to 850.2, this has an asymmetric structure with a bright elongated
region (sheared by lensing) and a western extension. The overall appearance is
of a dusty, disk-like galaxy (Figure 2). In contrast to the very red colors of
the bulk of 850.1, both the high-surface brightness region of 850.2 and the
western extension are detected in F606W, corresponding to restframe $\sim$
1100Å, with neither region detected in F435W, restframe $\sim$ 830Å,
indicating the presence of a strong Lyman break in the SED (Figure 4).
However, in addition to these UV-bright components there is also faint red
emission to the north that shows up as a band of higher-$A_{V}$ in Figure 6.
This northern region includes a compact red component, seen in F277W, F356W
and F444W (Figures 2 & 4), that appears as a distinct entity in the source
plane (Figure 2f). The peak of the SMA 880-$\mu$m emission lies between this
red clump and the UV-bright region, suggesting that this may be an interacting
system with both obscured and unobscured components, and with the unobscured
part corresponding to the higher-frequency peak seen in the CO spectrum of
this source.

The pixel-level SED fits for 850.2 give $A_{V}$ $\sim$ 0.3–1.0 over most of
the UV-detected region but rising to $A_{V}\sim$ 3 at the red clump on the
northern side. This behaviour contrasts with that seen in local galaxies,
where extinction typically rises towards the galaxy center (e.g., Holwerda et
al. 2005; Keel et al. 2023). From the stellar mass map the median stellar-mass
density within the effective radius in 850.2 was (0.10$\pm$0.01) $\times$ 1010
M⊙ kpc-2 with a peak of 0.7 $\times$ 1010 M⊙ kpc-2. Thus 850.2 has a more
uniform distribution of stellar mass density and $A_{V}$ than 850.1.

Figure 7: The stellar and interstellar medium (ISM) properties of 850.1 and
850.2. For comparison several panels show samples of 870-$\mu$m selected ALMA-
identified galaxies from the AS2UDS (Stach et al. 2019; Dudzevičiūtė et al.
2020) and AS2COSMOS (Simpson et al. 2019; Ikarashi et al. 2023) surveys and
the CO properties of a sample of sub-millimeter-continuum selected galaxies
from Birkin et al. (2021). a) The trend of specific SFR with stellar mass
(SFR/$M_{\ast}$–$M_{\ast}$) where the dotted line shows the starburst boundary
at $z$ $=$ 4 from Speagle et al. (2014). b) $A_{V}$ versus $M_{\ast}$, where
the dashed and dotted lines show the best fit relation and 1-$\sigma$ scatter
for a $K$-selected $z>$ 4 galaxies sample with $M_{\ast}$ $>$ 1010 M⊙ drawn
from the UKIDSS UDS survey (Lawrence et al. 2007) analysed by Dudzevičiūtė et
al. (2020). c) The dust temperature versus far-infrared luminosity ($L_{\rm
IR}$–$T_{d}$) relation. d) CO-derived gas fraction, $M_{\rm g}/(M_{\rm
g}+M_{\ast})$, as a function of redshift, including low-redshift SMG-analogs
from Oteo et al. (2017). e) The ratio of the [Ci]/CO(4–3) line luminosities
versus the [Ci] to far-infrared luminosity ratio, which also include sources
and PDR model tracks from Alaghband-Zadeh et al. (2013) and Valentino et al.
(2020). The literature sources are flagged if they are estimated to have
$\geq$ 20% contributions from an AGN to their far-infrared luminosities
(Valentino et al. 2020). f) Dust-to-stellar mass ratio, $M_{\rm d}/M_{\ast}$,
as a function of gas consumption timescale, $M_{\rm g}/{\rm SFR}$, including
two model tracks from Calura et al. (2017) for halos with baryonic masses of
1011 M⊙ and 1012 M⊙ roughly matching 850.2 and 850.1, respectively. The tracks
were rescaled from their assumed Larson IMF with $m_{\rm c}$ $=$ 1.2 for
comparison to the magphys derived values.

Although 850.2 is very blue compared to 850.1, the UV light traces only $\sim$
10% of the star formation in this system. In addition, there is a slight
excess of emission in the Spitzer 5.8 and 8 $\mu$m filters above the best-fit
model SED in Figure 5 that may indicate a steeply rising SED for the dustier
red northern region (but this is far from dominant in F444W). Taken together
with the strong far-infrared/sub-millimeter emission, indicating the presence
of highly obscured star formation within the system and the velocity offsets
seen between the optical and CO spectroscopy, this suggests that there could
be two spatially distinct components within this source. However, in contrast
to B2 and 850.1 in 850.2 the low-obscuration component dominates from the
optical out to at least 4.4 $\mu$m, with the fainter and more obscured
component only appearing at $>$ 2 $\mu$m.

The galfit analysis quantified the stellar continuum structure of the two
sources. The analysis of 850.1 in the F444W passband yielded a Sérsic
parameter $n$ $=$ 0.9 $\pm$ 0.1 (with lower, but increasingly uncertain,
values in bluer bands). A fixed $n=$ 1 was therefore adopted to simplify
comparison of the derived sizes between the bands. This showed a constant
effective radius with wavelength of $R^{\rm F444W}_{\rm e}$ $\sim$ 3.8 $\pm$
0.4 kpc after correcting for lensing (see also Chen et al. 2022; Kamieneski et
al. 2023). The similar analysis of 850.2 yielded $n$ $\sim$ 1.9 $\pm$ 0.1 in
F444W, with higher values in the bluer bands, although a combination of a de
Vauncoleurs (Sérsic $n=$ 4) and an $n$ $=$ 1 Sérsic did a better job of
reproducing the asymmetric, extended emission in the source. However, for
comparison to 850.1, $n$ $=$ 1 was adopted, which gave sizes that modestly
decrease in bluer bands, but with low significance, and $R^{\rm F444W}_{\rm
e}$ $\sim$ 2.5 $\pm$ 0.3 kpc in F444W (also corrected for lensing). These
sizes are comparable to those derived in the F444W waveband for sub-millimeter
galaxies with flux densities of $S_{\rm 850\mu m}$ $\geq$ 1 mJy in the
literature: $R^{\rm F444W}_{\rm e}$ $=$ 3.9 $\pm$ 0.8 kpc (Cheng et al. 2022,
2023; Chen et al. 2022). There was no evidence for a trend in restframe near-
infrared size with either dust mass or redshift for the combined samples.

The dust continuum radii, $R^{\rm dust}_{\rm e}$, derived from the SMA
observations (corrected for lensing magnification) were 2.8 $\pm$ 0.7 kpc for
850.1 and 1.9 $\pm$ 1.0 kpc for 850.2. Compared to the F444W sizes, these gave
identical ratios of dust to F444W effective radii of 0.7 $\pm$ 0.2 and 0.7
$\pm$ 0.4 respectively, similar to previous estimates (e.g., Lang et al. 2019;
Gullberg et al. 2019; Chen et al. 2022). These indicate the presence of
compact dust-enshrouded regions in the dense cores of both galaxies, linked to
the high $A_{V}$ regions (Simpson et al. 2017).

Finally, galfit was applied to the stellar mass distributions from the
resolved, pixel-level SED modelling to attempt to derive stellar-mass radii
with a Sérsic profile with $n$ $=$ 1\. However, this proved very unstable and
so instead simple numerical integration was used to derive the half-mass radii
for the two sources: $R^{\rm mass}_{\rm e}$ $=$ 1.0 $\pm$ 0.2 kpc for 850.1
and 2.2 $\pm$ 0.4 kpc for 850.2, both corrected for lensing magnification.
While the stellar mass radius determined for 850.2 was in agreement with that
measured at F444W, for 850.1 the stellar-mass size was around $\sim$ 4
$\times$ smaller than seen at F444W. This demonstrates that a significant
fraction of the stellar mass in this massive system lies in a compact and very
highly obscured nuclear region with the ratios of dust to stellar mass sizes:
2.8 $\pm$ 0.9 and 0.9 $\pm$ 0.5 for 850.1 and 850.2, respectively.

###  3.4 Comparisons to dust-mass-selected populations

Figure 7 compares the derived properties of 850.1 and 850.2 with a
representative sample of dust-mass-selected galaxies derived from ALMA
interferometric identifications of $\sim$ 1,000 sources found in $\sim$ 3
degrees2 of wide-field 850-$\mu$m SCUBA-2 surveys in the UDS and COSMOS
fields: AS2UDS (Stach et al. 2019) and AS2COSMOS (Simpson et al. 2020). These
samples have been analysed in an identical manner using magphys, allowing us
to perform a simple comparison of their derived properties (Dudzevičiūtė et
al. 2020; Ikarashi et al. 2023). After accounting for lensing magnification,
Figure 7a shows that 850.1 and 850.2 lie at either end of the mass range seen
in 850-$\mu$m-selected galaxies at $z$ $>$ 4: 850.1 being one of the more
massive galaxies found (from dust-selected samples or other methods) at these
redshifts, while 850.2 is one of the least massive in dust-selected samples
(see also, Pope et al. 2023). Looking at their specific SFR, 850.1 lies below
and 850.2 lies just inside the claimed “starburst” regime at this redshift.
The strong linear trend in Figure 7a likely reflects the fact that these
samples were selected primarily on their cold dust mass, the formation of
which requires an equivalent characteristic stellar mass of $\sim$ 1011 M⊙,
with an observed dispersion of only $\sim$ 0.4 dex.

Figure 7b shows the variation of $A_{V}$ with stellar mass for 850.1 and 850.2
compared with the $\sim$ 40 ALMA-identified $z$ $>$ 4 dusty star-forming
galaxies selected from SCUBA-2 mapping in the UDS and COSMOS fields as well as
less active $K$-selected star-forming galaxies at $z$ $>$ 4\. This figure
demonstrates one of the more strikingly differences between 850.1 and 850.2:
they lie at diametrically opposite ends of the trend of $A_{V}$ with stellar
mass for dusty galaxies at $z$ $\sim$ 4\. Thus these two closely-associated
sources span the full range in reddening seen in the high-redshift sub-
millimeter population, underlining the diversity in the restframe UV-to-
visible properties for dusty star-forming galaxies, which was already present
in the population, even in the same environment, at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle>$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 4\. The two sources broadly follow the
trend seen in the $z$ $>$ 4 $K$-selected galaxy population, which has the form
$\log_{10}(A_{V})$ $=$ 0.42 $\log_{10}(M_{\ast}/10^{11})+$ 0.23 with a median
absolute deviation of $\pm$ 0.32, although this has a steeper slope than seen
at $z$ $\sim$ 0 (Garn & Best 2010, but see also Gómez-Guijarro et al. 2023;
Shapley et al. 2022).

To test if there were other observed properties that would provide a better
predictor of $A_{V}$ for this sample of $z$ $>$ 4 SMGs, the correlation of
various properties ($M_{\ast}$, $M_{\rm d}$, $L_{\rm FIR}$, SFR, $T_{\rm d}$,
Age, $\Sigma_{\rm d}$) with $A_{V}$ was analysed using maximal information-
based non-parametric exploration (mine, Reshef et al. 2011). This showed that
the most significant correlation with $A_{V}$ for the SMG sample was indeed
with stellar mass, with a 0.3% chance of random correlation using a jackknife
test on the sample and the MINE likelihoods were calibrated with a Monte Carlo
simulation. There was only a minor reduction in scatter when additional
corrections were included for either $L_{\rm FIR}$ or $M_{\rm d}$ (Gómez-
Guijarro et al. 2023), and so it appeared that stellar mass was the most
likely driver of the differences in dust reddening between 850.1 and 850.2
(although $L_{\rm FIR}$ or SFR did nearly as well).

The final panel in Figure 7 showing purely SED properties is Figure 7c, which
compares the dust temperature and far-infrared luminosities from the magphys
analysis of 850.1/850.2 to the wider dusty $z>$ 4 population in UDS and
COSMOS, where $T_{\rm d}$ and $M_{\rm d}$ were derived in very similar ways
with magphys. The two galaxies lie within the scatter of the $T_{\rm
d}$–$L_{\rm FIR}$ trend at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle>$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 4, with 850.1 having a higher far-
infrared luminosity and a marginally higher dust temperature (although with
considerable uncertainties, even with the inclusion of the 450 $\mu$m SCUBA-2
photometry). The similarity in the dust temperatures is due to the similar
ratios of far-infrared luminosity to dust mass (or equivalently ISM mass, as
850.1 and 850.2 have very similar dust-to-gas ratios, see below) for the two
sources: (8$\pm$4) $\times$ 103 L⊙ M${}^{-1}_{\odot}$ and (7$\pm$1) $\times$
103 L⊙ M${}^{-1}_{\odot}$ for 850.1 and 850.2 respectively, suggesting their
far-infrared luminosities predominantly arise in similarly optically-thick
starburst regions (Scoville 2013).

###  3.5 Gas and dynamical masses

The lines detected in the NOEMA 3-mm spectra of the sources, along with their
dust continuum properties from SMA and SCUBA-2, provide a wealth of
information about the properties of the galaxies including both their gas
dynamics and the characteristics of their interstellar medium (ISM). Looking
first at their ISM properties, the CO line luminosity ratios for the two
galaxies are very similar: $r_{54}=$ 0.54 $\pm$ 0.01 and [Ci]/CO(4–3) $=$ 0.29
$\pm$ 0.01 for 850.1; $r_{54}=$ 0.61 $\pm$ 0.02 and [Ci]/CO(4–3) $=$ 0.29
$\pm$ 0.04 for 850.2. These compare to $r_{54}=$ 0.69 $\pm$ 0.06 and
[Ci]/CO(4–3) $=$ 0.82 $\pm$ 0.06 for the well-studied $z$ $=$ 2.3 SMG the
Cosmic Eyelash (Swinbank et al. 2010; Danielson et al. 2011), indicating that
850.1 and 850.2 have marginally less excited CO spectral line energy
distributions (SLEDs) compared to the Eyelash (i.e. suggesting a
flattening/turnover of the SLED around $J_{\rm up}$ $=$ 4–5).

Gas masses were estimated by converting the CO(4–3) line luminosities to
CO(1–0) using the Eyelash ratio of $r_{41}=$ 0.50 $\pm$ 0.04, noting that its
CO SLED appears slightly more excited than either 850.1/850.2 and hence this
may underestimate the CO(1–0) line luminosity. Adopting $\alpha_{\rm CO}$ $=$
1, consistent with the available constraints on dust-mass-selected samples and
previous studies (Birkin et al. 2021), gives cold gas masses, corrected for
lensing, $M_{\rm g}$ $=$ (1.05 $\pm$ 0.03) $\times$ 1011 $\alpha_{\rm CO}$ M⊙
for 850.1 and $M_{\rm g}$ $=$ (0.29 $\pm$ 0.02) $\times$ 1011 $\alpha_{\rm
CO}$ M⊙ for 850.2 (Table 6).

The NOEMA spectra also provide kinematic information about the motion of cool
gas in the galaxies. The Gaussian FWHM of the CO lines is the best measure of
the kinematics of the gas in 850.1. For 850.2, both CO transitions show
evidence for double-peaked lines and so the mean velocity difference between
the peaks, 330 $\pm$ 70 km s-1, was used as the measure of the projected
circular velocity. Inclination was estimated from the apparent axial ratio of
the source measured in the F444W band and corrected for lensing shear (Table
2). To calculate the dynamical masses a fixed radius of 10 kpc was chosen as
this should be sufficient to contain the majority of the stellar and cold-gas
mass in these systems.111When estimating the mass of the cold H2 reservoir
using a CO(1–0) luminosity extrapolated from moderate-$J$ CO lines using a
galaxy-integrated conversion factor, $r_{ij}$, it is critical to appreciate
that this does not mean that the extrapolated H2 (or CO(1–0)) reservoir has
the same physical extent as the moderate-$J$ CO emission. Gradients in the
excitation or optical depth will lead to different physical extents for the
different tracers (Weiß et al. 2005). Using $r$ $=$ 10 kpc and $M_{\rm
dyn}(<r)=r\times(V_{\rm c}/\sin(i))^{2}/G$ gives estimated dynamical masses of
3.8${}^{+5.1}_{-1.5}\times$ 1011 M⊙ and 0.8${}^{+0.3}_{-0.3}\times$ 1011 M⊙
for 850.1 and 850.2, respectively. These compare to the total baryonic masses
of 6.6${}^{+1.3}_{-0.5}\times$ 1011 M⊙ and 0.50${}^{+0.03}_{-0.03}\times$ 1011
M⊙, confirming that the baryonic masses are broadly consistent with the galaxy
kinematics derived from the CO line width, given the uncertainties on the
inclination corrections. If the calculation had instead used a Milky-Way-like
$\alpha_{\rm CO}$ $=$ 4.3 to estimate the cold gas mass, then the baryonic
masses would rise to 10.0${}^{+1.3}_{-0.5}\times$ 1011 M⊙ and
1.4${}^{+0.7}_{-0.7}\times$ 1011 M⊙, which both exceed the dynamical
estimates, although they are still formally in agreement due to the large
uncertainties on the latter from the inclination corrections and adopted
sizes.

Using the estimated gas masses assuming $\alpha_{\rm CO}$ $=$ 1, Figure 7d
illustrates the variation in gas fraction of galaxy populations as a function
of redshift for dust-mass-selected samples. The gas fraction of 0.16 $\pm$
0.03 for 850.1 is at the low end of the distribution for dust-selected systems
at $z$ $\sim$ 4, while the estimate for 850.2 of 0.62 $\pm$ 0.07 is at the
upper end and so more consistent with the rough trend for higher gas fractions
at higher redshifts.

###  3.6 ISM properties

Both 850.1 and 850.2 show fairly lower [Ci] luminosities relative to either
their CO(4–3) or far-infrared luminosities compared to many of the high-
redshift dust-mass selected galaxies or other [Ci]-detected star-forming
galaxies shown in Figure 7e. Following Alaghband-Zadeh et al. (2013), a simple
PDR model can be used assess the physical origin of these differences. The
ratio [Ci]/CO(4–3) traces the characteristic density of the ISM and
[Ci]/$L_{\rm FIR}$ indicates the strength of the radiation field, although the
absolute values are sensitive to the chosen model, compare the tracks from
Alaghband-Zadeh et al. (2013) and Valentino et al. (2020) in Figure 7e. The
Kaufman et al. (1999) PDR model grids indicate well-constrained ISM densities
for 850.1 and 850.2, $\log_{10}(n)$ $=$ 4.5 $\pm$ 0.1 cm-3, and radiation
fields of $\log_{10}(G_{0})$ $\sim$ 3.5–4.3 (in Habing units), depending upon
the adopted tracks (see the discussion in Valentino et al. 2020). These ISM
properties are within the ranges previously estimated for sub-millimeter
galaxies and local ULIRGs (see Alaghband-Zadeh et al. 2013; Valentino et al.
2020).

Assuming a simple uniform density sphere for the geometry of the ISM in these
sources, then the ISM densities and gas masses indicate a radius of $\sim$ 0.4
kpc for 850.1 and $\sim$ 0.25 kpc for 850.2 (see Yan & Ma 2016). Adopting a
more plausible thin disk geometry would result in a larger radius by a factor
of 1.9–2.5 $\times$ for disks with thickness of $\sim$ 0.1–0.05 $\times$ their
diameter. This would correspond to diameters of $\sim$ 1.8 kpc and $\sim$ 1.1
kpc, a factor of $\sim$ 1.6 $\times$ smaller than the measured dust continuum
sizes in both sources (Table 6). However, a more realistic model would involve
a multi-phase ISM, with much of the cool gas in a component that is
potentially distinct from the star-forming material, thus arguing for using a
lower gas mass (relevant for the star-forming phase) in these calculations and
hence a smaller volume. Even without this extra complication, it is clear that
the dense ISM traced by the moderate-$J$ CO lines in these galaxies is limited
to a relatively small volume, either distributed as smaller clouds spread
throughout the disks or more likely mostly restricted to a compact region in
the galaxy centers.

A similar calculation can be undertaken using the ISM radiation fields. For
solar metallicities, a Salpeter IMF, and constant-SFR bursts with ages of
50–500 Myr, $\sim$ 10% of the bolometric luminosity arises in ionizing
radiation with 6–13.6 eV. Assuming a centrally illuminated spherical shell
geometry then the estimated radiation field from the PDR models implies sheel
radii $r$ $\sim$ 1.1–2.9 kpc for 850.1 and $r$ $\sim$ 0.6–1.4 kpc for 850.2.
These sizes were smaller than those measured at F444W (Table 6) but are closer
to those derived for the total stellar mass (and dust continuum) in §3.2 as
well as by previous dust continuum studies (Simpson et al. 2015; Gullberg et
al. 2019), although these assume different geometries.

###  3.7 Gas masses and fractions

Figure 7f shows the variation of dust-to-stellar mass ratio with the gas
consumption timescale at the current SFR for the two galaxies. The trend in
Figure 7f reflects the fact that galaxy-integrated dust mass provides a crude
proxy for gas mass, hence $M_{\rm d}\propto M_{\rm g}$, with similar dust-to-
gas ratios of 69 $\pm$ 16 and 61 $\pm$ 8 for 850.1 and 850.2, slightly lower
than the median of 120 $\pm$ 50 for the $z$ $>$ 4 subset from Birkin et al.
(2021), with 60 $\pm$ 37 for their whole sample. The gas consumption
timescales of both galaxies are $\sim$ 100 Myr, so they will consume their
current gas reservoirs by $z$ $\sim$ 4, unless further material is accreted.
While the dust-to-stellar mass ratio for 850.2 is an order of magnitude higher
than for 850.1, at $M_{\rm d}/M_{\ast}$ $=$ 0.026 $\pm$ 0.004 compared to
0.0027 $\pm$ 0.0009, reflecting the very different stellar masses of the two
galaxies. To interpret this difference Figure 7f shows two tracks for proto-
spheroid galaxy models (top-hat collapse) from Calura et al. (2017). These
show the variation of dust-to-stellar mass ratio with gas consumption
timescale for halos with baryonic masses of 1012 M⊙ and 1011 M⊙, which roughly
bracket the estimates for 850.1 and 850.2. These evolutionary tracks start at
high $M_{\rm g}/{\rm SFR}$ and initially show rapidly increasing $M_{\rm
d}/M_{\ast}$ until a peak at $\sim$ 150–200 Myrs before declining, the tracks
then terminate at the point that an outflow is expected to remove the
remainder of the cold gas in the system. The higher mass model peaks at a
lower dust-to-stellar mass ratio owing to the shorter star-formation timescale
leading to increased dust destruction, before the galactic wind terminates all
star formation. These theoretical models demonstrate that the observed dust-
to-stellar ratios for 850.1 and 850.2 can be achieved by models at typical
ages of $\sim$ 100–300 Myrs, with their relative dust-to-stellar mass ratios
being consistent with 850.1 being older than 850.2.

The ISM properties of 850.1 and 850.2 are surprisingly similar in terms of
dust temperature, ionization field, and density, given the very different
stellar masses and restframe UV–visible obscuration of the stellar populations
in the two galaxies. This similarity has its basis in the comparable ratios of
dust mass to far-infrared luminosity (or star-formation rate) for the two
systems, which indicates similar relative amounts of ISM mass available to
absorb and then reradiate the luminosity arising from the on going star
formation (Scoville 2013). In contrast, the dust-to-stellar mass ratios of the
two galaxies are very different, reflecting their different evolutionary
stages. 850.1 is a much more massive system and is relatively gas poor,
suggesting that it may be in the process of quenching its activity. While
850.2 is a young and gas-rich system that is rapidly growing its stellar and
dust mass.

Figure 8: Comparisons between the a) stellar-mass effective radii and b)
stellar-mass surface densities of 850.1 and 850.2 with $z$ $\sim$ 0 quiescent
early-type galaxies from the ATLAS-3D survey (Cappellari et al. 2011, 2013),
color-coded by their formation redshift as determined from their SSP ages in
McDermid et al. (2015). In addition, a sample of quiescent galaxies at $z$
$\sim$ 1.5 with F160W-derived sizes and high stellar masses from Longhetti et
al. (2007) are shown. Also plotted are the F444W-derived sizes and stellar
densities for $z$ $\sim$ 1–3 SMGs from Chen et al. (2022). The red arrow on
both panels shows the effect of the expected (empirically and theoretically)
size evolution from $z$ $=$ 4.26 to $z$ $\sim$ 0 and the tick indicates the
evolution to $z$ $=$ 1.5 for a massive galaxy such as 850.1 (Cimatti et al.
2012; Oser et al. 2012). This demonstrates that 850.1 will likely evolve to
have a stellar size and surface mass density similar to those of massive,
compact quiescent galaxies at $z$ $\sim$ 1.5, and then go on to match the
properties of the most massive, and oldest, early-type galaxies at the present
day.

###  3.8 Evolutionary links

850.1 is one of the most massive galaxies known at $z$ $>$ 4 in terms of
stellar or total baryonic mass at $\sim$ 1011.8 M⊙ (e.g., Caputi et al. 2011;
Stefanon et al. 2015; Marsan et al. 2022), with a space density of $\sim$ 3
$\times$ 10-7 Mpc-3 estimated from the surface density of comparably bright
850-$\mu$m sources and assuming a redshift range of $z$ $=$ 3–6 for the
population. This is consistent with expected limits for such massive systems
from $\Lambda$CDM ($<$ 5 $\times$ 10-6 Mpc-3, Behroozi & Silk 2018). Unlike
most previous studies of high-redshift massive galaxies, the high-resolution
JWST imaging used here tightly constrains the restframe optical/infrared SED,
and also excludes significant contamination from an obscured AGN in our
estimates of the stellar mass. Likewise, the sub-millimeter imaging from SMA
provided both the identification of this massive (but optically undetected)
high-redshift galaxy and also constrained the influence of dust on 850.1’s SED
necessary to derive a robust stellar mass (see also Ikarashi et al. 2022).

This analysis also benefited from the high-spatial resolution and long-
wavelength coverage of JWST to go beyond what has been previously possible in
resolved studies of the stellar content of dusty galaxies at high redshifts
(e.g., Lang et al. 2019). The resolved stellar-mass maps from the pixel-level
SED fitting indicated stellar mass surface density estimates within the
central regions of 850.1 and 850.2 that are comparable to the stellar density
of early-type galaxies at the present day, long believed to be the potential
descendants of these systems. Moreover, the small stellar-mass sizes for the
two sources, 1–2 kpc, are comparable to those claimed for the most compact,
massive “quiescent/evolved” galaxies at $z$ $\sim$ 1–3, $\sim$ 1–5 kpc (e.g.,
Waddington et al. 2002; Trujillo et al. 2006; Longhetti et al. 2007; Buitrago
et al. 2008; Damjanov et al. 2009; Mowla et al. 2019; Lustig et al. 2021). The
stellar-mass sizes for 850.1 and 850.2 are also consistent with the
expectations from hydrodynamic simulations (e.g., Wellons et al. 2015), which
indicate sizes of $\sim$ 1 kpc for systems with masses of $\sim$ 1011 M⊙.

Figure 8 shows the intrinsic sizes and stellar-mass densities for 850.1 and
850.2 compared to their lensing-corrected stellar masses. These are plotted
alongside estimates from integrated SED fitting and JWST NIRCam F444W sizes
for $z$ $=$ 1–3 SMGs with $S_{\rm 850\mu m}$ $\sim$ 2–4 mJy from Chen et al.
(2022), although the F444W-based sizes may overestimate the true stellar-mass
sizes for these dust-obscured systems. The figure also includes a sample of
high mass spectroscopically-classified quiescent galaxies at somewhat lower
redshifts, $z$ $\sim$ 1.2–1.7, with estimated stellar masses above 3 $\times$
1011 M⊙, and sizes derived from HST F160W imaging (Longhetti et al. 2007).
These comparisons show that 850.2 has a stellar-mass size and stellar-mass
surface density consistent with those seen in early-type galaxies at the
present day, as do the other lower redshift sub-millimeter galaxies with
comparable 850-$\mu$m flux densities from Chen et al. (2022). However, 850.1
is an outlier in both panels, with a size that is a factor of $\sim$ 10
smaller than comparably massive early-type galaxies at $z$ $\sim$ 0\.
Nevertheless, the peak stellar-mass surface density for 850.1, 1.4 $\times$
1011 M⊙ kpc-2, is similar to the characteristic maximum stellar-mass density
estimated in the central regions local early-type galaxies (e.g., Hopkins et
al. 2009), suggesting a potential link.

Empirical estimates of the size evolution of massive, early-type galaxies out
to $z$ $\sim$ 3 indicate that these increase at lower redshifts as
$(1+z)^{-1.39\pm 0.13}$ (e.g., Cimatti et al. 2012; Trujillo et al. 2007;
Holwerda et al. 2020). This implies a size increase of $\sim$ 4 $\times$ for
850.1 from $z$ $=$ 4.26 to $z$ $=$ 1.5 and $\sim$ 10 $\times$ to the present
day. The evolution in the size of massive galaxies is driven by minor mergers
(such as the potential accretion of B2 and 850.2 onto 850.1) that increase the
total stellar mass of the system modestly but boost the effective radius
considerably (Naab et al. 2009; Hopkins et al. 2009; Oser et al. 2012). The
size evolution for lower mass galaxies is less extreme (Genel et al. 2018) and
may be moot for 850.2 if it subsequently merges with 850.1. As Figure 8 shows,
if 850.1 undergoes size evolution as suggested by simulations and observations
at $z$ $<$ 3, then at $z$ $\sim$ 1.5 it will end up in the region of the
$R_{\rm e}$–$M_{\ast}$ and $\Sigma_{\ast}$–$M_{\ast}$ planes that are
populated by the high-mass quiescent galaxies from Longhetti et al. (2007),
before evolving to match the properties of the most massive early-type
galaxies at $z$ $\sim$ 0\. These massive galaxies at $z$ $\sim$ 0 are also
those that appear to have formed the bulk of their stars at $z$ $>$ 4, again
in good agreement with the observations of 850.1. Overall, this appears to be
strong circumstantial evidence that 850.1 represents the formation phase of an
ultra-massive early-type galaxy, which will subsequently evolve through the
massive, passive galaxy population found at $z$ $\sim$ 1–3 (Trujillo et al.
2006; Buitrago et al. 2008; Damjanov et al. 2009).

To summarise, 850.1, 850.2, and B2 reside in a probable group-like halo
serendipitously discovered behind the core of A 1489. The two sub-millimeter
sources represent extremes of the dust-mass-selected population at $z$ $>$ 4,
with 850.1 being one of the most massive galaxies known at these redshifts.
The gas reservoirs in the two galaxies contribute very different fractions of
their total baryonic content at this early phase of the Universe (age of 1.4
Gyr): gas fractions of 16% and 62%, suggesting different evolutionary states.
These differences were also reflected in the different characteristic ages of
their best-fit SEDs and dust/stellar ratios, although both were predicted to
consume their current gas reservoirs by $z$ $\sim$ 4\. The properties of these
gas reservoirs are surprisingly similar, both indicating high densities,
strong radiation fields, and comparable dust temperatures. Thus 850.1 appears
to represent a massive, highly obscured, but relatively gas-poor
(“quenching”?) galaxy with possibly two closaree neighbors that are both much
less massive and more gas-rich, as well as younger.

If these galaxies do reside in a single group-sized $\sim$ 1013 M⊙ halo, as
was claimed for sub-millimeter galaxies (e.g., Stach et al. 2021), then 850.1
is very likely to be the central galaxy with B2 and 850.2 as satellites that
will eventually be accreted through minor mergers. These mergers will increase
the stellar-mass radius of 850.1, which at $\sim$ 1 kpc is more compact than
typical examples of “quiescent/passive” galaxies at $z$ $\sim$ 2–3, into which
it will likely evolve, so that the stellar-mass surface density of this galaxy
will eventually decline by two orders of magnitude to match those of the most
massive early-type galaxies at the present day. Such minor mergers may also be
the trigger for the formation of the apparent stellar bar visible in the
central regions of 850.1 (Figure 4). Similar features have been reported from
JWST imaging of galaxies at $z$ $\sim$ 1–3 (e.g., Guo et al. 2023), and likely
indicate dynamical disturbance of disks. Disks can form bars relatively
quickly if they were cold and gas rich and the central regions of the halos
were baryon dominated (e.g., Bland-Hawthorn et al. 2023). These non-
axisymmetric structures then serve to remove angular momentum from gas in the
disk and efficiently funnel it into the galaxy center potentially fueling the
central dusty starburst that alerted us to the existence of these systems in
the first place.

##  4 Conclusions

This pair of sub-millimeter galaxies demonstrates the remarkable diversity of
galaxies hosting massive dust and gas reservoirs at high redshifts. These two
galaxies are associated in a single structure at $z$ $=$ 4.26, but their
observed and intrinsic properties span the full range known for this
population at $z$ $>$ 4\. The main conclusions of this work are:

1\. 850.1 is an example of a rare population of very luminous sub-millimeter
sources with an intrinsic $S_{\rm 850\mu m}$ $=$ 13.5 $\pm$ 0.4 mJy,
corresponding to a population with a surface density of $\sim$ 10 $\pm$ 5
degree-2, while 850.2 with $S_{\rm 850\mu m}$ $=$ 3.8 $\pm$ 0.3 mJy comes from
the much more numerous population around the knee in the 850-$\mu$m counts,
having a surface density $\sim$ 50 $\times$ higher. 850.1 is one of the most
massive galaxies known at $z$ $>$ 4 in terms of stellar or total baryonic mass
at $\sim$ 1011.8 M⊙ with an estimated formation redshift $z_{\rm form}$ $\sim$
6 and a space density of $\sim$ 3 $\times$ 10-7 Mpc-3, consistent with
expected limits from $\Lambda$CDM. The high-resolution JWST imaging can
exclude significant contamination from obscured AGNs in these mass estimates.

2\. Without lensing 850.1 would have $H$ $\sim$ 29 and $K$ $\sim$ 27\. Neither
HST nor ground-based $K$-band surveys would have been able to detect this
class of source, let alone less-massive or higher-redshift examples. This
reflects the high dust reddening, $A_{V}$ $\sim$ 3–6, over the full $\sim$
7-kpc detected extent of the disk in 850.1 (with the exception of the UV-
bright companion B2, if it is actually part of 850.1). This wide spread and
relatively uniform dust reddening in a modestly inclined galaxy may be a
result of either efficient mixing of dust within the galaxy disk, or more
speculatively a dusty wind emanating from the $\sim$ 1 kpc stellar core.

3\. The high-quality multi-band JWST/HST images available for this field also
allows a pixel-level SED analysis that provides unique information on the
internal structures of the galaxies. This resolved SED analysis shows good
correspondence with the integrated SED fits, but only because it showed the
need to exclude the foreground/companion UV-bright sources, B1 and B2, from
the integrated photometry fits for 850.1. If these sources had been included
in a single SED fit then that results in a significantly worse fit and biased
derived properties. Similar issues may affect integrated photometry modelling
of systems comprising a mix of low- and high-obscuration regions or
components. This effect may also explain previous reports of mid-infrared
power-law excesses in $\sim$ 10% of dusty galaxies that were previously
attributed to buried AGN. Such resolved analyses benefit considerably from the
inclusion of resolved long-wavelength constraints on the pixel-level SEDs, as
provided by sensitive sub-millimeter interferometry.

4\. 850.1 and 850.2 are just $\sim$ 100 kpc ($\sim$ 15′′) apart in the source
plane and offset in velocity by only $\sim$ 30 km s-1, indicating that they
likely reside in a single structure, a small group at $z$ $\sim$ 4.26. Without
the lensing effect of the foreground cluster they would appear as a single,
blended, bright ($S_{\rm 850\mu m}$ $\sim$ 17 mJy) source in a single-dish
sub-millimeter survey. There are a number of similar lensed groups at $z$
$\sim$ 2–4, comprising one or more sub-millimeter galaxies and other
companions, reported in the literature. The frequency with which these systems
are being found may reflect the correspondence between the spatial scale of
group-sized halos and the size of high-magnification region of massive
clusters at $z$ $\mathrel{\raise
1.50696pt\hbox{$\scriptstyle<$}\kern-6.00006pt\lower
1.72218pt\hbox{{$\scriptstyle\sim$}}}$ 1, see also Frye et al. (2023). This
adds evidence that at least a subset of the high-redshift sub-millimeter
population resides in group-scale halos. These are expected to be the most
active environments for the accretion of gas and companion galaxies from the
surrounding intergalactic medium, potentially providing both the fuel and the
trigger needed to power their active star formation,

5\. The properties of 850.1 suggest it is the central galaxy of this group.
Its stellar size and central stellar-mass surface density will match those
measured for massive quiescent galaxies at $z$ $\sim$ 1.5 and $z$ $\sim$ 0 if
it follows the expected size evolution due to the accretion, through minor
mergers, of satellite galaxies such as 850.2 (which itself may be an
interacting system) and potentially B2.

6\. The exquisite JWST imaging of the A 1489 field, combined with the natural
magnification of the foreground cluster lens, provides a striking view of the
stellar structure of these two strongly star-forming galaxies. The structure
of 850.1 is particularly noteworthy, with features that appear to correspond
to arms and a central bar in a very massive galaxy at $z$ $=$ 4.26. It is
tempting to identify these features as a response to dynamical perturbations
from the group environment, including the presence of 850.2 as a close
companion. Bars with $\sim$ 1-kpc radii have been reported from high-
resolution dust continuum imaging of high-redshift sub-millimeter galaxies by
Hodge et al. (2019), as well as from statistical analyses of the dust
continuum shapes of the sub-millimeter galaxy population (Gullberg et al.
2019). These features have similar sizes to the dust continuum peak and the
massive stellar core in 850.1, suggesting that this galaxy is experiencing a
bar-driven inflow that is fueling a central starburst in its baryon-dominated
core.

## Acknowledgments

We thank Harold Pena for his help with the JCMT observations and reduction and
Rob Ivison and Anna Puglisi for valuable comments and discussions.

All of the Durham co-authors acknowledge STFC through grant number
ST/T000244/1 and ST/X001075/1. AZ acknowledges support by the Ministry of
Science & Technology, Israel, and by Grant No. 2020750 from the United States-
Israel Binational Science Foundation (BSF) and Grant No. 2109066 from the
United States National Science Foundation (NSF). RAW, SHC, and RAJ acknowledge
support from NASA JWST Interdisciplinary Scientist grants NAG5-12460,
NNX14AN10G and 80NSSC18K0200 from GSFC. Work by CJC acknowledges support from
the European Research Council (ERC) Advanced Investigator Grant EPOCHS
(788113). MAM acknowledges the support of a National Research Council of
Canada Plaskett Fellowship, and the Australian Research Council Centre of
Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through
project number CE17010001. CNAW acknowledges funding from the JWST/NIRCam
contract NASS-0215 to the University of Arizona. CC is supported by the
National Natural Science Foundation of China, No. 11803044, 11933003,
12173045. This work is sponsored (in part) by the Chinese Academy of Sciences
(CAS), through a grant to the CAS South America Center for Astronomy
(CASSACA). We acknowledge the science research grants from the China Manned
Space Project with NO. CMS-CSST-2021-A05. JMD acknowledges support from
PGC2018-101814-B-100. The James Clerk Maxwell Telescope is operated by the
East Asian Observatory on behalf of The National Astronomical Observatory of
Japan; Academia Sinica Institute of Astronomy and Astrophysics; the Korea
Astronomy and Space Science Institute; the National Astronomical Research
Institute of Thailand; Center for Astronomical Mega-Science (as well as the
National Key R&D Program of China with No. 2017YFA0402700). Additional funding
support is provided by the Science and Technology Facilities Council of the
United Kingdom and participating universities and organizations in the United
Kingdom and Canada. Additional funds for the construction of SCUBA-2 were
provided by the Canada Foundation for Innovation. The data used in this
project came from programs M23AP006 and M15AI29. The Submillimeter Array is a
joint project between the Smithsonian Astrophysical Observatory and the
Academia Sinica Institute of Astronomy and Astrophysics and is funded by the
Smithsonian Institution and the Academia Sinica. This work is based on
observations carried out under project number S22CX with the IRAM NOEMA
Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN
(Spain). This work is based on observations made with the NASA/ESA/CSA James
Webb Space Telescope. The data were obtained from the Mikulski Archive for
Space Telescopes (MAST) at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy, Inc.,
under NASA contract NAS 5-03127 for JWST. These observations are associated
with JWST program 1176. This research is based on observations made with the
NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science
Institute, which is operated by the Association of Universities for Research
in Astronomy, Inc., under NASA contract NAS 5–26555. These observations are
associated with program 15959. This work is based in part on archival data
obtained with the Spitzer Space Telescope, which was operated by the Jet
Propulsion Laboratory, California Institute of Technology under a contract
with NASA. This research has made use of Canadian Astronomy Data Centre
(CADC), NASA’s Astrophysics Data System (ADS) and the NASA Extragalactic
Database (NED). We recognize that Maunakea is a culturally important site for
the indigenous Hawaiian people; we are privileged to study the cosmos from its
summit. We also acknowledge the indigenous peoples of Arizona, including the
Akimel O’odham (Pima) and Pee Posh (Maricopa) Indian Communities, whose care
and keeping of the land has enabled us to be at ASU’s Tempe campus in the Salt
River Valley, where much of our work was conducted.

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=20mm

Table 2: Kinematic properties of sources

Source | $z$ | $\nu_{\rm CO(4-3)}$ | FWHMCO(4-3) | $\nu_{\rm[CI]}$ | FWHM[CI] | $\nu_{\rm CO(5-4)}$ | FWHMCO(5-4) | FWHM |  $b/a_{\rm corr}$a | $i$ | $M_{\rm dyn}(10\,\rm kpc)$
---|---|---|---|---|---|---|---|---|---|---|---
|  | [GHz] | [km s-1] | [GHz] | [km s-1] | [GHz] | [km s-1] | [km s-1] |  | [deg] | [1011 M⊙]
850.1 | 4.2599 $\pm$ 0.0001 | 87.6519 | 380 $\pm$ 15 | 93.5606 | 400 $\pm$ 30 | 109.558 | 375 $\pm$ 15 | 380 $\pm$ 10 | 0.88 $\pm$ 0.11 | 28 $\pm$ 12 | 3.8${}^{+5.1}_{-1.5}$
850.2 | 4.2593 $\pm$ 0.0003 | 87.6607 | 620 $\pm$ 70 | 93.6452 | 1100 $\pm$ 500 | 109.570 | 670 $\pm$ 80 |  640 $\pm$ 50 | 0.40 $\pm$ 0.03 | 66 $\pm$ 2 | 0.8 $\pm$ 0.3b

Note. — a Axis ratio measured from the F444W isophote shape and corrected for
lensing shear. b To estimate the dynamical mass, we used the velocity
difference between the two components of the CO(4–3) and CO(5–4) lines, 330
$\pm$ 70 km s-1, rather than the FWHM.

Table 1: Sub-/millimeter positions and observed flux densities

|  |  | Spitzer |  | SCUBA-2 | SMA | NOEMA
---|---|---|---|---|---|---|---
Source (short name) | R.A. Dec. | 5.8 $\mu$m | 8.0 $\mu$m | 24 $\mu$m | 450 $\mu$m | 850 $\mu$m | 880 $\mu$m | $\mu^{0.5}\rm FWHM_{\rm d}$ | 2.8 mm | 3.4 mm