# A high incidence of dusty H$\alpha$ emitters at $z>3$ among UltraVISTA
dropout galaxies in COSMOS
revealed by JWST

Sophie E. van Mierlo Kapteyn Astronomical Institute, University of Groningen,
PO Box 800, 9700 AV Groningen, The Netherlands  Karina I. Caputi  Kapteyn
Astronomical Institute, University of Groningen, PO Box 800, 9700 AV
Groningen, The Netherlands  Cosmic Dawn Center (DAWN), Denmark  Matthew L. N.
Ashby Optical and Infrared Astronomy Division Center for Astrophysics $|$
Harvard & Smithsonian Cambridge, MA 02138 USA  Vasily Kokorev Kapteyn
Astronomical Institute, University of Groningen, PO Box 800, 9700 AV
Groningen, The Netherlands  Rafael Navarro-Carrera Kapteyn Astronomical
Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The
Netherlands  Pierluigi Rinaldi Kapteyn Astronomical Institute, University of
Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

###### Abstract

We have characterized 26 Spitzer/IRAC-selected sources from the SMUVS program
that are undetected in the UltraVISTA DR5 $H$\- and/or $K_{\mathrm{s}}$-band
images, covering 94 square arcmin of the COSMOS field which have deep multi-
wavelength JWST photometry. We analyzed the JWST/NIRCam imaging from the
PRIMER survey and ancillary HST data to reveal the properties of these
galaxies from spectral energy distribution (SED) fitting. We find that the
majority of these galaxies are detected by NIRCam at $\lambda<2\,\rm\mu m$,
with only four remaining as near-infrared dropouts in the deeper JWST images.
Our results indicate that the UltraVISTA dropouts candidates are primarily
located at $z>3$ and are characterized by high dust extinctions, with a
typical colour excess $E(B-V)=0.5\pm 0.3$ and stellar mass
$\log(M_{*}/M_{\odot})=9.5\pm 1.0$. Remarkably, $\sim 75$ % of these sources
show a flux enhancement between the observed photometry and modelled continuum
SED that can be attributed to H$\alpha$ emission in the corresponding NIRCam
bands. The derived (H$\alpha$\+ N[II] + S[II]) rest-frame equivalent widths
and H$\alpha$ star formation rates (SFRs) span values $\sim 100$–2200 Å and
$\sim 5$–375 $M_{\odot}\,\mathrm{yr^{-1}}$, respectively. The locations of
these sources on the SFR-$M_{*}$ plane indicates that 35 % of them are
starbursts, 40 % are main-sequence galaxies and the remaining 25 % are located
in the star-formation valley. Our sample includes one AGN and three sub-
millimeter sources, as revealed from ancillary X-ray and sub-mm photometry.
The high dust extinctions combined with the flux boosting from H$\alpha$
emission explain why these sources are relatively bright Spitzer galaxies and
yet unidentified in the ultra-deep UltraVISTA near-infrared images.

galaxies: high-redshift — galaxies: photometry — galaxies: fundamental
parameters

##  1 Introduction

Over the past decade, deep near-/mid-infrared galaxy surveys have been used to
study galaxy evolution up to high redshifts with ever increasing statistics
(e.g., Laigle et al. 2016; Deshmukh et al. 2018; Weaver et al. 2022). A number
of these galaxy surveys have been conducted with the Infrared Array Camera
(IRAC; Fazio et al., 2004) on board the Spitzer Space Telescope (Werner et
al., 2004), in different cosmological fields (e.g., Ashby et al., 2015; Ashby
et al., 2018). Other galaxy surveys have been carried out from the ground at
near-infrared wavelengths, with infrared telescopes such as the VISTA
telescope (e.g. McCracken et al., 2012; Jarvis et al., 2013). A key factor to
make these galaxy surveys successful has been the availability of ancillary
photometry of matching depth at optical wavelengths, either from the Hubble
Space Telescope (HST) or ground-based telescopes. This multi-wavelength
coverage allow for properly constraining the galaxy spectral energy
distribution (SED) properties and particularly deriving secure photometric
redshifts.

Not all infrared galaxies are detected at optical wavelengths, though, even
when deep images are available (for recent studies on this population, see
e.g., Barrufet et al. 2023; Pérez-González et al. 2023; Smail et al. 2023).
Some galaxies can be relatively bright in the Spitzer mid-infrared images
($\lambda>3\,\rm\mu m$), but faint even at near-infrared ($\lambda=1-3\,\rm\mu
m$) wavelengths. A number of studies have been devoted to study the nature of
these very red sources. They concluded that these galaxies are mostly at
$z=3-6$, with a subset of candidates for higher redshifts (e.g., Caputi et
al., 2012; Wang et al., 2019). Although a minority amongst all high-redshift
galaxies, mid-infrared-bright, near-infrared-faint galaxies account for a half
of the most massive galaxies at $z=4-6$ (Caputi et al., 2015). From their
follow up at sub-/millimeter wavelengths, some of these galaxies have been
found to be dusty star-forming galaxies (Ikarashi et al., 2017; Shu et al.,
2022; Smail et al., 2023; Zavala et al., 2023), which contribute significantly
more to the cosmic star-formation-rate density than equally massive UV-bright
galaxies at $z>3$ (Zavala et al., 2021).

More difficult to constrain are the small percentage of Spitzer sources for
which no counterpart at all has been found in deep near-infrared images. To
date, the nature of these sources has remained completely unknown. The advent
of the James Webb Space Telescope (JWST) allows us now to investigate the
nature of these sources by providing, for the first time, ultra-deep multi-
wavelength coverage at infrared wavelengths.

In the red sources detected both at mid- and near-infrared wavelengths, the
enhanced mid-infrared with respect to near-infrared flux is explained by a
combination of factors, typically high dust extinction combined with an
intermediate/high redshift (Barrufet et al., 2023; Pérez-González et al.,
2023). The presence of emission lines can also potentially produce such red
colours, although only a few cases of this kind have been found to date
(Alcalde Pampliega et al., 2019).

Indeed emission lines can produce a significant enhancement even in broad-band
fluxes if their equivalent widths are sufficiently high. This is particularly
the case for H$\alpha$ ($\lambda 6563$ Å). A number of works have successfully
exploited the Spitzer/IRAC images to identify intense H$\alpha$-emitters at
$z\approx 4$–5 from flux excess in the IRAC 3.6 $\mu$m band (Smit et al.,
2016; Caputi et al., 2017; Faisst et al., 2019), with rest-frame equivalent
widths ($\rm EW_{0}$) ranging from $\sim 100$–10000 Å. These studies are
biased towards galaxies well detected in the near-infrared, as good
constraints on the continuum bluewards of the 3.6$\mu$m band are necessary to
identify any flux excess. No particular connection between line emitters and
dropout galaxies has been derived from these studies.

In this work, we exploit the JWST/NIRCam imaging in the PRIMER survey together
with ancillary HST/ACS and HST/WFC3 imaging, to follow up IRAC-selected
galaxies from the Spitzer Matching survey of the UltraVISTA ultra-deep Stripes
(SMUVS) (Ashby et al., 2018) that are undetected in the UltraVISTA DR5 $H$\-
and $K_{\mathrm{s}}$ bands. The improved depth and wavelength coverage of the
new JWST data enables us to reveal the true nature of these so far elusive
galaxies.

This paper is organized as follows. In Sect. 2 we describe the ground- and
space-based imaging used in this work, and Sect. 3 describes our initial
selection of dropout candidates. In Sect. 4, we describe the derived HST and
JWST aperture photometry for the JWST-detected counterparts of the dropout
candidates. In Sect. 5.1, we describe the subsequent SED fitting of our
sample, and present their general properties. In Sect. 5.2 we present our
results on the H$\alpha$ line-complex equivalent widths and SFRs. Finally, in
Sect. 6 we summarize our findings and discuss our conclusions. We adopt a
cosmology with $\rm{H_{0}=70\ km\ s^{-1}\ Mpc^{-1}}$, $\Omega_{\rm m}=0.3,$
and $\Omega_{\Lambda}=0.7$. All magnitudes and fluxes are total, with
magnitudes referring to the AB system (Oke & Gunn, 1983). Stellar masses
correspond to a Chabrier (2003) initial mass function (IMF).

##  2 Data

###  2.1 Ground-based and Spitzer/IRAC imaging

Our initial search for $H-$ and $K_{\rm s}$-band dropout galaxies is based on
the UltraVISTA VIRCAM imaging (McCracken et al., 2012) and Spitzer (Werner et
al., 2004) IRAC (Fazio et al., 2004) 3.6 and 4.5 $\mu$m imaging from the SMUVS
program (Ashby et al., 2018) in the COSMOS field (Scoville et al., 2007).

The UltraVISTA data considered in this work correspond to the fifth data
release (DR5), which contain ultra-deep imaging in three stripes on the sky
taken with the VIRCAM instrument on the VISTA telescope. Over the considered
PRIMER region in this work, the data reach average $5\sigma$ depths of $\sim
26$–27 mag in the $Y$, $J$, $H$, $K_{\rm s}$ and $NB118$ bands. We performed
source extraction on these data ourselves as described in Sect. 3.

The SMUVS program conducted observations to map three UltraVISTA ultra-deep
stripes, over a total area of 0.66 deg2. We make use of the existing IRAC 3.6
and 4.5 $\mu$m catalog from Ashby et al. (2018). Source detection and aperture
photometry measurements for this catalog were conducted using point spread
function (PSF)-fitting techniques implemented with StarFinder (Diolaiti et
al., 2000). Source positions were identified on both IRAC mosaics separately,
and subsequently combined into a single two-band catalog using a 1$\arcsec$
matching radius, whilst retaining single-detected sources as well for
completeness. Finally, the catalog contains a total of $\sim 350\,000$
galaxies down to 4$\sigma$ limits of roughly 25.0 AB mag in both IRAC bands.
Over the PRIMER area, the IRAC data reach average $5\sigma$ depths of $\sim$
26 mag.

###  2.2 HST and JWST imaging

In this work, we made use of archival HST and newly obtained JWST imaging to
uncover the true nature of potential $HK_{\rm s}$-dropout galaxies. We have
used the JWST/NIRCam data collected during the recent GO Cycle 1 program
PRIMER (PID: 1837; PI: James Dunlop). PRIMER observations are conducted in
eight NIRCam broad and medium bands, - F090W, F115W, F150W, F200W, F277W,
F356W, F410M, and F444W, and two MIRI bands, i.e. F770W and F1800W, over two
CANDELS legacy fields (Grogin et al., 2011; Koekemoer et al., 2011) in COSMOS
and UDS (Lawrence et al., 2007; Cirasuolo et al., 2007). Here, we consider all
observations of target COSMOS-2 , the first-released subsection of the full
PRIMER target area, taken up until January 6 2023, with a total integration
time of 36.1 hours covering an area of $\sim$94 square arcminutes. We did not
utilize the available MIRI data as they are offset from the NIRCam data, nor
did we use NIRCam/F150W-band imaging as it had not been taken yet.

We have processed the JWST/NIRCam data in the following way. First, we
retrieved the level-2 data products from the MAST archive 111Data available at
https://archive.stsci.edu/doi/resolve/resolve.html?doi=10.17909/bysp-ds64
(catalog 10.17909/bysp-ds64) and processed them with the grizli pipeline
(version 1.7.7; Brammer & Matharu 2021). We took particular care when
introducing additional corrections for the NIRCam photometric zeropoints
relative to the Calibration Reference Data System pipeline mapping (CRDS;
pmap) 1041, including variations across the detector itself
222https://doi.org/10.5281/zenodo.7143382 (catalog 10.5281/zenodo.7143382).
These were found to be consistent with the efforts by other groups (Boyer et
al., 2022; Nardiello et al., 2022). We additionally introduce corrections and
masking to account for the detrimental effect of the cosmic rays and stray
light in the detector (see Bradley et al., 2022). More detailed descriptions
of this procedure are presented in Kokorev et al. (2023) and Valentino et al.
(2023), and will be further expanded upon in Brammer et al. (in prep.).

We supplement the JWST observations with the available HST optical and near-
infrared data available in the Complete Hubble Archive for Galaxy Evolution
(CHArGE; Kokorev et al., 2022). These data include imaging in the HST/WFC3
F125W, F140W, F160W, and HST/ACS F435W, F606W, F814W bands. We do not include
the available data in WFC3/F105W and ACS/F475W bands as these cover only a
small fraction of the PRIMER region. All available JWST and HST exposures were
aligned to the Gaia DR3 (Gaia Collaboration et al., 2023) astrometry, co-
added, and drizzled (Fruchter & Hook, 2002) onto two sets of grids. We used
$0\farcs 02$ pixel scale for the Short Wavelength (SW) NIRCam bands and
$0\farcs 04$ for all the remaining JWST and HST filters. For comparison, the
Full Width at Half Maximum (FWHM) of the NIRCAM PSF adopted in this work is
$0\farcs 03$ in the F090W band.

The depth in each band considered throughout this work is shown in Fig. 1,
measured from 2$"$ empty apertures in the field.

Figure 1: Depth (5$\sigma$) and associated uncertainty in each filter
considered throughout this work, measured as the median and standard deviation
of the local background in 30 by $30\arcsec$ regions over the PRIMER field
respectively. In each region, the local background is the standard deviation
of the flux measurement in empty $2\arcsec$ circular diameter apertures. The
horizontal width of each datapoint indicates the Full Width at Half Maximum
(FWHM) for the filter.

##  3 Initial sample selection from IRAC and VIRCAM imaging

Our analysis is based on a selection of IRAC-detected sources in the PRIMER
region that are undetected in the UltraVISTA DR5 VIRCAM $H$\- and/or $K_{\rm
s}$-band images.

We created from scratch an initial UltraVISTA DR5 catalog to cross-match with
the IRAC combined 3.6 and 4.5 $\mu$m catalog from Ashby et al. (2018). We
performed source detection on the UltraVISTA $H$\- and $K_{\rm s}$-band DR5
images separately using SourceExtractor (Bertin & Arnouts, 1996). We adopted a
$1.5\sigma$ detection threshold over 5 contiguous pixels. We used
SourceExtractor in dual-image mode to measure supporting $Y$, $J$, and NB118
fluxes at the locations of the $H$\- and $K_{\rm s}$-detected sources. To
ensure we catch all the light for the flux measurements, we extracted both the
SExtractor ’mag_auto’ and the aperture magnitude measured from $2\arcsec$
apertures. Subsequently, for each band individually, we chose between these
those flux measurements based on the local $10\sigma$ depth, such that we
favour aperture fluxes for faint sources. The UltraVISTA images are not PSF-
matched, but this poses no concern as we derive aperture flux corrections for
each filter separately from the curve-of-growth of bright stars in the field.
We applied corrections for galactic dust extinction using the Schlafly &
Finkbeiner (2011) dust maps with the Fitzpatrick (1999) reddening law.

In total, the PRIMER region as considered in this work contains 11 651 and 10
633 $H$\- and $K_{\rm s}$-detected sources respectively, and 7057 IRAC-
detected galaxies from the Ashby et al. (2018) catalog. We cross-matched all
UltraVISTA sources that have $>2\sigma$ flux detections in the $Y$, $J$, $H$,
$K_{\rm s}$, and NB118 bands with the IRAC galaxies within a $1\arcsec$ search
radius. We kept all IRAC sources that are unmatched with any UltraVISTA source
within this search radius, yielding 1441 potential $K_{\rm s}$-dropout
candidates and 148 potential $H$-dropout candidates.

We proceeded to carefully inspect these candidates through various measures.
First, we used the JWST/NIRCam F444W imaging to check the validity of the IRAC
sources, as any IRAC source is expected to be visible in these data of
superior depth and resolution. We identified the JWST sources by running
SourceExtractor on the F444W image with a detection threshold of 1.5$\sigma$
over 3 contiguous pixels and a `gauss_3.0_3x3.conv` convolution mask. The IRAC
sources were cross-matched with the F444W-detected sources in a $1\arcsec$
search radius, and any IRAC source with no JWST source located in its aperture
was removed from the sample, that being 32 and 3 % of the initial $K_{\rm
s}$\- and $H$-dropout candidate samples respectively. From visual inspection,
we determined that 93 % of these removed candidates are falsely-detected
residuals of bright IRAC galaxies.

Our second check was to cross-match the dropout candidates with the HSC bright
star masks (Aihara et al., 2022) and Gaia DR3 star catalog (Gaia Collaboration
et al., 2023). Any IRAC source located within the HSC masks or within a
$3\arcsec$ radius of a Gaia source was removed from the sample, that being 16
and 41 % of the initial $K_{\rm s}$\- and $H$-dropout candidate samples
respectively. This was done to avoid flux contamination from the bright,
nearby star due to the broad IRAC PSF.

For our third and final check, we removed any dropout candidate that has
significant UltraVISTA fluxes. For example, an intrinsically faint galaxy with
strong emission lines may be undetected in the $K_{\rm s}$ image, but be
perfectly visible in the $Y$ or $J$ band. To measure the VIRCAM fluxes of the
dropout candidates, we performed forced aperture photometry on the VIRCAM
images at the IRAC positions, using the Python modules Astropy (version 5.0.4;
Astropy Collaboration et al. 2022) and Photutils (version 1.4.1; Bradley et
al. 2022), as contrary to SourceExtractor, one can directly supply a list of
source locations to these software. The photometry was measured on the $Y$,
$J$, $H$, $K_{\rm s}$, and NB118 bands in $2\arcsec$ apertures. We derived the
flux errors as the standard deviation of the $3\sigma$-clipped (five
iterations) background flux distribution, measured from empty apertures placed
over a 10 by 10$\arcsec$ region around the source. We required a minimum error
of 0.1 magnitude error, corrected the fluxes to total, and applied galactic
extinction corrections.

In principle, we discarded all $K_{\rm s}$-dropout candidates that have
$>2\sigma$ flux measurements in any of the $Y$, $J$, $H$, $K_{\rm s}$, and
NB118 bands. Similarly, we considered the $Y$, $J$, $H$, and NB118 flux
measurements for the $H$-dropout candidates. However, we carefully visually
inspected every dropout candidates discarded by this measure, as the flux
errors can be affected by a crowded background, and decided to keep 4 $K_{\rm
s}$\- 4 and $H$-dropout candidates in our final dropout candidate sample for
this reason. Moreover, upon visual inspection of the dropout candidates with
no significant VIRCAM fluxes, we find that between both the $K_{\rm s}$\- and
$H-$ dropout candidates, roughly a third are IRAC residual detections. These
detections survived our earlier checks, because they coincidentally match with
a very faint F444W-detected source. However, the focus of our work are
isolated IRAC galaxies with a direct correspondence to a JWST-detected source,
so that we discard these residual detections from the final sample. In
addition, we discarded another $\sim 20$ % of the galaxies with no significant
VIRCAM flux measurements because we visually confirmed the presence of a
convincing source in these bands, and another $10$ % because the VIRCAM and
IRAC imaging are highly contaminated by the presence of a nearby bright
source.

After implementing the above-mentioned checks, our final sample consists of 18
and 8 $K_{\rm s}$\- and $H$-band dropout candidates respectively. This amounts
to 0.4 % of all IRAC-detected sources in PRIMER from the Ashby et al. (2018)
catalog. The IRAC magnitudes of the final dropouts range between 23.1-27.0
mag, with the signal-to-noise ratio (SNR) varying between 3.5 and 10.9. For
comparison, considering all IRAC-detected sources over the PRIMER field, the
median 3.6 and 4.5 $\mu$m magnitudes are 23.6 and 23.7 with typical SNRs $\sim
40$.

##  4 HST and JWST photometry of the dropout candidates

Now that we have selected a sample of apparent dropout galaxies from VIRCAM
and IRAC data, we turn to the JWST and HST imaging to study these sources in
detail.

Given the superior resolution of the JWST imaging, instead of using the IRAC
priors to measure the NIRCam aperture photometry, we detected sources directly
on the F277W/F356W/F444W stacked image using SourceExtractor. Similar to the
extraction of sources on the singular F444W image in the previous section, we
required a detection threshold of 1.5$\sigma$ over 3 contiguous pixels and
convolved the data with a `gauss_3.0_3x3.conv` mask. In addition, we performed
identical runs on the F200W and F277W images alone, as some of the dropout
candidates suffer from source blending in the longest wavelength JWST
channels. We cross-matched our JWST-detected sources with the IRAC positions
of the dropout candidates, and retained all within a $1\arcsec$ radius of the
candidates, i.e., all that would fall within the IRAC $2\arcsec$ apertures
adopted throughout this work. In 11 out of the 26 UltraVISTA dropout
candidates, the JWST imaging shows multiple counterparts. For each
counterpart, we measure the HST and JWST  apertures fluxes individually. In
total, the cross-matching with the IRAC priors yields 51 JWST-detected
galaxies.

To measure the HST and JWST photometry of the cross-matched galaxies, we again
performed forced aperture photometry with Astropy and Photutils. We emphasize
that we did not create a fully realized JWST-detected source catalog over the
PRIMER field for this study. We measured fluxes in both $0\farcs 5$ and
$1\arcsec$ diameter circular apertures at the positions of the JWST-detected
sources on 13 optical and near-infrared bands, spanning the wavelength range
0.4–4.4 $\mu$m. We decided between the $0\farcs 5$ and $1\arcsec$ diameter
apertures on a source-by-source basis from careful visual inspection of the
NIRCam images. Similar to the routine described in the previous section, we
derived the flux errors as the standard deviation of the $3\sigma$-clipped
(five iterations) background flux distribution, measured from empty apertures
placed over a 10 by 10$"$ region around the source. We required a minimum
error of 0.05 magnitude error for both the HST and JWST photometry. Assuming a
point-source morphology, the HST aperture fluxes were corrected to total using
the median curve-of-growth of stars in the field. Aperture corrections for the
JWST fluxes were estimated using the WebbPSF 333WebbPSF is available at
https://webbpsf.readthedocs.io/en/latest/ software instead, as virtually all
stars in PRIMER-COSMOS are saturated in the NIRCam imaging. We applied
corrections for galactic dust extinction. In bands with non-detections, we
adopted $3\sigma$ flux upper limits derived from the empty aperture fluxes.

##  5 Results

###  5.1 SED fitting based on HST and JWST photometry

In this section, we describe how we derived redshifts and physical parameters
based on the HST and JWST photometry of the 26 dropout candidates. To ensure
the validity of the photometric redshifts for these sources, we ran three
separate redshifts codes: LePhare, EAZY, and Prospector. The latter two codes
are strictly used to verify the redshift results produced by LePhare. We
describe the routines adopted for the three codes in the following paragraphs.

First, the $\chi^{2}$-fitting algorithm LePhare (Arnouts et al., 1999; Ilbert
et al., 2006) uses a set of galaxy templates created from stellar population
synthesis to find the best-fit galaxy solution to the observed photometry
using a $\chi^{2}$-minimisation technique, thereby deriving the physical
parameters of the observed source. The galaxy models adopted in this work were
sampled from the GALAXEV library (Bruzual & Charlot, 2003).

To compile our galaxy models, we adopt different star formation histories
(SFHs): a single stellar population and two parametric SFHs, i.e. the star
formation rate (SFR) is an analytical function of time. For the latter, we
adopted an exponentially declining SFH ($\rm{SFR}\propto e^{-t/\tau}$) and a
delayed exponentially declining ($\rm{SFR}\propto te^{-t/\tau}$), using the
range of star formation timescales $\tau=0.01,0.1,0.3,1.0,3.0,5.0,10.0$, and
15 Gyr. We considered two metallicities: solar ($Z=Z_{\odot}$) and sub-solar
($Z=0.2Z_{\odot}$). This results in 36 different combinations of SFH and
metallicity for our considered SED models.

For dust attenuation, we adopted the Calzetti et al. (2000) reddening law and
left the color excess as a free parameter between $E(B-V)=0$ and 1, with a
step of 0.1. For the galaxy ages, we adopted a grid of values spanning
0.01–13.5 Gyr. We applied emission lines to the galaxy templates using
LePhare, which are modelled using scaling relations from Kennicutt (1998)
between the UV luminosity and [OII] line (see Ilbert et al. 2009 for a
detailed description). Absorption of emission at wavelengths shorter than
rest-frame $912$ Å by the intergalactic medium (IGM) was implemented following
Madau (1995). Finally, any templates that produce fluxes exceeding the
3$\sigma$ flux upper limits in non-detected bands are rejected by LePhare.

If our candidates drop out of the UltraVISTA bands because these sample the
Lyman break, the sources could be at redshifts as high as $z\sim 17$.
Therefore, we performed two runs with different redshift ranges: one spanning
$z=0$–9 and one extending to $z=17$. Similarly, the dropout nature could be
caused by extreme dust reddening, such that we ran one additional SED fit with
$z=0$–9 but $E(B-V)$ extending to 1.5. We subsequently compared the reduced
$\chi^{2}$ of the three fits for each source, where we adopt the results from
the run that yields the smallest $\chi^{2}_{\nu}$ value. We adopted this
approach in an effort to mitigate the effects of photometric redshifts
degeneracy, which red galaxies are particularly prone to.

Second, we used the Python version of EAZY (Brammer et al., 2008). EAZY
employs a set of non-negative linear combinations of basic templates
constructed with the Flexible Stellar Population Synthesis models (FSPS;
Conroy et al. 2009; Conroy & Gunn 2010). Specifically, this analysis used the
corr_sfhz_13 subset of models. These contain redshift-dependent SFHs, which at
a given redshift are restricted by the age of the Universe. In addition, the
maximum allowed dust attenuation is tied to a given epoch. We also included
the best-fit template to the JWST-observed extreme emission line galaxy at
$z=8.5$ (ID4590) from Carnall et al. (2022), which was re-normalized in
alignment with the FSPS models. This was done in order to model potentially
large equivalent width emission lines.

We adopted the EAZY template error function, to accommodate additional
uncertainty related to unusual stellar populations, using the default value of
0.2 for the template error. We considered a redshift range of $z=0.01$–17 for
this run. Because EAZY does not allow for the same treatment of non-detected
bands as LePhare, we instead adopted the $1\sigma$ flux upper limit as a flux
measurement for the non-detected bands in the fitting process.

Lastly, we made use of the Bayesian inference code Prospector (Johnson et al.,
2021). Similarly to EAZY, its stellar population models are based on the FSPS
code, but Prospector rather builds the galaxy SED models on the fly using the
Python-FSPS bindings. Prospector is well-known for its ability to model
complex, non-parametric SFHs (see e.g. Leja et al. 2017; Tacchella et al.
2022). However, given that our intend is to generate a photometric redshift
with Prospector in order to verify our LePhare results, we stick to a simple
delayed exponentially declining parametric SFH.

The parametric model involves seven free parameters, for which we chose broad
priors comparable to the LePhare run. For each parameter, we used the default
prior shapes with the following ranges: the redshift $z=0$–17, the formed
stellar mass $M_{*}=10^{6}$–$10^{12}M_{\odot}$, the metallicity
$\log(Z/Z_{\odot})=-2$–0.19, the e-folding time $\tau_{\rm SF}=0.001$–15 Gyr,
and the age $t_{age}=0.001$–13.8 Gyr. We modelled diffuse dust attenuation
following Calzetti et al. (2000) with $\tau_{dust}=0$–4. Lastly, we
implemented IGM absorption following Madau (1995) and nebular emission, using
the default parameters. Finally, Prospector also treats non-detections
differently, such that we set the flux to zero and the flux uncertainty as the
$1\sigma$ upper limit in the corresponding bands.

####  5.1.1 One-to-one comparison between IRAC- and JWST-detected galaxies

In Sect.5.1.2, we discuss the results from our SED fitting on the HST and JWST
photometry of all JWST-detected galaxies that are counterparts to our 26
UltraVISTA dropout candidates. As we remind the reader, the dropout candidates
cross-match with 51 JWST-detected galaxies. Therefore, for each system, the
one-to-one correspondence between the IRAC- and JWST-detected galaxies varies,
as we discover from the combined inspection of the JWST source detection, the
LePhare-derived photometric redshifts, and the image cutouts of the 26
UltraVISTA dropout candidates in the HST, JWST, IRAC, and ancillary ground-
based bands (VIRCAM and HSC). In the interest of clarity and structure, we
therefore first discuss the sub-classification of the 26 dropout candidates
before we present the photometric redshift evaluation in the next section.

The dropout image cutouts of the candidates can be found in Appendix A, in
Figures 10, 12, and 14.

First, we have checked if the UltraVISTA dropout candidates are true dropout
galaxies. Following the strategy applied on the VIRCAM photometry, we checked
if the JWST-detected counterparts have $>2\sigma$ flux measurements in the HST
and JWST bands blueward of the NIRCAM/F200W and WFC3/F160W bands for the
$K_{\rm s}$\- and $H$-dropout candidates respectively. We find that out of the
26 dropout candidates, 4 systems display no significant flux in these bands; 2
$K_{\rm s}$\- and 2 $H$-band dropout candidates are true dropout galaxies.

Second, in 15 out of the 26 candidates, the superior resolution of the HST and
JWST imaging compared to the IRAC data enables us to identify multiple sources
residing in the 2$"$ diameter IRAC apertures. In fact, only 11 out of 26
candidates appear as one galaxy in JWST, i.e., there is a one-to-one
correspondence with the IRAC detection. The other 15 candidates are composed
of multiple sources. Among these systems, 10 cases consist of a primary galaxy
with one or more non-associated foreground sources contaminating the aperture
photometry, boosting the flux of the primary source. The remaining 5
candidates are systems of galaxies that are partially associated with each
other. Throughout this paper, we refer to these categories as (11) secure
galaxies, (10) deboosted galaxies, and (5) associated galaxies.

As mentioned before, the cross-matching with the IRAC priors yields 51 JWST-
detected galaxies, although the contaminating sources of the deboosted sample
are omitted from our analysis, as these low-redshift foreground galaxies are
beyond the focus of our work. Therefore, the total number of JWST-detected
galaxies we discuss throughout this work is 35.

####  5.1.2 Evaluation of the HST- and JWST-derived photometric redshifts

Now that we have explained how the UltraVISTA dropout candidates are
classified in different categories based on their JWST-detected counterparts,
we discuss the redshift results from the three separate SED fitting routines
performed with LePhare, EAZY, and Prospector.

Table 1 shows the best-fit photometric redshifts obtained for each of the
relevant 35 JWST-detected galaxies, as well as the source position,
[F160W-F444W] color and size of the aperture used to measure the HST and JWST
photometry. We distinguish between candidates originally identified as $H$\-
and $K_{\rm s}$-dropouts candidates by means of their ID. We also group the
sources in Table 1 by their respective classes (secure, deboosted, and
associated) for clarity. The LePhare and EAZY photometric redshifts
corresponds to the model that minimizes the $\chi^{2}_{\nu}$; for Prospector,
the median of the probability distribution function of the redshift, PDF($z$),
is reported. In all three cases, the uncertainty on the photometric redshift
correspond to the 16 and 84 percentiles.

We visualize our redshift results in Fig. 2, where we show for each JWST-
detected galaxy the PDF($z$) from Prospector, as well as the best-fit
redshifts and uncertainty ranges from LePhare and EAZY. We also indicate the
secondary redshift form LePhare, i.e., the competing model that minimizes the
$\chi^{2}_{\nu}$ second-best.

Figure 2: Photometric redshift distribution of the dropout candidates derived
from SED fitting the HST and JWST photometry. Each panel shows one of the 35
JWST-detected counterparts to the UltraVISTA dropout candidates considered in
this work, depicting the probability density function of the redshift produced
by Prospector in blue. The best-fit photometric redshift solutions from
LePhare and EAZY are shown with pink dashed and green dotted vertical lines
respectively. We also show for the LePhare and EAZY redshift solutions the 16
and 84 percentiles by means of correspondingly colored shaded regions.
Finally, the secondary redshift from LePhare when available is shown in a
solid pink vertical line.

Table 1: Photometric redshifts of JWST-detected galaxies located within the
IRAC apertures

ID | RA | Dec | [F160W-F444W] | Aperture | Photometric redshift
---|---|---|---|---|---
|  |  | (AB mag) | $"$ | LePhare | EAZY | Prospector
PD-H-s-1 |  | 10:00:30.42 | +02:26:17.16 | 3.6 | 0.5 | $3.28^{+0.10}_{-0.10}$ | $3.53^{+0.06}_{-0.28}$ | $3.39^{+0.09}_{-0.12}$
PD-H-s-2 |  | 10:00:27.98 | +02:25:29.62 | 3.8 | 1.0 | $3.32^{+0.17}_{-0.13}$ | $3.53^{+0.18}_{-0.22}$ | $12.34^{+0.38}_{-8.84}$
PD-H-s-3 |  | 10:00:47.04 | +02:19:56.52 | 1.5 | 0.5 | $4.21^{+0.12}_{-0.13}$ | $1.67^{+2.33}_{-0.01}$ | $1.73^{+0.11}_{-0.09}$
PD-Ks-s-4 |  | 10:00:36.19 | +02:15:50.76 | 2.4 | 0.5 | $4.71^{+0.09}_{-0.15}$ | $4.34^{+0.22}_{-0.13}$ | $5.74^{+0.26}_{-0.1.00}$
PD-Ks-s-5 |  | 10:00:33.86 | +02:20:18.15 | 2.4 | 0.5 | $4.78^{+0.04}_{-0.02}$ | $4.45^{+0.87}_{-0.07}$ | $4.80^{+0.95}_{-0.06}$
PD-Ks-s-6 bbThis qualifies as a true dropout galaxy, such that it has no significant flux detections in the short-wavelength HST and JWST channels. |  | 10:00:24.15 | +02:20:05.40 | 4.1 | 1.0 | $4.68^{+2.03}_{-0.25}$ | $3.83^{+0.42}_{-1.29}$ | $4.48^{+1.78}_{-0.46}$
PD-Ks-s-7 |  | 10:00:40.80 | +02:26:37.37 | 3.5 | 0.5 | $4.75^{+0.06}_{-0.28}$ | $4.35^{+0.07}_{-1.37}$ | $4.62^{+0.15}_{-0.26}$
PD-Ks-s-8 |  | 10:00:35.34 | +02:28:26.63 | 2.9 aaThe [F160W-F444W] color of this source is a based on a flux upper limit in F160W so it represents a lower limit on the redness of this color.  | 1.0 | $5.63^{+0.00}_{-0.04}$ | $3.53^{+0.16}_{-0.73}$ | $10.97^{+0.66}_{-7.37}$
PD-Ks-s-9 |  | 10:00:28.73 | +02:16:01.25 | 4.9 | 0.5 | $4.75^{+0.06}_{-0.24}$ | $4.42^{+0.33}_{-0.11}$ | $4.71^{+0.08}_{-0.17}$
PD-Ks-s-10 |  | 10:00:48.53 | +02:27:48.77 | 2.2 | 0.5 | $5.85^{+0.39}_{-1.00}$ | $4.30^{+0.28}_{-2.32}$ | $5.47^{+0.47}_{-0.90}$
PD-Ks-s-11 bbThis qualifies as a true dropout galaxy, such that it has no significant flux detections in the short-wavelength HST and JWST channels. |  | 10:00:27.03 | +02:24:24.03 | 3.8 | 1.0 | $3.51^{+0.19}_{-0.35}$ | $3.53^{+0.23}_{-0.97}$ | $3.54^{+0.90}_{-0.27}$
PD-H-d-1  |  | 10:00:41.83 | +02:25:47.02 | 3.2 aaThe [F160W-F444W] color of this source is a based on a flux upper limit in F160W so it represents a lower limit on the redness of this color.  | 1.0 | $5.73^{+0.54}_{-0.14}$ | $3.23^{+0.48}_{-0.54}$ | $6.00^{+5.16}_{-2.28}$
PD-H-d-bbThis qualifies as a true dropout galaxy, such that it has no significant flux detections in the short-wavelength HST and JWST channels. 2 |  | 10:00:22.44 | +02:23:40.99 | 2.9 | 0.5 | $3.32^{+0.23}_{-0.16}$ | $3.39^{+0.18}_{-0.97}$ | $11.74^{+0.66}_{-8.52}$
PD-H-d-3 |  | 10:00:38.31 | +02:25:44.22 | 1.8 | 1.0 | $1.99^{+0.51}_{-0.09}$ | $1.45^{+1.16}_{-0.04}$ | $1.91^{+0.45}_{-0.50}$
PD-H-d-4 bbThis qualifies as a true dropout galaxy, such that it has no significant flux detections in the short-wavelength HST and JWST channels. |  | 10:00:28.94 | +02:25:05.13 | 6.2 | 1.0 | $3.25^{+0.26}_{-0.07}$ | $3.50^{+0.18}_{-0.24}$ | $3.49^{+0.12}_{-0.15}$
PD-Ks-d-5 |  | 10:00:37.48 | +02:15:57.82 | 2.7 | 1.0 | $4.44^{+0.47}_{-0.30}$ | $3.88^{+0.66}_{-0.00}$ | $4.75^{+0.08}_{-0.30}$
PD-Ks-d-6 |  | 10:00:45.86 | +02:22:02.26 | 2.0 | 0.5 | $3.62^{+2.64}_{-0.16}$ | $1.63^{+2.11}_{-0.02}$ | $3.43^{+0.32}_{-1.67}$
PD-Ks-d-7 |  | 10:00:31.20 | +02:25:04.07 | 1.6 | 1.0 | $5.21^{+0.06}_{-0.06}$ | $3.46^{+0.01}_{-0.22}$ | $5.19^{+0.07}_{-0.12}$
PD-Ks-d-8 |  | 10:00:26.31 | +02:19:38.36 | 0.6 | 0.5 | $3.82^{+0.20}_{-1.23}$ | $3.80^{+0.12}_{-1.08}$ | $3.83^{+0.34}_{-0.58}$
PD-Ks-d-9 |  | 10:00:45.96 | +02:18:52.85 | 2.7 | 0.5 | $3.47^{+0.24}_{-0.69}$ | $3.62^{+0.21}_{-1.38}$ | $3.22^{+0.51}_{-1.02}$
PD-Ks-d-10 |  | 10:00:27.78 | +02:25:52.04 | 2.9 aaThe [F160W-F444W] color of this source is a based on a flux upper limit in F160W so it represents a lower limit on the redness of this color.  | 1.0 | $3.93^{+0.87}_{-0.13}$ | $2.51^{+1.52}_{-0.12}$ | $5.97^{+0.47}_{-1.72}$
PD-H-a-1/2 | a | 10:00:37.73 | +02:16:55.07 | 0.0 | 0.5 | $2.82^{+0.48}_{-2.11}$ | $2.79^{+1.04}_{-1.15}$ | $2.47^{+2.14}_{-1.36}$
b | 10:00:37.78 | +02:16:54.53 | 2.2 | 0.5 | $3.61^{+0.09}_{-0.09}$ | $3.48^{+0.05}_{-0.60}$ | $5.17^{+0.03}_{-0.04}$
c | 10:00:37.75 | +02:16:54.42 | 0.7 | 0.5 | $3.61^{+1.59}_{-0.11}$ | $3.47^{+1.68}_{-0.14}$ | $5.16^{+0.03}_{-0.04}$
d | 10:00:37.72 | +02:16:54.34 | 1.3 | 0.5 | $5.21^{+0.00}_{-0.02}$ | $5.16^{+0.04}_{-0.03}$ | $5.20^{+0.06}_{-0.04}$
e | 10:00:37.71 | +02:16:54.01 | 2.3 | 0.5 | $5.21^{+0.00}_{-0.01}$ | $5.14^{+0.05}_{-0.02}$ | $5.18^{+0.03}_{-0.03}$
f | 10:00:37.69 | +02:16:53.56 | 2.3 | 0.5 | $5.18^{+0.04}_{-0.01}$ | $5.14^{+0.08}_{-0.04}$ | $5.16^{+0.05}_{-0.05}$
g | 10:00:37.68 | +02:16:52.74 | -0.3 | 0.5 | $0.77^{+0.11}_{-0.13}$ | $0.79^{+0.16}_{-0.14}$ | $0.70^{+0.13}_{-0.13}$
PD-Ks-a-3 | a | 10:00:40.04 | +02:23:16.75 | -0.1 | 0.5 | $1.52^{+0.10}_{-0.13}$ | $1.53^{+0.08}_{-0.17}$ | $1.52^{+0.19}_{-0.18}$
b | 10:00:40.02 | +02:23:16.57 | 0.0 | 0.5 | $1.57^{+0.05}_{-0.29}$ | $1.59^{+0.04}_{-0.20}$ | $1.47^{+0.13}_{-0.14}$
PD-Ks-a-4 | a | 10:00:45.05 | +02:24:41.74 | 0.6 | 0.5 | $2.86^{+0.33}_{-1.46}$ | $2.80^{+0.59}_{-0.98}$ | $3.02^{+0.61}_{-0.76}$
b | 10:00:45.10 | +02:24:41.38 | 0.2 | 0.5 | $3.36^{+1.81}_{-2.30}$ | $3.56^{+0.66}_{-1.73}$ | $1.38^{+3.55}_{-0.99}$
c | 10:00:45.07 | +02:24:40.66 | 1.8 aaThe [F160W-F444W] color of this source is a based on a flux upper limit in F160W so it represents a lower limit on the redness of this color.  | 0.5 | $4.76^{+0.09}_{-0.77}$ | $2.49^{+3.02}_{-0.02}$ | $4.55^{+1.76}_{-0.49}$
PD-Ks-a-5 | a | 10:00:31.03 | +02:16:56.00 | 1.5 | 0.5 | $5.74^{+0.03}_{-0.05}$ | $1.50^{+4.59}_{-0.01}$ | $6.89^{+0.14}_{-0.10}$
b | 10:00:31.03 | +02:16:55.77 | 2.3 | 0.5 | $5.75^{+0.27}_{-0.05}$ | $5.65^{+0.59}_{-3.50}$ | $6.82^{+0.09}_{-0.05}$

Note. — Photometric redshifts derived from SED fitting the HST and JWST of the
JWST-detected counterparts to the dropout candidates, using three separate
routines with SED fitting codes LePhare, EAZY, and Prospector. We also
indicate their right ascension and declination as measured on the JWST images,
the [F160W-F444W] color, and what diameter circular aperture was used to
measure the photometry. The ID of each entry indicates the type of the
associated IRAC-selected dropout candidate, i.e. $H$\- or
$K_{\mathrm{s}}$-band dropout. The three subsamples, i.e. secure, deboosted,
and associated, are divided by horizontal lines for readability. The
contaminant galaxies in the deboosted sample are omitted from this table.

We consider the Lephare best-fit redshift valid if it satisfies the criterion

$\frac{|z-z_{\rm LePhare}|}{1+z_{\rm LePhare}}<0.15\,$ (1)

where $z$ corresponds to either the EAZY or Prospector photometric redshift.
This is the case for the vast majority of our sample, with the exception of
the following four sources: PD-H-s-3, PD-Ks-s-8, PD-Ks-d-10, and PD-Ks-a-5a.
Given that their photometric redshifts and therefore physical parameters are
unreliable, we would discard them from any further analysis. However, PD-H-s-3
and PD-K-s-8 have secondary LePhare redshifts coinciding with the EAZY
results, so we adopt the LePhare physical parameters associated with the
secondary redshift solution. PD-Ks-d-10 is adjusted in a similar way, as its
secondary redshift coincides with the Prospector solution. Lastly, PD-Ks-a-5a
may have conflicting redshift results between the three SED fitting routines,
but this is not the case for companion galaxy PD-Ks-a-5b, for which the
LePhare redshift is backed up by the EAZY result. Given the similarity between
the two sources and the large uncertainty on the EAZY redshift of PD-Ks-a-5a,
we decide to keep the LePhare redshift for PD-Ks-a-5a. Therefore, none of the
four JWST-detected galaxies with uncertain redshifts are discarded from the
sample, but their LePhare-based physical parameters are modified accordingly.

####  5.1.3 General SED-derived properties of the JWST-detected galaxies

In this section, we qualitatively analyze the physical parameters of the 35
JWST-detected counterparts to the UltraVISTA dropout candidates as derived
with LePhare. We show the finalized LePhare photometric redshift distribution
as discussed in the previous section in Fig. 3. The vast majority of our
sample is located between $z=3$–6. Moreover, the galaxies are relatively
grouped in redshift space at $z\sim 3.3$, $z\sim 4.8$, and $z\sim 5.7$.

Figure 3: Photometric redshift distribution of the dropout candidates derived
from the LePhare SED fitting the HST and JWST photometry.

The physical parameters as determined with LePhare are shown in Table 2,
including the photometric redshift, $\chi^{2}_{\nu}$, color excess, stellar
mass, and age.

Table 2: Physical parameters of JWST-detected galaxies located within the IRAC
apertures

ID | Redshift | $\chi^{2}_{\nu}$ | Color Excess | Stellar Mass | Age
---|---|---|---|---|---
|  |  | $E(B-V)$ | ($\log[M/M_{\odot}]$) | ($\times 10^{7}$ yr)
PD-H-s-1 |  | $3.28^{+0.10}_{-0.10}$ | 0.97 | 0.7 | $10.02^{+0.14}_{-0.18}$ | 4.0
PD-H-s-2 aaDetected at X-ray wavelengths in Chandra/XMM-Newton (Cappelluti et al., 2009; Civano et al., 2016) |  | $3.32^{+0.17}_{-0.13}$ | 0.96 | 0.9 | $9.99^{+0.13}_{-0.14}$ | 3.5
PD-H-s-3 ccThe redshift and physical parameters presented for this source are based on the second best-fit model from LePhare, which does agree with the redshift results from EAZY and/or Prospector.  |  | $1.61^{+0.00}_{-0.02}$ | 1.29 | 0.7 | $8.39^{+0.29}_{-0.08}$ | 1.0
PD-Ks-s-4 |  | $4.71^{+0.09}_{-0.15}$ | 1.50 | 0.5 | $9.48^{+0.10}_{-0.09}$ | 5.7
PD-Ks-s-5 |  | $4.78^{+0.04}_{-0.02}$ | 2.70 | 0.6 | $10.19^{+0.09}_{-0.10}$ | 29
PD-Ks-s-6 bbDetected at sub-mm wavelengths with ALMA/SCUBA/VLA (Liu et al., 2019; Koprowski et al., 2016; Smolčić et al., 2017) |  | $4.68^{+2.03}_{-0.25}$ | 0.16 | 0.7 | $9.66^{+0.23}_{-0.28}$ | 1.0
PD-Ks-s-7 |  | $4.75^{+0.06}_{-0.28}$ | 0.36 | 0.7 | $9.98^{+0.14}_{-0.15}$ | 16
PD-Ks-s-8 b,cb,cfootnotemark:  |  | $3.48^{+0.23}_{-0.13}$ | 0.69 | 0.8 | $9.90^{+0.24}_{-0.25}$ | 2.1
PD-Ks-s-9 |  | $4.75^{+0.06}_{-0.24}$ | 2.84 | 0.7 | $10.10^{+0.12}_{-0.12}$ | 18
PD-Ks-s-10 |  | $5.85^{+0.39}_{-1.00}$ | 0.22 | 0.3 | $10.12^{+0.09}_{-0.11}$ | 16
PD-Ks-s-11 |  | $3.51^{+0.19}_{-0.35}$ | 0.31 | 0.8 | $9.96^{+0.25}_{-0.21}$ | 1.5
PD-H-d-1 bbDetected at sub-mm wavelengths with ALMA/SCUBA/VLA (Liu et al., 2019; Koprowski et al., 2016; Smolčić et al., 2017) |  | $5.73^{+0.54}_{-0.14}$ | 0.88 | 0.7 | $10.35^{+0.19}_{-0.18}$ | 1.0
PD-H-d-2 |  | $3.32^{+0.23}_{-0.16}$ | 0.23 | 0.5 | $9.78^{+0.17}_{-0.24}$ | 13
PD-H-d-3 |  | $1.99^{+0.51}_{-0.09}$ | 1.23 | 0.2 | $9.93^{+0.10}_{-0.24}$ | 300
PD-H-d-4 |  | $3.25^{+0.26}_{-0.07}$ | 0.88 | 1.0 | $10.27^{+0.17}_{-0.22}$ | 3.5
PD-Ks-d-5 |  | $4.44^{+0.47}_{-0.30}$ | 1.3 | 0.5 | $9.22^{+0.19}_{-0.15}$ | 1.0
PD-Ks-d-6 |  | $3.62^{+2.64}_{-0.16}$ | 0.49 | 0.4 | $9.13^{+0.23}_{-0.24}$ | 2.5
PD-Ks-d-7 |  | $5.23^{+0.07}_{-0.06}$ | 0.90 | 0.3 | $9.47^{+0.13}_{-0.13}$ | 4.0
PD-Ks-d-8 |  | $3.82^{+0.20}_{-1.23}$ | 0.20 | 0.1 | $8.53^{+0.21}_{-0.26}$ | 36
PD-Ks-d-9 |  | $3.47^{+0.24}_{-0.69}$ | 0.14 | 0.6 | $9.46^{+0.25}_{-0.22}$ | 3.0
PD-Ks-d-10 ccThe redshift and physical parameters presented for this source are based on the second best-fit model from LePhare, which does agree with the redshift results from EAZY and/or Prospector.  |  | $6.17^{+0.11}_{-0.18}$ | 0.84 | 0.4 | $10.70^{+0.09}_{-0.11}$ | 29
PD-H-a-1/2 | a | $2.82^{+0.48}_{-2.11}$ | 0.53 | 0.2 | $7.28^{+0.32}_{-0.31}$ | 1.0
b | $3.61^{+0.09}_{-0.09}$ | 2.11 | 0.3 | $8.41^{+0.11}_{-0.08}$ | 3.5
c | $3.61^{+1.59}_{-0.11}$ | 2.54 | 0.4 | $8.24^{+0.10}_{-0.10}$ | 2.1
d | $5.21^{+0.00}_{-0.02}$ | 1.22 | 0.5 | $8.82^{+0.04}_{-0.04}$ | 1.0
e | $5.21^{+0.00}_{-0.01}$ | 1.36 | 0.4 | $8.89^{+0.10}_{-0.08}$ | 1.0
f | $5.18^{+0.04}_{-0.01}$ | 1.16 | 0.5 | $9.65^{+0.28}_{-0.14}$ | 1.0
g | $0.77^{+0.11}_{-0.13}$ | 0.12 | 0.1 | $7.39^{+0.17}_{-0.22}$ | 11
PD-Ks-a-3 | a | $1.52^{+0.10}_{-0.13}$ | 0.63 | 0.1 | $7.09^{+0.16}_{-0.15}$ | 1.5
b | $1.57^{+0.05}_{-0.29}$ | 0.80 | 0.3 | $7.58^{+0.16}_{-0.15}$ | 1.0
PD-Ks-a-4 | a | $2.86^{+0.33}_{-1.46}$ | 0.47 | 0.0 | $8.47^{+0.27}_{-0.33}$ | 200
b | $3.36^{+1.81}_{-2.30}$ | 0.17 | 0.0 | $7.39^{+0.16}_{-0.15}$ | 3.0
c | $4.76^{+0.09}_{-0.77}$ | 0.94 | 0.6 | $9.19^{+0.29}_{-0.23}$ | 1.0
PD-Ks-a-5 | a | $5.74^{+0.03}_{-0.05}$ | 1.66 | 0.2 | $9.13^{+0.19}_{-0.18}$ | 3.5
b | $5.75^{+0.27}_{-0.05}$ | 3.84 | 0.5 | $9.41^{+0.19}_{-0.17}$ | 1.0

Note. — Physical parameters derived from SED fitting the HST and JWST of the
JWST-detected counterparts to the UltraVISTA dropout candidates, using
LePhare. The contaminant galaxies in the deboosted sample are omitted from
this table.

Figure 4: Stellar mass, color excess, age and 1.6-4.5 $\mu$m color against
redshift for our JWST-detected sample in blue. Ancillary UltraVISTA DR4
galaxies within the same redshift and mass range as the dropout candidates are
shown in tan data points, with their median value derived in $dz=0.5$ bins
indicated by the brown line. The 1.6-4.5 $\mu$m color for the dropout
candidates is based on the WFC3/F160W and NIRCam/F444W bands, and on the
VIRCAM $H$ and IRAC 4.5 $\mu$m bands for the ancillary sample. Upper and lower
limits on the redness of this color are indicated with downwards- and upwards-
pointing triangles respectively.

Fig. 4 shows the stellar mass, color excess, age, and observed [F160W-F444W]
color against redshift for our JWST-detected sample. To put these special
galaxies into the context of the general population, we compare them to the
UltraVISTA DR4 catalog from van Mierlo et al. (2022). This catalog contains
physical parameters for UltraVISTA $HK_{\rm s}$-detected galaxies at $z_{\rm
phot}=0$–9, derived from SED fitting on multi-wavelength photometry spanning
optical to IRAC imaging. To ensure a fair comparison, the catalog has been cut
to the PRIMER area, such that it contains 10 272 galaxies. The UltraVISTA DR4
galaxies shown in Fig. 4 are matched to the redshift and stellar mass range of
the JWST-detected sample, i.e. $z=0.7$–6.2 and
$\log(M_{*}/M_{\odot})=7.0$–10.8.

We find that $\sim 70$ % of our JWST-detected sample is defined as $z=3$–6
galaxies of intermediate stellar mass $\log(M_{*}/M_{\odot})\sim 8.2$–10.3,
that have red [F160W-F444W] colors explained by their dusty nature. In fact,
for the secure and deboosted samples (where there is good correspondence
between the IRAC- and JWST-detected sources), 95% of the galaxies are dusty
with $E(B-V)\geq 0.2$, and moreover, 71% are highly dusty with $E(B-V)\geq
0.5$. Their corresponding red colors explain straightforwardly why these
sources were initially identified as dropouts in UltraVISTA imaging.

Although our sample consists mostly of these dusty intermediate-$z$ sources,
the remaining JWST-detected galaxies display a variety in behavior. For
example, source PD-H-d-3, with $z=1.99$ and $E(B-V)=0.2$, appears to be an
$H$-dropout galaxy in the UltraVISTA imaging. The HST and JWST photometry
reveal that PD-H-d-3 is in fact not a dropout galaxy, but that its red color
can be attributed to its age of 3.0 Gyr. In addition, system PD-Ks-a-3 is very
different from any of the other dropout candidates we study throughout this
paper (see Fig. 14 for the image cutouts). It is in fact the faintest IRAC
source in our sample, with magnitude 27.0 ($3.5\sigma$) in the IRAC 4.5 $\mu$m
band and no detection at all in the 3.6 $\mu$m band. HST and JWST imaging
reveal that this IRAC source is made up out of two young galaxies at $z\sim
1.5$, that have very flat SEDs and low stellar masses of
$\log(M_{*}/M_{\odot})=7.1$ and 7.6. To understand why this duo enters our
dropout candidate sample, we search for galaxies of comparable redshift and
IRAC brightness in the UltraVISTA DR4 catalog. This yields only six galaxies,
all which have 3–$22\sigma$ detections in the VIRCAM $H$, $K_{\mathrm{s}}$ and
IRAC 3.6 $\mu$m bands. Given that the IRAC and VIRCAM imaging considered in
this work are of comparable depths, it is unclear why PD-Ks-a-3-a and -b are
absent from the VIRCAM imaging. Finally, we note the arc-like spatial
alignment as seen in PD-H-a-1 and PD-H-a-2, which are two IRAC-detected
galaxies at $1\farcs 4$ angular separation, that both drop out in the VIRCAM
$H$ band. However, the HST and JWST imaging reveal that these two IRAC sources
are in fact made up out of seven distinct galaxies that are partially
spatially aligned, two of them at $z=3.6$ and three at $z=5.2$, although PD-
H-a-1/2-c has a significant secondary solution at $z=5.2$ too.

In the context of the general galaxy population, Fig. 4 clearly shows that the
JWST-detected galaxies in this work are amongst the reddest and dustiest
intermediate-mass sources at $z\simeq 3-6$. However, there are several
UltraVISTA DR4 galaxies with similarly red colors, i.e. $[H-4.5]\gtrsim 2$,
which do not drop out of the VIRCAM images, despite being of comparable
brightness in the IRAC bands as our JWST-detected sample. Upon further
inspection, these UltraVISTA DR4 galaxies are mostly old galaxies with ages
exceeding 1 Gyr, such that given their redshift, the well-developed Balmer
break is located in between the $H$ and $K_{\mathrm{s}}$ bands, explaining
their red $[H-4.5]$ color and simultaneous presence in the UltraVISTA DR4
catalog (which was detected on the stacked $HK_{\mathrm{s}}$ image). In
addition, a small sample of the red UltraVISTA DR4 galaxies are dusty with
$E(B-V)=0.3$–0.7, but overall brighter than our JWST-detected galaxies ($\sim
1.2$ mag in the IRAC bands), such that they are just bright enough to be
detected in the UltraVISTA imaging.

We show the normalized color excess distributions of our JWST-detected
galaxies and the UltraVISTA DR4 galaxies in three redshift bins between
$z=3$–6 in Fig. 5. In each redshift bin, we find that the color excess
distribution between the UltraVISTA DR4 galaxies and our sample are distinctly
different, as confirmed by performing the two-sample Kolmogorov-Smirnov test
(Smirnov, 1939), which yields p-values $<3\times 10^{-3}$. The JWST-detected
galaxies are consistently significantly dustier, even though Deshmukh et al.
(2018) already showed that the fraction of $E(B-V)\geq 0.2$ galaxies at the
epoch $z=3$–6 is considerate at $\sim 30$–40%. Especially at $z=4$–5, all of
the JWST-detected galaxies are highly dusty, with $E(B-V)\geq 0.5$. This
clearly demonstrates that these dust-reddened galaxies are a special
population of galaxies that are missing from wide-area near-infrared surveys.

Figure 5: Normalized color excess distribution of the JWST-detected galaxies
in this work (turquoise) and the UltraVISTA DR4 galaxies (tan), in three
redshifts bins $z=3$–4, $z=4$–5, and $z=5$–6.

###  5.2 H$\alpha$ emission in the UltraVISTA dropout galaxies

From inspection of the LePhare best-fit SEDs, some of the JWST-detected
galaxies appear to have excess flux in bands coinciding with the (H$\alpha$\+
N[II] + S[II]) line complex considering their photometric redshifts. Following
Caputi et al. (2017), we set a minimum condition of $\mathrm{\Delta
mag=mag_{obs}-mag_{synth}<-0.1}$ in the band to consider the flux excess
significant. Here, $\mathrm{mag_{synth}}$ is the continuum emission in the
band, which we obtain by re-modelling the SEDs with LePhare whilst excluding
the affected band and emission line modelling, and adopting the fixed
photometric redshift obtained from the original LePhare run (with all bands
and emission lines). In addition, to ensure that flux excess is not the result
of a poorly constrained continuum, we imposed the condition that
$\mathrm{\Delta mag\leq 2\times mag_{err}}$ in at least one adjacent observed
band.

For all galaxies where the H$\alpha$ line coincides with one of the HST and
JWST bands, we calculated the rest-frame equivalent width ($\mathrm{EW_{0}}$)
of the (H$\alpha$\+ N[II] + S[II]) line following the approach outlined in
Rinaldi et al. (2023) (which is in turn based on Mármol-Queraltó et al. 2016),
such that

$\mathrm{EW_{0}=\frac{W_{rec}}{1+z}(10^{(-0.4\Delta mag)}-1),}$ (2)

where $\mathrm{W_{rec}}$ is the FWHM of the filter containing the emission
line of interest. We propagate the redshift error and the flux error into the
$\mathrm{EW_{0}}$ error. Subsequently, we converted the flux excess into an
H$\alpha$ emission line flux using the modelled continuum flux. We corrected
for dust extinction using the color excess from the original SED fitting and
assuming a Calzetti et al. (2000) dust reddening law. Here, we assumed that
the extinction is the same for both the lines and the continuum. We also
propagated the error on the dust extinction, which was derived from the
difference in $E(B-V)$ values between the best-fit SEDs at the lower and upper
redshift error for each source. We obtained the net H$\alpha$ flux considering
that $f$(H$\alpha$)=0.63$f$(H$\alpha$+N[II]+S[II]) (Anders & Fritze-v.
Alvensleben, 2003), and converted it to a SFR following Kennicutt (1998):

$\mathrm{SFR(H\alpha)}(M_{\odot}\,\mathrm{yr^{-1})=7.936\times 10^{-42}}\times
L_{\mathrm{{H\alpha}}}\mathrm{(erg\,s^{-1}),}$ (3)

where we divide the resulting SFRs by a factor 1.69 to re-scale them from a
Salpeter (1955) IMF over (0.1–100) $M_{\odot}$ to a Chabrier (2003) IMF.

Not all galaxies in our JWST-detected sample display flux excess at the
location of the H$\alpha$ line-complex; for those which do not, we calculated
upper limits on their H$\alpha$ properties from the minimum line-complex
$\mathrm{EW_{0}}$ that would produce $\mathrm{\Delta
mag=mag_{obs}-mag_{synth}=-0.1}$ in the band containing the emission line-
complex. Only for source PD-H-s-3 are we unable to calculate information on
the H$\alpha$ emission; due to its redshift, the line does not fall into any
of the HST and JWST filters considered in this study.

Table 3: H$\alpha$ $\mathrm{EW_{0}}$, $L$ and SFR of JWST-detected galaxies
located within the IRAC apertures, including upper limits for sources that
display no flux excess.

ID | Redshift | Band | (H$\alpha$\+ N[II] + S[II]) $\mathrm{EW_{0}}$ | $L_{\mathrm{{H\alpha}}}$ | SFR | sSFR
---|---|---|---|---|---|---
|  |  | Å | $10^{42}\,\mathrm{erg\,s^{-1}}$ | $\log(M_{\odot}\mathrm{yr^{-1}})$ | $\log(\mathrm{yr^{-1}})$
PD-H-s-1 |  | 3.28 | F277W | $326^{+15}_{-15}$ | $3.2\pm 0.1$ | $15\pm 6$ | -8.8
PD-H-s-2 aaThis source is likely an AGN because of ancillary X-ray emission, such that the SFR conversion from Kennicutt (1998) is not applicable.  |  | 3.32 | F277W | $722^{+34}_{-32}$ | $3.8\pm 0.2$ | - | -
PD-H-s-3 ddDue to its redshift, the H$\alpha$ emission line falls outside any of the bands considered in this work, such that estimating its flux excess is not possible. |  | 1.61 | - | - | - | - | -
PD-Ks-s-4 |  | 4.71 | F356W | $667^{+32}_{-30}$ | $13.9\pm 0.6$ | $65\pm 27$ | -7.7
PD-Ks-s-5 |  | 4.78 | F356W | $617^{+29}_{-28}$ | $23.2\pm 1.1$ | $109\pm 46$ | -8.1
PD-Ks-s-6 |  | 4.68 | F356W | $622^{+30}_{-21}$ | $9.0\pm 2.8$ | $42\pm 25$ | -8.0
PD-Ks-s-7 |  | 4.75 | F356W | $594^{+29}_{-27}$ | $9.4\pm 0.4$ | $44\pm 18$ | -8.3
PD-Ks-s-8 |  | 3.48 | F277W | $395^{+19}_{-17}$ | $2.0\pm 0.6$ | $9\pm 5$ | -8.9
PD-Ks-s-9 |  | 4.75 | F356W | $538^{+26}_{-25}$ | $11.4\pm 0.5$ | $54\pm 22$ | -8.4
PD-Ks-s-10 bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. |  | 5.85 | F444W | $155$ | $18.6$ | $87$ | -8.2
PD-Ks-s-11 |  | 3.51 | F277W | $653^{+33}_{-29}$ | $4.7\pm 1.5$ | $22\pm 13$ | -8.6
PD-H-d-1  |  | 5.73 | F444W | $400^{+19}_{-17}$ | $30.4\pm 1.4$ | $143\pm 59$ | -8.2
PD-H-d-2 bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. |  | 3.32 | F277W | $158$ | $1.7$ | $8$ | -8.9
PD-H-d-3 bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. |  | 1.99 | F200W | $229$ | $2.7$ | $13$ | -8.8
PD-H-d-4 |  | 3.25 | F277W | $437^{+20}_{-19}$ | $1.8\pm 0.1$ | $8\pm 3$ | -9.4
PD-Ks-d-5 |  | 4.44 | F356W | $358^{+17}_{-15}$ | $6.2\pm 1.9$ | $29\pm 17$ | -7.8
PD-Ks-d-6 |  | 3.62 | F277W | $311^{+17}_{-11}$ | $3.1\pm 0.9$ | $14\pm 9$ | -8.0
PD-Ks-d-7 |  | 5.23 | F410M | $484^{+26}_{-25}$ | $74.2\pm 3.9$ | $349\pm 146$ | -6.9
PD-Ks-d-8 bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. |  | 3.82 | F356W | $169$ | $2.1$ | $10$ | -7.5
PD-Ks-d-9 bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. |  | 3.47 | F277W | $153$ | $0.9$ | $4$ | -8.8
PD-Ks-d-10 |  | 6.17 | F444W | $318^{+15}_{-14}$ | $62.2\pm 2.9$ | $292\pm 122$ | -8.2
PD-H-a-1/2 | a | 2.82 | F277W | $1602^{+675}_{-268}$ | $1.4\pm 0.5$ | $7\pm 4$ | -6.5
b | 3.61 | F277W | $1091^{+51}_{-49}$ | $7.7\pm 2.4$ | $36\pm 21$ | -6.9
c ccThis source has poorly constrained continuum in bands F356W and F410M, and a significant secondary solution at $z=5.2$, such that the H$\alpha$ properties listed are uncertain at best. | 3.61 | F277W | $2169^{+147}_{-105}$ | $5.4\pm 1.7$ | $26\pm 15$ | -6.8
d | 5.21 | F410M | $1171^{+64}_{-64}$ | $23.9\pm 1.3$ | $112\pm 47$ | -6.8
e | 5.21 | F410M | $811^{+37}_{-37}$ | $39.6\pm 1.8$ | $186\pm 77$ | -6.6
f | 5.18 | F410M | $515^{+24}_{-24}$ | $79.8\pm 3.7$ | $375\pm 156$ | -7.1
g bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. | 0.77 | F115W | $460$ | $0.04$ | $0.2$ | -8.1
PD-Ks-a-3 | a | $1.52$ | F160W | $115$ | $0.2$ | $0.8$ | -7.2
b bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. | $1.57$ | F160W | $113$ | $0.1$ | 0.6 | -7.8
PD-Ks-a-4 | a | $2.86$ | F277W | $355^{+72}_{-41}$ | $2.4\pm 1.5$ | $11\pm 12$ | -7.7
b bbNo flux excess in the band where the H$\alpha$ line should be observed for this source. The $EW_{0}$, $L_{\mathrm{{H\alpha}}}$ and SFR are upper limits assuming $\mathrm{\Delta mag=0.1}$. | $3.36$ | F277W | $157$ | $0.6$ | $3$ | -7.0
c | $4.76$ | F356W | $446^{+24}_{-20}$ | $4.5\pm 5.6$ | $21\pm 55$ | -7.9
PD-Ks-a-5 | a | $5.74$ | F444W | $1641^{+76}_{-75}$ | $76.9\pm 3.5$ | $361\pm 151$ | -6.6
b | $5.75$ | F444W | $1577^{+73}_{-70}$ | $50.0\pm 2.2$ | $225\pm 94$ | -7.1

Note. — Measurements for the (H$\alpha$\+ N[II] + S[II]) rest-frame equivalent
width, the H$\alpha$ luminosity and the derived (specific) star formation rate
for the JWST-detected counterparts to our UltraVISTA dropout candidates.

Table 3 lists the (H$\alpha$\+ N[II] + S[II]) $\mathrm{EW_{0}}$, the H$\alpha$
luminosity and derived SFR for each JWST-detected counterpart to the
UltraVISTA dropout candidates, including the upper limits for the sources
which show no flux excess. We also list the band in which the flux excess is
observed, and the specific star formation rate (sSFR). For this and any
further plots involving the stellar mass, we consider the stellar mass as
reported in Table 4, i.e., derived from the original SED fitting with emission
lines and the filter containing the H$\alpha$ line. In total, we identify
H$\alpha$-emitters from the HST and JWST photometry amongst 19 of the 26 IRAC-
selected dropout candidates, such that $\sim 75$ % of the sources that enter
our original dropout candidate selection are boosted redwards of
$K_{\mathrm{s}}$ band due to the presence of H$\alpha$ line-complex emission.

Fig. 6 shows the derived (H$\alpha$\+ N[II] + S[II]) $\mathrm{EW_{0}}$ versus
stellar mass for our JWST-detected sample. We exclude here and from any
further plots the sources PD-H-s-2 and PD-H-a-1/2 c; the former because its
detection at X-ray wavelengths means it is most likely an AGN, such that the
SFR conversion from Kennicutt (1998) cannot be applied. The latter we exclude
as both its redshift and the continuum in the bands redwards to the H$\alpha$
line-complex are poorly constrained, such that its listed H$\alpha$ properties
in Table 3 are uncertain at best. We highlight in our plots three sources that
are detected at sub-/millimeter wavelengths and are all H$\alpha$ emitters,
namely PD-Ks-s-6, PD-Ks-s-8, and PD-H-d-1, which we discuss further in Sect.
5.2.1. The rest-frame EWs span values between $\sim 100$–2200 Å. We find a
general decline in the median $\mathrm{EW_{0}}$ values with stellar mass: they
are $\sim 950$ Å for galaxies with $\log(M_{*}/M_{\odot})=8.0$–9.0,but only
$\sim 440$ Å for galaxies with $\log(M_{*}/M_{\odot})=10.0$–10.5. The anti-
correlation between the main optical emission lines and stellar mass is well
known at different redshifts (e.g, Reddy et al. 2018; Endsley et al. 2021;
Rinaldi et al. 2023; Caputi et al. 2023). This trend is shown in Fig. 6,
including the median (H$\alpha$\+ N[II] + S[II]) $\mathrm{EW_{0}}$
measurements from Smit et al. (2016) (the photometric–$z$ sample) and Caputi
et al. (2017), for galaxies at $z=3.8$–5.0 and $z=3.9$–4.9 respectively. We
also show the empirical relation between (H$\alpha$\+ N[II]) $\rm EW_{0}$ and
$M_{*}$ from Reddy et al. (2018), derived from spectroscopic line measurements
for galaxies at $z=1.0$–2.6. The difference in $\rm EW_{0}$ between the
(H$\alpha$\+ N[II]+S[II]) and (H$\alpha$\+ N[II]) line-complex is on average
only 0.09 dex, such that we can safely include their result in our plot. The
$\mathrm{EW_{0}}$ measurements in this work are located above this empirical
relation, as expected given that emission line equivalent widths increase with
redshift at fixed stellar mass (Reddy et al., 2018). We attribute the
difference between the results from Smit et al. (2016) and Caputi et al.
(2017) to cosmic variance and a different level of statistics, as the latter
study covers a field that is $\sim 7$ times larger than that of the former.
Finally, we note that the observed negative $\rm EW_{0}$-$M_{*}$ correlation
for our JWST-detected galaxies makes sense from the perspective that these are
IRAC-selected sources with no UltraVISTA $H$\- and/or $K_{\rm s}$-band
counterparts; the less massive galaxies require significant flux boosting from
H$\alpha$ emission to enter our sample.

Figure 6:  (H$\alpha$\+ N[II] + S[II]) rest-frame equivalent width versus
stellar mass for all JWST-detected galaxies in our JWST-detected sample.
$\mathrm{EW_{0}}$ upper limits are indicated with downwards-pointing arrows.
The orange data points represent the three sub-mm detected galaxies in the
sample. We also show the median (H$\alpha$\+ N[II] + S[II]) $\mathrm{EW_{0}}$
results from Smit et al. (2016) (photometric–$z$ sample) and Caputi et al.
(2017), from galaxies at $z=3.8$–5.0 and $z=3.9$–4.9, in pink circles and
magenta diamonds respectively. The purple line shows the empirical relation
between (H$\alpha$\+ N[II]) and $M_{*}$ from Reddy et al. (2018), derived from
spectroscopic line measurements for galaxies at $z=1.0$–2.6.

We show the best-fit SEDs (derived from the original LePhare run) for three of
the most prominent H$\alpha$-emitters in our sample in Fig. 7. We show the SED
for PD-Ks-a-5-b, which together with its associated galaxy are amongst the
highest redshift galaxies in our sample that display flux excesses, with
$z=5.75$ and $\mathrm{EW_{0}}\sim 1600$ Å. In addition, we show the SED of PD-
H-a-1/2-d, that is the brightest in this system of sources, where all but the
one contaminating foreground galaxy are strong H$\alpha$-emitters, with mean
$\mathrm{EW_{0}}\sim 1040$ Å. Lastly, we include the SED of one of the sources
detected at sub-mm wavelengths, i.e. PD-Ks-s-6, which due to extreme dust
content combined with its H$\alpha$ excess, has a red slope perfectly
demonstrating why this relatively IRAC-bright galaxy is undetected in the
ground-based VIRCAM imaging.

Figure 7: Best-fit SED obtained with LePhare on HST and JWST photometry for
three of the most prominent H$\alpha$-emitters among the JWST-detected
galaxies. Photometric measurements are shown in blue diamonds, flux upper
limits in cyan with downward-pointing arrows and the template fluxes (with
emission line contributions) with red circles.

Finally, we show the H$\alpha$ emitters on the SFR-$M_{*}$ plane in Fig. 8. We
also indicate the main sequence (MS) and starburst (SB) SFR-$M*$ relations
from Caputi et al. (2017) and Rinaldi et al. (2022), the former derived from
H$\alpha$ emitters at $3.9\leq z\leq 4.9$, the latter from star-forming
galaxies in COSMOS at $4\leq z\leq 5$. The H$\alpha$ SFRs span values between
$\sim 5$–400 $M_{\odot}\,\mathrm{yr^{-1}}$, and as expected, broadly increase
with stellar mass. We find that the our H$\alpha$-emitters are scattered over
the SFR-$M*$ plane: out of the 23 JWST-detected galaxies with secure H$\alpha$
SFRs (that is, excluding upper limit estimates and the aforementioned poorly
constrained source PD-H-a-1/2 c), 8 are starbursts with
$\mathrm{sSFR}\geq-7.1$, and 9 are main-sequence galaxies with
$\mathrm{sSFR}\leq-8.1$. The remaining 6 sources have sSFRs between -8.0 and
-7.7, which following the classification from Caputi et al. (2017) places them
in the so-called star formation valley, such that they are likely
transitioning from starbursts into main-sequence galaxies.

Figure 8: SFR derived from the H$\alpha$ luminosity versus stellar mass for
all JWST-detected galaxies in our sample. SFR upper limits are indicated with
downwards-pointing arrows. The orange data points represent the three sub-mm
detected galaxies in the sample. The solid and dotted lines display the main
sequence and starburst SFR-$M_{*}$ relations respectively, taken from Caputi
et al. (2017) in dark green and Rinaldi et al. (2022) in light green
respectively. The faint, light green lines correspond to the SFR-$M_{*}$
relations at $z=2.8$–4 and $z=5$–6 from Rinaldi et al. (2022).

####  5.2.1 X-ray and sub-mm counterparts to the JWST-detected galaxies

Given the highly dusty nature of many of our JWST-detected galaxies, we search
for counterparts in the literature specifically at X-ray and sub-mm
wavelengths. We find that source PD-H-s-2 has a $0\farcs 7$ offset X-ray
association from both the Chandra-COSMOS Legacy Survey (Civano et al., 2016)
and the XMM-Newton Wide-Field Survey (Cappelluti et al., 2009). Interestingly,
as shown in Fig. 9, the optical and near-infrared continuum of PD-H-s-2 is
well-fitted by stellar population templates, with $\chi^{2}_{\nu}=0.96$ and no
preference from LePhare for an AGN solution. Compared to the best-fit SED, the
photometry of PD-H-s-2 shows flux excess in the F200W and F356W bands, which
sample the (H$\beta$+O[III]) and (H$\alpha$\+ N[II] + S[II]) lines
respectively. Finally, we notice that PD-H-s-2 and PD-H-s-1 have very similar
SED properties and brightness (F444W $\sim$ 23.4 mag), but PD-H-s-1 lacks any
ancillary detections at X-ray or radio wavelengths nor does it have similarly
broad emission lines.

Figure 9: Best-fit SED obtained with LePhare on HST and JWST photometry for
PD-H-s-2. Photometric measurements are shown in blue diamonds, flux upper
limits in cyan with downward-pointing arrows and the template fluxes (with
emission line contributions) with red circles.

In addition, we find that sources PD-Ks-s-6, PD-Ks-s-8, and PD-H-d-1 (all of
which are H$\alpha$ emitters as shown in the previous section) are detected at
sub-mm wavelengths with ALMA from the A3COSMOS catalog (Liu et al., 2019),
with $\sim 2$ mJy fluxes from Band 6 ($\sim 1.2$ mm). PD-Ks-s-8 is also
detected at 450 and 850 $\mu$m from the SCUBA-2 Cosmology Legacy Survey
(Koprowski et al., 2016), and PD-H-d-1 with the VLA at 3 GHz (Smolčić et al.,
2017). Liu et al. (2019) report photometric redshifts of $z=5.9$ and $z=6.4$
and stellar masses of $\log(M_{*}/M_{\odot})\sim 11.2$ for sources PD-Ks-s-6
and PD-H-d-1 respectively. These parameters are discrepant with our own
solutions for these galaxies, although the probability distribution function
of PD-Ks-s-6’s redshift, PDF($z$), is degenerate with a secondary solution at
$z\sim 6.7$. Even when we re-run LePhare on our photometry with the redshift
fixed to $z=5.9$, we only retrieve a stellar mass of
$\log(M_{*}/M_{\odot})=10.4$ for this source.

##  6 Discussion and conclusions

In this work we have studied a sample of Spitzer IRAC-detected galaxies that
are part of the SMUVS program in the COSMOS field, but which are completely
undetected in the latest data release (DR5) of the UltraVISTA VIRCAM $H$\- and
$K_{\rm s}$-band imaging. These sources are rare, as we identify only 26 over
an area of 94 square arcminutes (i.e., the area with multi-wavelength JWST
coverage considered in this work). We used the available JWST/NIRCam data in 7
bands from the PRIMER program, together with ancillary HST/ACS and HST/WFC3
data, to study the properties of these galaxies, with vastly improved angular
resolution and sensitivity with respect to the original Spitzer and ground-
based data. We find that more than half of the IRAC sources are in fact
systems of multiple galaxies: 10 consisting of a so-called deboosted galaxy
that is accompanied by a low-redshift, faint foreground contaminant; and
another 5 consisting of associated galaxies at similar redshifts. In addition,
the majority of the dropout candidates display flux in the short-wavelength
JWST channels, such that only two $K_{\rm s}$-band dropout candidates and two
$H_{\rm s}$-band dropout candidates qualify as true dropout galaxies.

We find that our JWST-detected galaxies display a variety of properties, with
redshifts between $z=0.8$–6.2, stellar masses
$\log(M_{*}/M_{\odot})=7.0$–10.7, and color excess between $E(B-V)=0.0$ and
1.0. Most of the sample consists of intermediate-mass galaxies that are red
because of dust attenuation, with median values of $z=3.6$,
$\log(M_{*}/M_{\odot})=9.5$, [F160W-F444W]$=2.2$, and $E(B-V)=0.5$. We
compared the properties of our JWST-detected galaxies at $z=3$–6 with those of
UltraVISTA DR4 $HK_{\rm s}$-detected galaxies in the same region. We find that
our JWST-detected galaxies are amongst the dustiest galaxies at this epoch; in
fact, $\sim 65$ % of our sample at $z=3$–6 has $E(B-V)\geq 0.5$, compared to
7% at most amongst the UltraVISTA DR4 galaxies.

For $\sim 75$% of our IRAC-selected dropout candidates, the JWST photometry
shows flux excess in bands encompassing the H$\alpha$ emission line-complex,
which enabled us to calculate (H$\alpha$\+ N[II] + S[II]) rest-frame
equivalent widths and H$\alpha$ star formation rates. The (H$\alpha$\+ N[II] +
S[II]) $\mathrm{EW_{0}}$ span values between $\sim 100$–2200 Å, and the SFRs
between $\sim 5$–375 $M_{\odot}\,\mathrm{yr^{-1}}$. We studied them in the
SFR-$M_{*}$ plane, and found 35 % of the JWST-detected galaxies with secure
H$\alpha$ SFRs to be starbursts, 40 % to be main-sequence galaxies and the
remaining 25 % to be located in the star-formation valley. Given their
redshifts, the H$\alpha$ emission contributes strongly to the continuum
measurements redwards of the $K_{\rm s}$ band, explaining, together with their
dusty nature, why these galaxies enter our initial dropout sample.

We checked if the H$\alpha$-emitting nature of our sample is apparent from the
IRAC photometry as well. For seven dropout candidates, the H$\alpha$ line
falls in the 3.6 $\mu$m band, such that we would expect to see a flux excess
with respect to the 4.5 $\mu$m band. However, for only three of these galaxies
do we observe $\mathrm{m_{3.6}-m_{4.5}}<0$, such that we conclude the presence
of H$\alpha$ emission is not obvious from the IRAC bands alone. We note that
although there are multiple dropout candidates for which the H$\alpha$ line is
observed with the 4.5$\mu$m band, we cannot perform a similar check, as a flux
excess in this band with respect to the $3.6\mu$m band can be explained by red
continuum emission just as well.

We have cross-matched our sources with ancillary data at X-ray and sub-mm
wavelengths, and find one AGN candidate and three ALMA-detected galaxies
amongst them. For the ALMA-detected sources we obtained photometric redshifts
between $z\sim 3.5$ and 5.7 and stellar masses
$\log(M_{*}/M_{\odot})=9.7$–10.4 from the HST and JWST photometry. These
sources are among the (H$\alpha$\+ N[II] + S[II]) line emitters discussed
above.

Our galaxies are not as Spitzer/IRAC-bright and massive as other studies
targeting optically faint sources. The brightest IRAC source in our sample is
23.9 and 23.1 mag in the $3.6\mu$m and $4.5\mu$m bands respectively, and the
median IRAC magnitude of our dropout candidates is $\sim 24.6$ mag. In
addition, the most massive galaxy in our sample is
$\log(M_{*}/M_{\odot})=10.7$ and the median stellar mass of the sample is
$\log(M_{*}/M_{\odot})=9.5$. Similar endeavors to ours have been focused
mostly on bright IRAC sources ($[4.5]<24$ mag), finding them to be massive
$\log(M_{*}/M_{\odot})>10.5$ star-forming galaxies at $z\sim 3$–6. Alcalde
Pampliega et al. (2019) do find H$\alpha$ emitters among relatively Spitzer-
bright ($[4.5]<24.5$ mag) HST/F160W-band dropout sources, although their
incidence is only a third in their sample, versus our finding of
H$\alpha$-emitters among $\sim 75$ % of our dropout candidates. More recently,
Barrufet et al. (2023) looked for JWST-detected HST-dropout galaxies in CEERS.
Their sample spans a similar NIRCam/F444W magnitude range as our JWST-detected
galaxies ($23-28$ mag), which they find to be dusty $E(B-V)\sim 0.5$,
relatively massive $\log(M_{*}/M_{\odot})\sim 10$ star-forming galaxies at
$z\sim 2$–8. The presence of H$\alpha$ line emission is unfortunately not
discussed in their work.

Lastly, our dropout candidates, with an observed surface density of $\sim 0.3$
arcmin-2, appear to be relatively rare. Their incidence is comparable to that
of mid-IR bright, true HST-dropout galaxies, for which studies have found
typical surface densities of $\sim 0.1$ arcmin-2 (e.g., Alcalde Pampliega et
al. 2019; Wang et al. 2019). However, a recent study by Barrufet et al. (2023)
reported a surface density of $\sim 0.8$ arcmin-2 for JWST-detected, HST-dark
galaxies.

In summary, we have revealed the nature of a representative sample of the IRAC
sources that were left unidentified from the Spitzer era. In the future, once
JWST/NIRCam imaging becomes available for other well-studied extra-galactic
fields on the sky, we will be able to follow up more of these sources and
characterize the overall population of emission-line galaxies with significant
dust extinction at intermediate/high redshifts.

KIC and VK acknowledge funding from the Dutch Research Council (NWO) through
the award of the Vici Grant VI.C.212.036. 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 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. All the JWST
data used in this paper can be found in MAST: 10.17909/bysp-ds64.

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##  Appendix A HST and JWST image cutouts of 26 dropout candidates

Figure 10: Image cutouts for the 11 secure dropout candidates. From left to
right, the horizontal panels show the following imaging in 3 by 3$"$ windows:
IRAC 3.6 and 4.5 $\mu$m average; HST/ACS F435W, F606W, and F814W average;
JWST/NIRCam F090W and F115W average; HST/WFC3 F125W and F140W average;
JWST/NIRCam F200W; JWST/NIRCam F277W, F356W, and F444W rgb image. The red
circle represents the $0\farcs 5$ diameter circular aperture at the position
of the JWST-detected galaxy.  Figure 11: Figure 10 continued.  Figure 12:
Image cutouts for the 10 deboosted dropout candidates. The horizontal panels
for each source show the same imaging as in Fig. 10. The circles represent the
$0\farcs 5$ diameter circular aperture at the positions of the JWST-detected
galaxies, showing the deboosted source in red and the contaminating foreground
galaxies in blue.  Figure 13: Figure 12 continued.  Figure 14: Image cutouts
for the 5 associated dropout candidates. The horizontal panels for each source
show the same imaging as in Fig. 10. The red circles represent the $0\farcs 5$
diameter circular aperture at the positions of the JWST-detected galaxies,
which are annotated with their IDs on the F200W image cutout. As PD-H-a-1/2 is
a system consisting of two IRAC detections, we indicate the $2\arcsec$ IRAC
apertures on the 3.6 and 4.5 $\mu$m image stack in blue.