# Schrodinger’s Galaxy Candidate: Puzzlingly Luminous at $z\approx 17$, or
Dusty/Quenched at $z\approx 5$?

Rohan P. Naidu Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden
Street, Cambridge, MA 02138, USA  Pascal A. Oesch Department of Astronomy,
University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland  Cosmic Dawn
Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128,
København N, DK-2200, Denmark  David J. Setton Department of Physics and
Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA
Jorryt Matthee Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 27,
8093 Zürich, Switzerland  Charlie Conroy Center for Astrophysics $|$ Harvard &
Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA  Benjamin D. Johnson
Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden Street,
Cambridge, MA 02138, USA  John. R. Weaver Department of Astronomy, University
of Massachusetts, Amherst, MA 01003, USA  Rychard J. Bouwens Leiden
Observatory, Leiden University, NL-2300 RA Leiden, Netherlands  Gabriel B.
Brammer Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of
Copenhagen, Jagtvej 128, København N, DK-2200, Denmark  Pratika Dayal Kapteyn
Astronomical Institute, University of Groningen, 9700 AV Groningen, The
Netherlands  Garth Illingworth Department of Astronomy and Astrophysics,
University of California, Santa Cruz, CA 95064, USA  Laia Barrufet Department
of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix,
Switzerland  Sirio Belli Dipartimento di Fisica e Astronomia, Università di
Bologna, via Gobetti 93/2, 40122 Bologna, Italy  Rachel Bezanson Department of
Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA
15260, USA  Sownak Bose Institute for Computational Cosmology, Department of
Physics, Durham University, Durham, DH1 3LE, UK  Kasper E. Heintz Cosmic Dawn
Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128,
København N, DK-2200, Denmark  Joel Leja Department of Astronomy &
Astrophysics, The Pennsylvania State University, University Park, PA 16802,
USA  Institute for Computational & Data Sciences, The Pennsylvania State
University, University Park, PA, USA  Institute for Gravitation and the
Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
Ecaterina Leonova GRAPPA, Anton Pannekoek Institute for Astronomy and
Institute of High-Energy Physics, University of Amsterdam, Science Park 904,
1098 XH Amsterdam, The Netherlands  Rui Marques-Chaves Department of
Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
Mauro Stefanon Departament d’Astronomia i Astrofìsica, Universitat de
València, C. Dr. Moliner 50, E-46100 Burjassot, València, Spain  Unidad
Asociada CSIC ”Grupo de Astrofísica Extragaláctica y Cosmología” (Instituto de
Física de Cantabria - Universitat de València)  Sune Toft Cosmic Dawn Center
(DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, København
N, DK-2200, Denmark  Niels Bohr Institute, University of Copenhagen, Jagtvej
128, København N, DK-2200, Denmark  Arjen van der Wel Astronomical
Observatory, Ghent University, Krijgslaan 281, Ghent, Belgium  Pieter van
Dokkum Astronomy Department, Yale University, 52 Hillhouse Ave, New Haven, CT
06511, USA  Andrea Weibel Department of Astronomy, University of Geneva,
Chemin Pegasi 51, 1290 Versoix, Switzerland  Katherine E. Whitaker Department
of Astronomy, University of Massachusetts, Amherst, MA 01003, USA  Cosmic Dawn
Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128,
København N, DK-2200, Denmark

###### Abstract

JWST’s first glimpse of the $z>10$ Universe has yielded a surprising abundance
of luminous galaxy candidates. Here we present the most extreme of these
systems: CEERS-1749. Based on $0.6-5\mu$m photometry, this strikingly luminous
($\approx$26 mag) galaxy appears to lie at $z\approx 17$. This would make it
an $M_{\rm{UV}}\approx-22$, $M_{\rm{\star}}\approx 5\times
10^{9}M_{\rm{\odot}}$ system that formed a mere $\sim 220$ Myrs after the Big
Bang. The implied number density of this galaxy and its analogues challenges
virtually every early galaxy evolution model that assumes $\Lambda$CDM
cosmology. However, there is strong environmental evidence supporting a
secondary redshift solution of $z\approx 5$: all three of the galaxy’s nearest
neighbors at $<2.5\arcsec$ have photometric redshifts of $z\approx 5$.
Further, we show that CEERS-1749 may lie in a $z\approx 5$ protocluster that
is $\gtrsim 5\times$ overdense compared to the field. Intense line emission at
$z\approx 5$ from a quiescent galaxy harboring ionized gas, or from a dusty
starburst, may provide satisfactory explanations for CEERS-1749’s photometry.
The emission lines at $z\approx 5$ conspire to boost the $>2\mu$m photometry,
producing an apparent blue slope as well as a strong break in the SED. Such a
perfectly disguised contaminant is possible only in a narrow redshift window
($\Delta z\lesssim 0.1$), implying that the permitted volume for such
interlopers may not be a major concern for $z>10$ searches, particularly when
medium-bands are deployed. If CEERS-1749 is confirmed to lie at $z\approx 5$,
it will be the highest-redshift quiescent galaxy, or one of the lowest mass
dusty galaxies of the early Universe detected to-date ($A_{5500}\approx 1.2$
mag, $M_{\rm{\star}}\approx 5\times 10^{8}M_{\rm{\odot}}$). Both redshift
solutions of this intriguing galaxy hold the potential to challenge existing
models of early galaxy evolution, making spectroscopic follow-up of this
source critical.

High-redshift galaxies (734), Galaxy formation (595), Galaxy evolution (594),
Early universe (435)

††facilities: JWST, HST††software:  IPython (Pérez & Granger, 2007),
matplotlib (Hunter, 2007), numpy (Oliphant, 2015), scipy (Virtanen et al.,
2020), jupyter (Kluyver et al., 2016), Astropy (Astropy Collaboration et al.,
2013, 2018), grizli (Brammer, 2018)

##  1 Introduction

The first weeks of JWST data have already produced an overwhelming number of
science publications. At the highest redshifts, JWST has finally pushed our
observational frontier beyond $z\sim 11$, into the last unknown epoch of our
cosmic timeline. Several teams have reported a surprisingly large number of
particularly luminous sources very early in the Universe’s history (e.g.,
Naidu et al., 2022a; Castellano et al., 2022; Donnan et al., 2022; Atek et
al., 2022; Yan et al., 2022; Finkelstein et al., 2022; Harikane et al.,
2022a).

Some of these early $z>10$ galaxy candidates have proven quite surprising
given that the current area surveyed with JWST/NIRCam used for these first
studies is still rather limited ($<$60 arcmin2). Standard extrapolations of
the UV luminosity function’s (UV LF) evolution and galaxy simulations predict
a much larger survey area is required to yield such a bounty of luminous
candidates. Even though the overall evolution of the UV LF at $z>10$ had been
debated from the limited information available from HST data (e.g., Oesch et
al., 2018; McLeod et al., 2016), evidence had been growing for a differential
evolution of the galaxy population: a high number density of the most UV-
luminous sources seems to be in place rather early (e.g., Stefanon et al.,
2019; Bowler et al., 2020; Morishita et al., 2020; Harikane et al., 2022b;
Bagley et al., 2022), while the number density of the fainter galaxy
population continues to decline at $z>8$ (e.g., Oesch et al., 2018; Ishigaki
et al., 2018; Bouwens et al., 2022).

Theoretical models based on forming galaxies embedded in dark matter halos
have not yet successfully reproduced this differential evolution, should it be
confirmed. Most models thus underpredict the number density of the most UV-
luminous observed galaxies (e.g., Bowler et al., 2020; Leethochawalit et al.,
2022; Finkelstein et al., 2022). Already with HST, the discovery of GN-z11 in
a very small search volume was very puzzling (Oesch et al., 2016; Waters et
al., 2016; Mutch et al., 2016). Now, the first results from JWST imaging are
further challenging models.

However, before changing the theoretical models, it is important to first
confirm the high-redshift nature of these galaxies. Since JWST provides us
with the first-ever deep, high-resolution view of the Universe at $>2$ µm, it
is conceivable that we are discovering other, hitherto unknown galaxy
populations. In particular, the most distant Lyman break galaxies (LBGs) are
typically selected based on one single spectral feature: the complete break at
the redshifted Ly$\alpha$ line due to absorption by the neutral inter-galactic
medium. This break can be confused with a Balmer break at lower redshift
(e.g., Vulcani et al., 2017), or with extremely strong emission line sources
(e.g., Atek et al., 2011; van der Wel et al., 2011), especially when limited
filter coverage is available longward of the break (e.g., Brammer et al.,
2013). Additionally, dust-obscured sources can cause very red colors rendering
galaxies undetected at shorter wavelengths, even in JWST data (e.g., Barrufet
et al., 2022; Labbe et al., 2022; Nelson et al., 2022; Fudamoto et al., 2022).
However, such sources are usually guarded against in LBG selections by
requiring blue SEDs longward of the break. Nevertheless, when selecting high-
redshift galaxies in a novel regime, one has to be cautious.

Here, we present a detailed analysis of an extremely luminous $z\sim 17$
galaxy candidate identified in the Early Release Science (ERS) program CEERS
(Finkelstein et al., 2022). The source was first mentioned in Naidu et al.
(2022a) and presented as a likely $z=16.6$ candidate in Donnan et al. (2022)
as well as Harikane et al. (2022a). Here, we present a detailed analysis of
this source, including arguments for a possible lower redshift solution of
this galaxy at $z\sim 5$ (see also Zavala et al., 2022).

This paper is organized as follows. §2 describes the dataset and sample
selection. In §3 we present our results, followed by a discussion on the
implications follows in §4 and a summary in §5. Magnitudes are in the AB
system (e.g., Oke & Gunn, 1983). For summary statistics we report medians
along with 16th and 84th percentiles. We adopt a Planck Collaboration et al.
(2015) cosmology.

Figure 1: Summary of photometry and redshift solutions for CEERS-1749. Top:
Multi-wavelength photometry in $3\arcsec$x$3arcsec$ cutouts spanning $\approx
1-4.5\mu$m. The source is well-detected ($>9-20\sigma$) in all but the two
bluest bands, where it entirely drops out. Center: Redshift solution assuming
a $10\%$ error floor on photometry. The data are depicted in purple – upper
limits are shown as arrows, and represent 1-$\sigma$ limits. The best-fit
spectral energy distribution (SED) from EAZY is shown in dark orange,
representing a Lyman break galaxy (LBG) at $z\approx 16.6$. The best-fit SED
from our EAZY run constrained to $z<6$ is shown in silver, and corresponds to
a quiescent galaxy whose Balmer break produces a drop in flux across F277W and
F200W. The failure to match the photometry in F277W coupled with the non-
detections in the bluer bands together strongly favor the LBG solution. In the
right panel, we show the probability distributions for the redshift derived
using EAZY (orange solid) and Prospector (orange dashed). Both frameworks
place the source at $z>10$ with $>99.9\%$ probability. Bottom: Same as center
panels, but adopting a $20\%$ error floor on photometry. While the best-fit
EAZY SEDs in the left-panel are similar as the $10\%$ error floor case, a
notable feature in the right-panel is a solution occurring at $z\approx 5$
that appears only when this conservative error floor is adopted. See also
Figure 3 for other possible $z\sim 5$ SED solutions from Prospector.

##  2 Data & Methods

Table 1: Photometry in units of nJy – JWST/NIRCam followed by HST/WFC3.

Band |  CEERS-1749
---|---
$F$115$W$ | -1$\pm$5
$F$150$W$ | 3$\pm$6
$F$200$W$ | 26$\pm$3
$F$277$W$ | 127$\pm$5
$F$356$W$ | 110$\pm$5
$F$410$M$ | 107$\pm$9
$F$444$W$ | 91$\pm$5
$F$606$W$ | 5$\pm$6
$F$814$W$ | 5$\pm$8
$F$125$W$ | 5$\pm$11
$F$140$W$ | 9$\pm$24
$F$160$W$ | -4$\pm$10

Note. — We set an error floor of $20\%$ on the fluxes for all EAZY and
Prospector fits to conservatively account for systematic uncertainty that is
not reflected in the errors here.

###  2.1 Imaging Data and Catalogs

The data and analysis methods used here are the same as in Naidu et al.
(2022a). We refer the reader to that paper for details. Briefly, this work is
based on some of the first JWST/NIRCam imaging datasets from the Early Release
Science (ERS) programs CEERS (Finkelstein et al., 2022) and GLASS (Treu et
al., 2022). In particular, the sources analyzed in detail here were identified
in the CEERS NIRCam images that span $\sim$40 arcmin2.

The stage 2, calibrated images were obtained from the MAST archive and
processed with the grizli pipeline, which performs WCS alignment and image
mosaicking. Additionally, grizli masks ‘snowballs’ and mitigates the 1/f noise
that are most prominent in the short-wavelength data (Rigby et al., 2022). The
JWST data used here includes the six wide filters F115W, F150W, F200W, F277W,
F356W, F444W and the medium band filter F410M. The 5$\sigma$ depth as measured
in empty circular sky apertures of 0$\farcs$32 diameter ranges from 28.5 to
28.9 mag in the wide filters.

Additionally, we include ancillary HST data available in the AEGIS field, most
notably from the CANDELS survey (Koekemoer et al., 2011; Grogin et al., 2011).
Specifically, we use a re-reduction of the ACS F606W and F814W images, in
addition to F125W, F140W, and F160W taken with WFC3/IR. All images were
drizzled at 40mas and aligned to the GAIA DR3 catalog.

We use SExtractor to detect sources in the F444W filter and measure multi-
wavelength photometry in small circular apertures of 0$\farcs$32 diameter.
These fluxes are then corrected to total using the AUTO flux measurement in
our detection band, in addition to applying small corrections for remaining
flux lost based on the point-spread functions.

###  2.2 NIRCam Zeropoint Uncertainties

Given possible uncertainties with the NIRCam zeropoints (see, e.g., Rigby et
al., 2022), we perform several tests on the current photometry. In particular,
we use the available spectroscopic redshifts for galaxies in the CANDELS/EGS
field to derive iterative zeropoint corrections with EAZY. The derived
corrections depend on the exact choices of parameters. However, they typically
remain smaller than $20\%$. To allow for this systematic uncertainty, we
decided to set an error floor of 20% to all photometric bands, but to use the
original pipeline-provided zeropoints. As discussed later, this choice results
in the appearance of lower redshift solutions for some de-facto secure very
high-redshift candidates.

###  2.3 Selection of Luminous High-Redshift Galaxy Candidates

This paper follows an earlier analysis of Naidu et al. (2022a), who identified
the most luminous galaxy candidates in the existing ERS imaging data, based on
optical non-detection criteria and photometric redshift measurements using the
EAZY code (Brammer et al., 2008). As first noted in that paper, the CEERS
field revealed a seemingly reliable, but extremely luminous galaxy candidate
with photometric redshift at $z=16.6\pm 0.1$ (see also Donnan et al., 2022;
Harikane et al., 2022a). Given its extraordinary luminosity, if confirmed to
lie at $z\sim 17$, this source deserved special attention and analysis. We
will discuss it in detail below.

##  3 Results

###  3.1 Evidence for a $z\approx 17$ solution

We briefly discussed CEERS-1749 as a $z\approx 17$ candidate in our search for
luminous $z>10$ galaxies presented in Naidu et al. (2022a). CEERS-1749 is an
extremely luminous (26.3 mag in $F$277$W$) galaxy that is well-detected in all
bands at $\gtrsim 2\mu$m (Figure 1). It shows a sharp drop-off in flux between
F277W and F200W (1.7 mag), is undetected at lower wavelengths (F115W, F150W),
and appears to display a blue continuum slope characteristic of early Universe
galaxies at longer wavelengths. Both EAZY and the Prospector modeling
framework (Johnson et al., 2021) interpret the break in the SED as being due
to total absorption of $<1215$Å rest-frame photons at $z\approx 17$ by the
neutral IGM. The redshift probability distribution is almost entirely
contained at $z>10$ – $p(z>10)>99.9\%$ – unless a conservative error floor of
$20\%$ on the photometry is adopted (see center and bottom panels of Figure 1,
discussed below).

While this candidate satisfied all our quality cuts and search criteria in
Naidu et al. (2022a), we were concerned that the $F$150$W$ and $F$115$W$
photometry for this source, which are crucial to its candidacy, are based on
slightly lower SNR areas of the mosaic, with uneven depth apparent around the
object (see e.g., the F115W stamp in Figure 1). This, combined with current
NIRCam calibration uncertainties (Rigby et al. 2022) made judging a stringent
flux upper limit difficult. Stringent non-detections in these bands are
critical to the source’s candidacy because the break in the SED by itself is
unable to settle the case. Unlike in the cases of GLASS-z11 and GLASS-z13
(Naidu et al., 2022a; Castellano et al., 2022), the drop in flux across
immediately adjacent filters is not as dramatic ($\approx 5\times$ vs.
$\approx 10\times$), leaving more room for other possibilities (as we discuss
below).

We have since performed a battery of tests to check the robustness of the
$z\approx 17$ solution. The most crucial of these is the incorporation of HST
data, in particular, deep imaging in the F160W band (whose zero-point is well-
known) where the source is undetected with a stringent $<10$ nJy $1\sigma$
upper-limit, supporting the NIRCam/F150W non-detection that we report here. In
fact, the EAZY redshift derived by replacing the bluest JWST bands (F115W and
F150W) with HST/F160W yields $z=16.5$ as the most likely solution. Other
notable tests include recovering this source at $z\approx 17$ in an entirely
independent analysis of the CEERS images (Bouwens et al., in prep.), and
setting a conservative error-floor of $\approx 20\%$ on all fluxes to account
for systematic uncertainty in e.g., zeropoints across this analysis.

If confirmed at $z\approx 17$, CEERS-1749 would be one of the most luminous
galaxy candidates at $z>10$ ($M_{\rm{UV}}\approx-22$), second only to HD1
($M_{\rm{UV}}\approx-23$, Harikane et al. 2022b; Pacucci et al. 2022) and
comparable to the spectroscopically confirmed GNz11 (Oesch et al., 2016; Jiang
et al., 2021). A source of such luminosity simply does not exist at such a
potentially early time in a wide swath of empirical and theoretical models of
the $z>10$ Universe (discussed further in §4.1).

###  3.2 Evidence for a $z\approx 5$ solution

While the $z\approx 17$ solution is formally favored, there is non-zero
probability that the source lies at $z\approx 5$. We note that our $p(z)$
estimate does not include a prior based on the luminosity function (which is
poorly constrained at these epochs and luminosities). So, while the low
redshift solution is formally disfavored, it is not ruled out, and the
relative probability of the two solutions depends sensitively on our prior
belief of the source residing at $z\approx 5$ compared to $z\approx 17$. In
this section we consider the evidence for the lower redshift solution.

####  3.2.1 Environmental evidence: a $z\approx 5$ protocluster?

Figure 2: Environmental support for a $z\approx 5$ solution for CEERS-1749.
Top Left: $3.4\arcsec$x$3.4\arcsec$ F277W image centered on CEERS-1749. The
best-fit photometric redshifts of the three nearest neighbors are labeled in
white. All neighbors lie at $z\approx 5$, which happens to be the predicted
redshift for CEERS-1749 if it were a quiescent galaxy. Top Right: Best-fit
redshift distribution in 30” around CEERS-1749 (solid navy blue line) compared
to the redshift distribution of the whole CEERS field (dashed light blue line)
for galaxies detected at $>10\sigma$ in all LW bands. The redshift probability
distribution for CEERS-1749 at $z<10$ is overlayed in orange. The redshift
distribution in the immediate vicinity of CEERS-1749 shows a pronounced excess
at $z\approx 5$, with $\approx 20$ galaxies at $4.5<z<5.5$. That is, not only
are CEERS-1749’s immediate neighbors at $z\approx 5$, but it also lies amidst
a $z\approx 5$ protocluster candidate. Bottom: Best-fit EAZY SEDs of some of
the potential protocluster members show clear signatures of ancient stellar
populations (e.g., strong Balmer breaks) consistent with the early formation
epoch expected of such clusters, making it more plausible that we are indeed
observing a protocluster.

The first argument we consider for the $z\approx 5$ solution is based on the
local environment and large-scale structure near the source. The galaxy’s
three nearest neighbors all lie at precisely the redshift required by the
quiescent galaxy solution, i.e., $z\approx 5$ (Figure 2, top-left). The most
massive of these galaxies is the nearest neighbor only $1.7\arcsec$ away at
$z_{\rm{EAZY}}=4.9$ whose mass we fit to be a substantial
$M_{\rm{\star}}\approx 10^{11}M_{\rm{\odot}}$ (via the fiducial Prospector
setup described in Naidu et al. 2022a). If CEERS-1749 lies at the same
redshift, and therefore a physical separation of $<15$ kpc, it may be an
associated satellite galaxy of its massive neighbor analogous to the
Magellanic Clouds ($M_{\rm{\star}}\approx 5\times 10^{9}M_{\rm{\odot}}$, van
der Marel et al. 2009) and the present-day Milky Way ($M_{\rm{\star}}\approx
10^{11}M_{\rm{\odot}}$, Bland-Hawthorn & Gerhard 2016).

CEERS-1749 may also be part of a larger $z\approx 5$ protocluster. We find
that the $30\arcsec$ region around CEERS-1749 is overdense in galaxies with
best-fit photometric redshifts of $z\approx 5$ compared to the overall CEERS
field (Figure 2). Across the full 40 arcmin2 analyzed we find $\approx 300$
galaxies at $4.5<z<5.5$ with SNR$>10$ in all LW bands (F444W, F356W, F277W).
Strikingly, in $30\arcsec$ around CEERS-1749 we find $\approx 20$ such
sources, which translates to a $\approx 4\times$ higher density than the field
average. Even taking a model-independent point of view, selecting sources akin
to CEERS-1749 with a red F200W-F277W color ($>0.75$) that are detected at
$<2\sigma$ in F606W (the dropout filter at $z\approx 5$) shows an overdensity
of $\approx 9\times$ in the $30\arcsec$ around the source. Intriguingly, the
median redshift of these potential protocluster galaxies
($z_{\rm{cluster}}=4.9$) is an excellent match to the predicted range for a
lower-redshift solution.

In models of hierarchical structure formation, protoclusters are expected to
be among the first sites of star-formation in the Universe, as they form from
the first overdensities that collapse into stars. Intriguingly, in the
$z\approx 5$ sample proximal to CEERS-1749 we do find galaxies with strong
Balmer breaks characteristic of old stellar populations (see bottom row of
Figure 2). A handful of these galaxies have an F200W-F277W color almost as red
as CEERS-1749 within errors, and a handful even show a second mode in their
redshift probability distributions at $z\approx 16$. The existence of such
proximal ancient galaxies at the right redshift makes it more plausible that
CEERS-1749 may also belong to this protocluster.

####  3.2.2 Plausible SEDs at $z\approx 5$

Figure 3: Prospector posterior SEDs for CEERS-1749’s three classes of $z\approx 5$ solutions found by searching around the overdensity redshift ($z=4.9\pm 0.1$). The median spectrum is shown in dark orange, with the region between the $16^{\rm{th}}$ and $84^{\rm{th}}$ percentiles shaded. $\chi^{2}$ values are computed assuming a $20\%$ error floor on photometry. Top: Quiescent galaxy solutions where the Balmer break straddles F200W and F277W. The F277W prediction falls short of the observed flux by $\approx 50\%$. Center: The highest likelihood quiescent solution, now with emission arising from ionized gas around an AGN. This type of emission helps explain the F200W-F277W color. While the required AGN luminosity is an outlier as per local scaling relations, any mechanism that ionizes gas (e.g., shock heating in the protocluster environment) could produce such an SED. Bottom: A dusty starburst with intense nebular emission. All the $>2\mu$m bands, particularly F277W containing [OIII]$\lambda 5007,4959$Å+H$\beta$, are boosted by line emission, while flux in the bluest bands is suppressed by a steep attenuation curve that mimics a $z>10$ dropout. Table 2: Predicted JWST/MIRI photometry in units of nJy shows significant differences at longer wavelengths across the scenarios discussed in §3.2.2. Band | $z\approx 17$ | $z\approx 5$ | $z\approx 5$
---|---|---|---
|  | Quiescent | Starburst
F560W | 99${}^{+11}_{-10}$ | 133${}^{+19}_{-21}$ | 109${}^{+22}_{-7}$
F770W | 109${}^{+55}_{-20}$ | 136${}^{+26}_{-24}$ | 83${}^{+28}_{-5}$
F1000W | 87${}^{+68}_{-29}$ | 136${}^{+37}_{-32}$ | 84${}^{+37}_{-5}$
F1280W | 106${}^{+93}_{-39}$ | 118${}^{+37}_{-27}$ | 169${}^{+21}_{-11}$
F1500W | 71${}^{+102}_{-27}$ | 86${}^{+36}_{-21}$ | 91${}^{+22}_{-7}$
F1800W | 78${}^{+107}_{-26}$ | 82${}^{+82}_{-23}$ | 199${}^{+49}_{-16}$
F2100W | 77${}^{+96}_{-37}$ | 69${}^{+81}_{-21}$ | 228${}^{+49}_{-17}$

Taking into account that CEERS-1749 may be a part of a $z\approx 4.9$
protocluster, we can refine our models for the source by fixing the redshift
to that of the overdensity (a Gaussian with $z=4.9\pm 0.1$), which
significantly shrinks the parameter space search volume.

Under our fiducial $\approx 20\%$ error floor assumption, the galaxies we find
are all quiescent systems (see top-row of Figure 3). The only data point that
this class of solutions struggles to explain is the F277W flux, where the
prediction falls short by $\approx 50\%$ – this is why these solutions are
disfavored compared to the $z\approx 17$ scenario in e.g., Figure 1. Perhaps
there is a calibration or zero-point issue in our F277W photometry (though
Donnan et al. 2022 report a similar F200W-F277W color as in this work). Or
perhaps, the models we have employed here are missing some physics (e.g., line
emission from AGN or shocks) that manifests around rest-frame 4300-5300 Å.

Inspired by this, we add an AGN emission line template (following line ratios
from Richardson et al. 2014, their Table 3) to the quiescent galaxy
solution111https://github.com/bd-j/prospector/blob/agnlines/. We are also
motivated by the finding that ionized gas, likely due to AGN, is routinely
observed in quiescent and post-starburst systems (e.g., Belli et al., 2019).
The resulting emission lines boost the F277W flux, providing a better fit
(center panel, Figure 3). However, the required AGN luminosity –
(H$\beta/L_{\rm{\odot}})/(M_{\rm{\star}}/M_{\rm{\odot}})\approx 10^{-2.5}$ –
is $\gtrsim 100\times$ higher than expected for the galaxy’s stellar mass
based on $z\lesssim 2$ scaling relations and observations (e.g., Heckman &
Best, 2014). Nonetheless, the key takeaway from this exercise is that ionized
gas regardless of its origin (e.g., from shock heating in the overdense
environment) that produces emission line luminosities and ratios atypical for
star-forming galaxies, and is therefore missed by standard models, may help
explain the SED.

Finally, we perform a search by adopting a $10\%$ error-floor on all
photometry. This yields a starburst solution (bottom panel, Figure 3) in a
relatively low-mass ($\approx 5\times 10^{8}M_{\rm{\odot}}$) system. The SED
is dominated by young stars, and shows a Balmer jump instead of a Balmer
break. The $>2\mu$m photometry is remarkably well-explained by nebular
emission – this is a challenging constraint to match, given the required blue
slope and the sensitivity of the F410M medium-band. In bluer bands the
predicted flux lies barely under the $\approx 1\sigma$ detection limits due to
significant dust attenuation ($A_{\rm{5500}}\approx 1.2$ mag). It is
perplexing as to why this solution, with a $\chi^{2}$ lower than the quiescent
galaxies, was missed in our broad $z=0-20$ search despite using conservative
dynesty sampling settings. A clue might be the extremely tight redshift
posterior of $z=4.87^{+0.00}_{-0.02}$. This is consistent with a highly
“spiky” likelihood space, where there is a very precise combination of
redshift and nebular parameters that produce the perfect conspiracy of
emission lines to mimic the $z\approx 17$ system. Constraining the redshift to
the overdensity mean, thereby greatly reducing the prior volume, likely helped
reveal this solution, but this is a critical issue for future study.

###  3.3 Physical Properties

We derive physical properties for the source using Prospector (Leja et al.,
2017; Johnson et al., 2021). The priors and parameter choices are as described
in Naidu et al. (2022a) (their §4.1; closely following Tacchella et al. 2022).
Briefly, we fit a non-parametric star-formation history assuming a prior on
the redshift that follows the EAZY photometric redshift constraint. For our
fiducial run we assume a “continuity” prior on the star-formation history and
a formation redshift of $z=20$ that produces smooth histories disfavoring
abrupt jumps from bin to bin (Leja et al., 2019). The resulting properties are
summarized in Table 3. In what follows we split the discussion assuming the
$z\approx 17$ and $z\approx 5$ solutions.

####  3.3.1 CEERS-1749 at $z\sim 17$

Broadly, CEERS-1749 at $z\sim 17$ has properties characteristic of galaxies
found at $z\gtrsim 8$ (e.g., Stefanon et al., 2022; Tacchella et al., 2022;
Leethochawalit et al., 2022), with a star-formation rate expected of its
stellar mass and a blue $\beta_{\rm{UV}}\approx-2.3$ (see also Bagpipes fits
in Donnan et al., 2022). As noted earlier, its $M_{\rm{UV}}\approx-22$ would
place it among the most UV-luminous sources at $z\gtrsim 6.5$. Perhaps the
most striking property from the SED fitting is the stellar mass – we infer
$\log({M_{\rm{\star}}/M_{\rm{\odot}}})\approx 9.6$ in stars to have formed in
a mere $\sim 220$ Myrs after the Big Bang. We caution that estimates of the
stellar mass from rest-UV photometry alone come with significant systematic
uncertainties since the light is dominated by young stars.

To understand the range of allowed stellar masses we consider the following
changes to our fiducial fitting setup – we assume a “bursty” prior for the
star-formation history that allows for rapid fluctuations (Tacchella et al.,
2022); we consider three different initialization points for the star-
formation history ($z=18,20,30$); we consider a Kroupa IMF in addition to the
fiducial Chabrier (2003) IMF. Across all these various parameter choices, we
recover stellar masses of $\approx 10^{9.4}-10^{9.8}M_{\rm{\odot}}$ with
$\lesssim$0.3 dex uncertainties on each individual fit. As a limiting case we
also model the galaxy as a single stellar population set to an age $<200$ Myr
(the age of the Universe at $z=16.6$ is 230 Myrs), which yields a stellar mass
of $\log(M_{\rm{\star}}/M_{\rm{\odot}})\approx 10.0^{+0.2}_{-0.2}$ and an age
of $70^{+30}_{-10}$ Myrs. Based on these experiments, we conservatively adopt
a mass of $\log(M_{\rm{\star}}/M_{\rm{\odot}})\approx 9.6^{+0.5}_{-0.5}$ or
$\approx 5\times 10^{9}M_{\rm{\odot}}$ for this source for the discussion in
the rest of the paper.

CEERS-1749 was also recently reported by Donnan et al. (2022, see also ), in
their search for $z>10$ candidates across the JWST ERS data, as lying at
$z=16.6$. They infer a slightly lower mass of
$\log(M_{\rm{\star}}/M_{\rm{\odot}})=9.0\pm 0.4$. On top of differences
between inference frameworks – Bagpipes vs. Prospector, and importantly their
underlying stellar isochrones, Parsec vs. MIST, see e.g., Whitler et al. 2022
– their reported fluxes are significantly lower than those reported here (by
$\approx 2\times$), which can likely be traced to aperture corrections.
CEERS-1749 is an extended source, and the point-source correction applied for
its flux in Donnan et al. (2022) may have been insufficient.

####  3.3.2 CEERS-1749 at $z\sim 5$

We run fits for CEERS-1749 by fixing the redshift to the protocluster redshift
($z=4.9\pm 0.1$) as described in the previous section. We first discuss the
quiescent case followed by the low-mass starburst scenario.

The quiescent galaxy solution displays very modest ongoing star-formation –
averaged over the last 50 Myrs, its SFR is
$0.1^{+3.6}_{-0.1}M_{\rm{\odot}}$/yr, and over the last 10 Myrs an even more
humble $0.02^{+4.08}_{-0.02}M_{\rm{\odot}}$/yr. The constraint against recent
star-formation is a direct result of having to produce a strong Balmer break
to match the red F200W-F277W color. We estimate its stellar mass to be
$\approx 5\times 10^{9}M_{\rm{\odot}}$. No quiescent galaxy with such a low
stellar mass has been spectroscopically confirmed at $z\gtrsim 2$ yet, but
this has been largely due to the impracticality of continuum spectroscopy at
such luminosities. The low-mass quenched population certainly exists, as
evidenced by Marchesini et al. (2022)’s recent confirmation of a $\approx
8\times 10^{9}M_{\rm{\odot}}$ quiescent system at $z\approx 2.4$ via
JWST/NIRISS spectroscopy. Other candidate quiescent systems at $z>2$ of even
lower mass have been observed as part of the same program, and may be
confirmed as NIRISS calibrations improve.

The dusty star-forming galaxy scenario features a system that has formed
$\approx 15\%$ of its total stellar mass in only the last $<10$ Myrs. Its
young stars are producing intense nebular emission that strongly boosts the
broadband photometry and comfortably accounts for the F200W-F277W color. Such
extreme emission has been inferred to be routine (e.g., De Barros et al.,
2019), and is now beginning to be directly observed during these epochs (e.g.,
Schaerer et al., 2022). The high attenuation ($A_{\rm{5500}}\approx 1.2$ mag),
on the other hand, may seem unexpected for a galaxy of such low stellar mass
based on $<2\mu$m-selected samples (e.g., Cullen et al., 2018). However, as
fainter members of the “HST-dark” population (galaxies detected purely at
$>2\mu$m) come into view (e.g., Barrufet et al., 2022; Nelson et al., 2022;
Fudamoto et al., 2022), galaxies like CEERS-1749 may be found to be common. In
fact, the colors of the overdensity suggest it may be the perfect place to
prospect for the low-mass end of this population. The dust-curve we infer is
slightly steeper than that of the Small Magellanic Cloud ($A_{\rm
1500}/A_{\rm{5500}}\approx 6$) – such steep curves are often observed in the
local Universe (Salim & Narayanan, 2020), but typically in lower
$A_{\rm{5500}}$ systems (Chevallard et al., 2013).

We end this section by observing that broadly, the properties of CEERS-1749
are physically plausible, but fall firmly in observational regimes that are
only beginning to come into view with JWST (e.g., low-mass quiescent, low-mass
HST-dark). Their novelty represents an exciting opportunity for surprises, as
well as a potential challenge for $z>10$ searches.

Table 3: Summary of properties for the high- and lower-redshift solutions.

CEERS-1749$\dagger$$\dagger$footnotemark:  at | $z\sim 17$ | $z\sim 5$ | $z\sim 5$
---|---|---|---
|  | (quiescent) | (starburst)
R.A. | 14:19:39.48
Dec. | +52:56:34.95
Best-fit Redshift $z_{\rm{Prospector}}$ | $16.0$ | $4.8$ | $4.9$
Redshift $z_{\rm{Prospector}}$ | $16.0^{+0.6}_{-0.6}$ | $4.8^{+0.1}_{-0.1}$ | $4.87^{+0.00}_{-0.02}$
Best-fit Redshift $z_{\rm{EAZY}}$ | $16.6$ | $4.6$ | –
Redshift $z_{\rm{EAZY}}$ | $16.3^{+0.4}_{-1.1}$ | $4.4^{+0.6}_{-1.0}$ | –
UV Luminosity ($M_{\rm{UV}}$) | $-22.0^{+0.1}_{-0.1}$ | $-15.6^{+1.1}_{-0.7}$ | $-14.3^{+0.2}_{-1.0}$
UV Slope ($\beta$; $f_{\rm{\lambda}}\propto\lambda^{\beta}$) | $-2.3^{+0.1}_{-0.1}$ | $1.1^{+1.1}_{-1.2}$ | $3.0^{+0.3}_{-0.5}$
Stellar Mass $\log$($M_{\rm{\star}}/M_{\rm{\odot}}$) | $9.6^{+0.2}_{-0.2}$ | $9.6^{+0.2}_{-0.5}$ | $8.7^{+0.1}_{-0.1}$
Age ($t_{\rm{50}}$/Myr) | $54^{+27}_{-27}$ | $564^{+105}_{-492}$ | $580^{+17}_{-236}$
SFR${}_{\rm{50\ Myr}}$ ($M_{\rm{\odot}}$/yr) | $34^{+17}_{-12}$ | $0.1^{+3.6}_{-0.1}$ | $1.7^{+1.8}_{-0.2}$
$r_{\mathrm{eff}}$, $F$444$W$ [kpc] | 0.4 | 0.9
$r_{\mathrm{eff}}$, $F$200$W$ [kpc] | 0.2, 0.3 | 0.5, 0.7

††footnotetext: This is the same source as CEERS-93316 in Donnan et al. (2022)
and CR2-z17-1 in Harikane et al. 2022a.

Note. — Quantities derived via SED fitting assume a continuity prior (Leja et
al. 2019) on the star-formation history and a Chabrier (2003) IMF. Variations
on this fiducial assumption, with a focus on the recovered stellar mass, are
tested in detail in §3.3. The two effective radii quoted in $F$200$W$ are for
the two component fit in Fig. 4.

Figure 4: Results of the GALFIT morphology analysis for CEERS-1749. The
different columns from left to right correspond to the original data, the
model, and the residual, and the rows correspond to the analysis of the image
in F200W, F277W, and F444W respectively. The images are scaled so that color
corresponds to negative flux, black corresponds to postive flux, and white
corresponds to 0 flux. The galaxy is consistent in all band with a disky
($n\sim 1$) Sérsic profile. In the bluer bands, the structure of the galaxy is
asymmetric, and a clumpy feature can be seen to the SE (illustrated with a
black circle in the residuals). However, in the reddest band, F444W, this
feature is absent.

###  3.4 Clues from Morphology

In order to characterize the morphology of the galaxy while accounting for the
effect of the PSF, we fit two-dimensional Sérsic profiles to the candidate in
the F200W, F277W, and F444W imaging using GALFIT (Peng et al., 2010a). We
create 150-pixel cutouts around the galaxy, then use photutils and astropy to
create a segmentation map to identify nearby galaxies using the F444W image.
We mask all nearby galaxies using the resultant segmentation maps and fit only
CEERS-1749. In all our fits, we measure a scalar sky background estimated from
the median of the sky pixels identified in the segmentation map and fix the
flux of the sky to this value in GALFIT. We use a theoretical PSF model
generated from WebbPSF at our 0$\farcs$04 pixel scale; we oversample the PSF
by a factor of 9 in order to minimize artifacts as we rotate the PSF to the
CEERS observation angle calculated from the APT file, then convolve with a 9x9
pixel square kernel and downsample to the mosaic resolution. In all fits, we
constrain Sérsic index $n$ to be between 0.01 and 8, the magnitude to be
between 0 and 45 mag, and the half-light radius $r_{e}$ to be between 0.3 and
200 pixels (0$\farcs$012 - 8$\farcs$0).

The results of these fits are shown in Figure 4. In all 3 bands, the galaxy is
well described by a disky ($n\sim 1$) Sérsic profile. However, in the F200W
image and resultant residuals, there is a clear indication of non-Sérsic flux
to the southeast of the main galaxy, illustrating that the galaxy may be a
clumpy disk or a merging pair. This same residual structure is marginally
visible in the F277W image, but in F444W, the galaxy profile is consistent
with being completely smooth.

Assuming that CEERS-1749 is at $z\approx 17$, its morphology is consistent
with bright galaxies catalogued across the Epoch of Reionization, which often
break up into clumpy substructure in their rest-UV (e.g., Bowler et al., 2017;
Matthee et al., 2019). The size in $F$444$W$ (0.4 kpc) as well as that of the
individual components in $F$277$W$ (0.2 and 0.3 kpc) is quite compact, similar
to that measured recently in the CEERS field for the most massive star-forming
galaxies at $z\approx 7-10$ (Labbe et al., 2022), hinting at an evolutionary
link between the most luminous $z>10$ systems and the most massive galaxies at
lower redshifts.

For the case of CEERS-1749 at $z\approx 5$, the nearby clump in the images
appears across a wide-range of rest-optical wavelengths ($\approx
3000-7500$Å). Clumpy star-formation occurring in the same disk is a
possibility. However, these clumps are typically evident in the UV, with
optical profiles behaving more smoothly.

We might perhaps be witnessing a merger. If CEERS-1749 lives in a dense
protocluster environment at $\approx 5$, mergers are quite likely. Mergers are
a key channel for setting off the kind of vigorous starbursts required for the
dusty starburst scenario (e.g., Mihos & Hernquist, 1996). Further, several
quenching mechanisms, particularly at $z\gtrsim 2$, invoke mergers (e.g., Man
& Belli, 2018) – e.g., an AGN is triggered by fresh inflows of gas to the
centre of the galaxy, and the subsequent feedback from the freshly fed AGN
quenches star-formation. It is interesting to note that the merging clump in
the images appears only in the bluer bands, and is not apparent in F444W. In
the $z\approx 17$ scenario, strong wavelength-dependent morphology across such
a narrow rest-$1600$Å to rest-$2150$Å range is surprising, but is perhaps more
easily explained by a blue galaxy merging with a quiescent/dusty one at
$z\approx 5$.

Another possibility is that CEERS-1749 may be in the process of tidal
disruption by its massive neighbor if they lie at the same redshift. The
galaxy lies well within the expected virial radius of its neighboring
$M_{\rm{\star}}\approx 10^{11}M_{\rm{\odot}}$ system that is only $<15$ kpc
away. The faint substructure in the images may be stripped tidal debris
trailing the galaxy. Starbursts often occur in the disrupting galaxy (e.g., Di
Cintio et al., 2021), following which it loses all its gas and is quenched
(e.g., Tepper-García & Bland-Hawthorn, 2018). A circular polar orbit that
decays gradually may allow for a significant period between infall and tidal
dissolution that may allow us to observe the galaxy in this transient state
(see e.g., the gallery of mergers in Naidu et al. 2021).

##  4 Discussion

Figure 5: How CEERS-1749 at $z\sim 16.5$ would challenge our understanding of
galaxy formation. Left: Implied constraints on the bright end of the UV LF at
$z\sim 16.5$. If this source is indeed confirmed to lie at $z\sim 16.5$, it
would defy virtually every model of early galaxy evolution. The solid light
gray lines show predictions from two models (Dayal et al., 2014; Behroozi et
al., 2019) at $z\sim 11$, while the darker gray lines are for $z\sim 15-16$.
At the observed number density, CEERS-1749 would be 4mag too bright for the
model predictions. Even more impressive is the comparison with the DM halo
mass function. The dotted gray line shows the predicted LF of an extreme model
where all baryons in a given halo are converted into stars. This is the only
way to reproduce the UV LF of this galaxy. For context, similarly luminous
galaxies at $z\approx 6-10$ have star-formation efficiencies inferred to be
$<10\%$ (e.g., Tacchella et al., 2018; Stefanon et al., 2021). As an empirical
comparison we also show extrapolations of the double-power law UVLF (”DPL”)
from Bowler et al. (2020) to $z\approx 11$ and $z\approx 16$. Right: Galaxy
stellar mass threshold vs. redshift expected for $\Lambda$CDM cosmology,
adopted from Behroozi & Silk (2018). The stellar mass threshold is derived
from halo mass functions that assume a $100\%$ star-formation efficiency,
i.e., $M_{\rm{\star}}/M_{\rm{halo}}=f_{\rm{baryon}}$, where
$f_{\rm{baryon}}=0.16$ is the cosmic baryon fraction. CEERS-1749, and the
tentative implied number density of its analogues ($\approx 10^{-5}$ Mpc-3),
places it in a regime that significantly deviates from the norm for
$\Lambda$CDM. If confirmed to lie at $z\approx 17$, and if analogues of
CEERS-1749 prove to be as common as the first JWST extragalactic fields imply,
this may provide a compelling constraint on cosmology.

###  4.1 Implications of the $z\approx 17$ scenario

If CEERS-1749 is confirmed to lie at $z\approx 17$, it would force a major
revision of early galaxy evolution models, and potentially even our underlying
cosmological framework (see, e.g., Steinhardt et al., 2016; Mason et al.,
2022; Boylan-Kolchin, 2022). It is very challenging to produce such
extraordinarily luminous and massive galaxies only $\sim$200 Myrs after the
Big Bang under standard assumptions in the framework of $\Lambda$CDM cosmology
(see also the candidates reported in Atek et al. 2022; Yan et al. 2022).

This situation is demonstrated in Figure 5. First, we consider the number
density implied by an $M_{\rm{UV}}\approx-22$ source at $z\approx 16.5$ found
in a search area of a mere 40 arcmin2 and $\Delta z=1$. No theoretical UVLF or
empirical extrapolation comes close to matching this implied number density
within one to several orders of magnitude (Behroozi et al., 2019; Bowler et
al., 2020; Naidu et al., 2022a; Dayal et al., 2022). Even more strikingly, the
only way to match such a high number density is by coupling dark matter halo
mass functions to a $100\%$ instantaneous star-formation efficiency (see also
Mason et al., 2022). That is, every baryon allocated to a dark matter halo in
accordance with the cosmic baryon fraction is converted into stars immediately
following a Salpeter (1955) IMF between $0.1-100\,M_{\rm{\odot}}$. For
context, the typical efficiency inferred from a variety of arguments at
$z\approx 6-10$ is $<10\%$ (e.g., Tacchella et al., 2018; Stefanon et al.,
2021).

The right panel of Figure 5 illustrates this extraordinary situation in terms
of a stellar mass threshold implied by halo mass functions under a similar
assumption of a $100\%$ star-formation efficiency calculated in Behroozi &
Silk (2018). CEERS-1749 (and its implied number density) places it in a region
of the diagram that is in tension with $\Lambda$CDM cosmology.

Numerous caveats underlie these comparisons, and there are several possible
solutions to this apparent tension (CEERS-1749 at $z\sim 5$ being the most
likely one). Here we detail several other possibilities.

Lensing is expected to have a substantial effect on the bright end of the UVLF
at high redshifts given the increasing optical depth. This is particularly
pertinent for CEERS-1749 given that it sits $<2\arcsec$ from a $z\approx 5$
$M_{\rm{\star}}\approx 10^{11}M_{\rm{\odot}}$ galaxy and that the sightline
includes a protocluster. However, lensing corrections even in the most
overdense regions tend to be $<2\times$ (e.g., Mason et al., 2015), which does
little to alleviate the situation in Figure 5.

Another relevant class of ideas revises the relationship between light and
mass. For instance, modifying the IMF to be extremely top-heavy produces much
higher UV luminosities for a given stellar mass (up to $\approx 10\times$
higher compared to our assumptions of a “normal” IMF, e.g., Fardal et al.
2007). Pop III stars and binary stars occurring at low metallicities similarly
produce different translations between light and mass. And finally, a
possibility that can not be ignored is that some fraction of the luminosity of
CEERS-1749 may not be of stellar origin at all, but could arise from accretion
onto early black holes (e.g., Pacucci et al., 2022).

###  4.2 Implications of the $z\approx 5$ scenario

Figure 6: Demonstration of the narrow redshift range allowed for a $z\approx
5$ interloper with strong emission lines to mimic a $z\approx 17$ system.
Left: The redshift probability distribution allowed for the dusty star-forming
galaxy solution is very narrow because of the stringent requirement to produce
a break at $\sim 2\mu m$ as well as a blue UV slope across a very wide
wavelength range ($\approx 2-5\mu$m), which includes a medium band filter
(F410M). Right: SEDs of dusty star-forming galaxies with strong emission lines
at the most-likely redshift of $z=4.87$ (center, black border), a slightly
lower redshift of $z=4.72$ (top, blue border), and a slightly higher redshift
of $z=4.92$ (bottom, red border). The medium band at 4.1$\mu$m sensitively
discriminates even among these very similar redshifts – notice the strongly
fluctuating $\chi^{2}$ across the panels. This makes an emission line galaxy
solution possible only in an very narrow redshift range, severely limiting the
volume for such contaminant in $z>10$ searches, particularly when medium bands
are used in the filter set.

We emphasize that the redshift solution for CEERS-1749 across multiple
studies, which use diverse data reduction choices and $z>10$ selection
techniques, seems unambiguous: $z\approx 17$, with $p(z>10)>99.9\%$, and
little room permitted for any other possibility (Naidu et al., 2022a; Donnan
et al., 2022; Harikane et al., 2022a). There is no hint of a $z\approx 5$
solution that may be upweighted into relevance by e.g., a luminosity prior. If
not for the conservative error floor on the photometry adopted here, and the
fortuitous environmental evidence, there would be little reason to place this
source at $z\approx 5$ (but see also Zavala et al., 2022).

The difficulty of identifying the $z\approx 5$ solution for CEERS-1749 could
be construed to imply that some fraction of the seemingly secure $z>10$
candidates may be interlopers of the kind discussed in this work. Galaxies
with relatively weaker breaks in their SED are the most vulnerable – e.g., the
dusty starburst scenario could account both for their break as well as the
slope of their longer wavelength photometry. Such interlopers may help resolve
the tension described in the prior section. At slightly lower redshifts
($z\approx 6-10$) the occurrence of the strongest rest-optical lines as well
as the presence of both Balmer breaks as well as Lyman breaks in the NIRCam
coverage provide additional safeguards (e.g., Labbe et al., 2022). Further,
MIRI photometry (e.g., see how the dusty galaxy stands out in Table 2), an
additional medium band (for example, in the JADES GTO program filter-set,
Rieke 2020), or any spectroscopy would comfortably protect against such
interlopers.

We also emphasize that we are dealing with an extraordinary situation given
the foreground protocluster. The redshift range in which strong emission lines
in a dusty system perfectly conspire to mimic a Lyman break as well as the
blue UV-slope of a $z>10$ galaxy is very narrow. For instance, in our dusty
starburst scenario the full redshift posterior collapses to an extremely tight
$z=4.87^{+0.00}_{-0.02}$. The implied foreground contaminant volume for such a
narrow redshift range is therefore far less dire for $z>10$ searches than may
seem at first glance, particularly when a medium-band is included in the
filter-set. We illustrate this situation in Figure 6.

If this source turns out to be a quiescent galaxy, it would be the highest
redshift quiescent system known, with the current most distant galaxy at
$z\approx 4$ (Valentino et al., 2020). Finding quenched galaxies at such early
times places stringent constraints on galaxy evolution models and the physics
of feedback. Since the galaxy has a relatively low stellar mass, it is
expected to be undergoing “environmental quenching” (e.g., Peng et al.,
2010b), which is consistent with its location in an overdensity. However, it
being at such high redshift, only $\approx 1$ Gyr after the Big Bang,
challenges many theoretical scenarios that have been developed at lower
redshifts, including the general result that satellites experience “delayed-
then-rapid” quenching (e.g., Wetzel et al., 2013; Fillingham et al., 2019;
Naidu et al., 2022b), which starts several Gyrs after infall.

On the other hand, confirmation of the dusty starburst scenario would extend
the HST-dark population to lower masses, raising intriguing questions about
how such low-mass galaxies got so dusty so fast in only the first billion
years of the Universe (e.g., Ferrara et al., 2016; Popping et al., 2017;
Leśniewska & Michałowski, 2019; Dayal et al., 2022). Further, protoclusters at
high redshift are expected to be among the first sites of star-formation and
reionization in the Universe – the dozens of potentially associated neighbors
around CEERS-1749 are therefore exciting targets that make multi-object
spectroscopic follow-up an even more compelling proposition.

##  5 Summary & Outlook

Within the first few weeks of JWST’s initial data release, the facility has
already delivered a major expansion of our cosmic frontier, with dozens of
$z>10$ candidates being reported. Several of these sources are unexpectedly
luminous ($M_{\rm{UV}}\lesssim-21$), and are far more common than state-of-
the-art projections for the full Cycle 1 yield across all scheduled programs.
In this paper we present potentially the most extreme of these systems:
CEERS-1749. We discuss two redshift solutions for the source, each of which
has wide-ranging implications.

  * •

Across a variety of SED-fitting choices, we find $z\approx 17$ to be the most
likely redshift with no lower-$z$ solutions found. Other independent analyses
and state-of-the-art techniques find a similarly confident, unambiguous
solution (e.g., Donnan et al., 2022; Harikane et al., 2022a). Only a
conservative $\approx 20\%$ error-floor on all photometry to account for
systematic uncertainties shows hints of a $z\approx 5$ solution. [Figure 1]

  * •

If the galaxy is at $z\approx 17$, then it has physical properties (e.g.,
$\beta_{\rm{UV}}$, SFR) and a morphology (clumpy in the rest-UV) expected of
$z>10$ galaxies. However, most strikingly, its stellar mass ($\approx 5\times
10^{9}M_{\rm{\odot}}$) and UV luminosity ($M_{\rm{UV}}\approx-22$) are
unexpected for a system lying a mere $\sim 220$ Myrs from the Big Bang. [Table
3, Figure 5]

  * •

We show that the SED of the galaxy can be explained by a $\approx
10^{9}-10^{10}M_{\rm{\odot}}$ quiescent galaxy with line emission arising from
ionized gas, or a $\approx 5\times 10^{8}M_{\rm{\odot}}$ dusty starburst whose
nebular emission lines boost the photometry and conspire to produce an
apparently blue slope in the $>2\mu$m photometry. The morphology of the
source, which shows hints of a merger and/or tidal disturbance supports both
these scenarios. [§3.2.2, Figure 3, 4]

  * •

The $z\approx 5$ scenario has strong environmental support. The three nearest
neighbors of the galaxy lie at precisely the redshift required by the low-$z$
solution, i.e., $z\approx 5$. Further, there is a hint of a substantial
$z\approx 5$ overdensity in the CEERS field – $\approx 25$ sources
$<5^{\prime}$ from the candidate have photometric redshifts of $z\approx 5$,
some of them displaying mature stellar populations expected of an early-
forming protocluster. [§3.2, Figure 2]

  * •

These results suggest that at certain specific redshifts, $z>10$ candidates
from JWST may contain a class of lower-$z$ interlopers. If not for the various
lines of circumstantial evidence, there would have been little reason to doubt
the $z\approx 17$ solution. However, we stress that the perfect storm of
parameters required to mimic both the break and continuum of a $z>10$
candidate is possible only in a very narrow redshift range, especially when
medium-bands are employed, implying that such interlopers may not be a major
concern for $z>10$ searches. [§4.2, Fig. 6]

Spectroscopic follow-up of this remarkable galaxy is of critical urgency to
JWST’s mission of expanding the cosmic frontier. A $z\approx 5$ solution might
provide new insights into the physics of quenching and dust production, as
well as an important class of interloper galaxies to strengthen $z>10$
searches. On the other hand, if this source does lie at $z\approx 17$, we may
embark on the grand enterprise of revising the physics of galaxy evolution at
the earliest epochs.

We are grateful to the CEERS team for planning these early release
observations and speedily making resources available to the community that
have made this work possible. RPN acknowledges funding from JWST programs
GO-1933 and GO-2279. We acknowledge support from: the Swiss National Science
Foundation through project grant 200020_207349 (PAO, LB, AW). The Cosmic Dawn
Center (DAWN) is funded by the Danish National Research Foundation under grant
No. 140. RJB and MS acknowledge support from NWO grant TOP1.16.057. S. Bose is
supported by the UK Research and Innovation (UKRI) Future Leaders Fellowship
[grant number MR/V023381/1]. MS acknowledges support from the
CIDEGENT/2021/059 grant, and from project
PID2019-109592GB-I00/AEI/10.13039/501100011033 from the Spanish Ministerio de
Ciencia e Innovación - Agencia Estatal de Investigación. S. Belli is supported
by the Italian Ministry for Universities and Research through the _Rita Levi
Montalcini_ program. K.E.H. acknowledges support from the Carlsberg Foundation
Reintegration Fellowship Grant CF21-0103. PD acknowledges support from the NWO
grant 016.VIDI.189.162 (“ODIN”) and from the European Commission’s and
University of Groningen’s CO-FUND Rosalind Franklin program. Cloud-based data
processing and file storage for this work is provided by the AWS Cloud Credits
for Research program. 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. These
observations are associated with programs # 1324 and # 1345.

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