# Brown Dwarf Candidates in the JADES and CEERS Extragalactic Surveys

Kevin N. Hainline Steward Observatory, University of Arizona, 933 N. Cherry
Ave, Tucson, AZ 85721, USA  Jakob M. Helton Steward Observatory, University of
Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA  Benjamin D. Johnson Center
for Astrophysics $|$ Harvard & Smithsonian, 60 Garden St., Cambridge MA 02138
USA  Fengwu Sun Steward Observatory, University of Arizona, 933 N. Cherry Ave,
Tucson, AZ 85721, USA  Michael W. Topping Steward Observatory, University of
Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA  Jarron M. Leisenring
Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ
85721, USA  William M. Baker Kavli Institute for Cosmology, University of
Cambridge, Madingley Road, Cambridge CB3 0HA, UK  Cavendish Laboratory,
University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK  Daniel
J. Eisenstein Center for Astrophysics $|$ Harvard & Smithsonian, 60 Garden
St., Cambridge MA 02138 USA  Ryan Hausen Department of Physics and Astronomy,
The Johns Hopkins University, 3400 N. Charles St. Baltimore, MD 21218  Raphael
E. Hviding Steward Observatory, University of Arizona, 933 N. Cherry Ave,
Tucson, AZ 85721, USA  Jianwei Lyu Steward Observatory, University of Arizona,
933 N. Cherry Ave, Tucson, AZ 85721, USA  Brant Robertson Department of
Astronomy and Astrophysics, University of California, Santa Cruz, 1156 High
Street, Santa Cruz CA 96054, USA  Sandro Tacchella Kavli Institute for
Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge
CB3 0HE, UK  Christina C. Williams NSF’s National Optical-Infrared Astronomy
Research Laboratory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
Christopher N. A. Willmer Steward Observatory, University of Arizona, 933 N.
Cherry Ave, Tucson, AZ 85721, USA  Thomas L. Roellig NASA Ames Research
Center, MS 245-6, Moffett Field, CA 94035, USA

###### Abstract

By combining the JWST/NIRCam JADES and CEERS extragalactic datasets, we have
uncovered a sample of twenty-one T and Y brown dwarf candidates at best-fit
distances between 0.1 - 4.2 kpc. These sources were selected by targeting the
blue 1$\mu$m - 2.5$\mu$m colors and red 3$\mu$m - 4.5$\mu$m colors that arise
from molecular absorption in the atmospheres of T${}_{\mathrm{eff}}<$ 1300K
brown dwarfs. We fit these sources using multiple models of low-mass stellar
atmospheres and present the resulting fluxes, sizes, effective temperatures
and other derived properties for the sample. If confirmed, these fits place
the majority of the sources in the Milky Way thick disk and halo. We observe
proper motion for seven of the candidate brown dwarfs with directions in
agreement with the plane of our galaxy, providing evidence that they are not
extragalactic in nature. We demonstrate how the colors of these sources differ
from selected high-redshift galaxies, and explore the selection of these
sources in planned large-area JWST NIRCam surveys. Deep imaging with
JWST/NIRCam presents an an excellent opportunity for finding and understanding
these very cold low-mass stars at kpc distances.

Brown dwarfs(185) — Halo stars(699) — Infrared astronomy(786) — James Webb
Space Telescope(2291)

††facilities: JWST(NIRCam, NIRSpec), HST(ACS, WFC3), Spitzer(IRAC)††software:
astropy (Astropy Collaboration et al., 2013, 2018), astroquery (Ginsburg et
al., 2019), matplotlib (Hunter, 2007), numpy (Harris et al., 2020), scipy
(Virtanen et al., 2020), Photutils (Bradley et al., 2023), EAZY (Brammer et
al., 2008).

##  1 Introduction

At the intersection of stellar and exoplanet science lies the study of
ultracool brown dwarfs, low-mass stars (M $<0.07$ M⊙) at relatively cool
effective temperatures (T${}_{\mathrm{eff}}<$ 2500 K). Over the last several
decades researchers have uncovered many hundreds of brown dwarfs, which extend
the stellar classification system from hot O-type stars down to L
(T${}_{\mathrm{eff}}=1300-2000$K), T (T${}_{\mathrm{eff}}=700-1300$K), and Y
(T${}_{\mathrm{eff}}<700$K) dwarfs (Kirkpatrick et al., 1999; Cushing et al.,
2011; Kirkpatrick et al., 2021). Selecting and characterizing these sources is
vital for exploring the stellar initial mass function, understanding binary
stellar evolution, and for increasing the census of stars around which
potentially habitable planets may lie.

However, at very low temperatures, brown dwarf stars emit most of their flux
in the infrared (IR) and are optically quite faint with spectral energy
distributions (SEDs) that feature broad molecular and narrow resonance
features (from H2O, CH4, NH3, CO, and CO2, among others). The observational
depths for existing large area IR surveys restrict the recovered brown dwarfs
to those in the stellar neighborhood, although there are individual sources
that have been found in the Milky Way thick disk and halo (Burgasser et al.,
2003b; Lodieu et al., 2010; Murray et al., 2011; Burningham et al., 2014;
Zhang et al., 2019; Schneider et al., 2020). It is vital to expand on these
samples, as brown dwarf stars at these distances help us understand the
history of star formation in the Milky Way.

Low-temperature brown dwarf candidates have been serendipitously discovered in
deep near-IR surveys from the Hubble Space Telescope (HST), as they serve as a
potential contaminant in the search for high-redshift galaxies and quasars
(Bunker et al., 2004; Wilkins et al., 2014; Finkelstein et al., 2015; Bouwens
et al., 2015; Bañados et al., 2016). Several authors have used the accumulated
area of multiple deep HST near-IR surveys to perform systematic searches for
brown dwarf candidates with observations out to $\sim 2\mu$m (Ryan et al.,
2011; Masters et al., 2012; Aganze et al., 2022). Models of brown dwarf
emission, however, show that while they do emit at 1 - 2$\mu$m, cold T and Y
dwarfs peak at $4-5\mu$m, outside of the wavelength coverage by HST, with
faint fluxes requiring very deep observations.

With the launch of the James Webb Space Telescope (JWST), a new chapter in
finding low-mass stars has begun. The NIRCam instrument on JWST offers imaging
across a FOV of 9.7 arcmin2 at $0.7-5.0\mu$m in the near-IR. Current and
upcoming extragalactic surveys, which will cover tens to many hundreds of
square arcminutes, will explore flux limits ideal for selecting cold brown
dwarfs (Holwerda et al., 2018; Hainline et al., 2020). Indeed, multiple
studies have predicted the number counts of brown dwarfs in JWST deep surveys
(Ryan & Reid, 2016; Aganze et al., 2022; Casey et al., 2022), and in the first
year of JWST, there have been discoveries made of individual brown dwarf
candidates in both the JWST Director’s Discretionary Early Release Science
Program ERS 1324, GLASS, (Nonino et al., 2023) and the Cosmic Evolution Early
Release Science (CEERS) Survey (Wang et al., 2023). Excitingly, Langeroodi et
al. (2023) and Burgasser et al. (2023) both independently confirmed a number
of brown dwarf candidates using spectroscopy taken as part of the Ultra-deep
NIRCam and NIRSpec Observations Before the Epoch of Reionization (UNCOVER,
Bezanson et al., 2022), including the source from Nonino et al. (2023). These
spectroscopic confirmations demonstrate the usage of deep extragalactic data
for finding these rare sources. It should be noted that deep fields are chosen
to point away from the disk of the Milky Way to avoid stellar contamination,
and as a result the populations of brown dwarfs observed in these surveys may
not be broadly applicable to the full Milky Way population.

Following these studies, we aim to use the JWST Advanced Deep Extragalactic
Survey (JADES) observations (Eisenstein et al., 2023) of the Great
Observatories Origins Deep Survey (GOODS) Southern and Northern regions
(Giavalisco et al., 2004) to explore the brown dwarf population at
T${}_{\mathrm{eff}}<1300$K. These data, which we combine with deep ancillary
HST observations at optical wavelengths, probe near-IR depths down to 29 -
30th magnitude AB. In addition, we supplement these observations with NIRCam
observations from the CEERS Survey (Finkelstein et al., 2022) to increase the
area covered at similar observational depths. Sources detected in these
surveys offer a unique opportunity to understand brown dwarfs at kpc distances
in the Milky Way thick disk and halo, and can be used to explore the types of
subdwarf populations that will be observed in future very large area JWST
surveys such as COSMOS-Web (Casey et al., 2022) and PRIMER (Dunlop et al.,
2021).

The outline of this paper is as follows. In Section 2 we discuss the JADES and
CEERS data we are using for finding brown dwarf candidates, the reduction of
these data, and the extraction of photometry. We then describe the selection
procedure for separating stellar candidates from galaxies. In Section 3 we
present our full sample of 21 brown dwarf candidates, discuss our brown dwarf
model fitting procedure, and the recovered properties of these objects,
including their temperatures and distances. We also describe the measured
proper motions for a subset of our sources. In Section 4 we explore the
positions of these sources within the galaxy, demonstrate methods to separate
these sources from high-redshift galaxy candidates, and discuss the selection
of low-mass stars in other JWST deep extragalactic surveys. Finally, we
conclude in Section 5. Throughout this paper we will be using magnitudes in
the AB system (Oke, 1974).

##  2 Data and Sample

###  2.1 JWST NIRCam Observations

In order to explore the types of brown dwarf candidates that can be recovered
in deep extragalactic fields, we looked at both the JADES and CEERS datasets,
which consist of deep NIRCam observations in multiple filters spanning
$1-5\mu$m that we use to separate these sources from galaxies. These survey
data have been explored both to find (Finkelstein et al., 2022; Robertson et
al., 2023; Hainline et al., 2023) and spectroscopically-confirm (Curtis-Lake
et al., 2023; Arrabal Haro et al., 2023; Fujimoto et al., 2023) the farthest
high-redshift galaxies as of the writing, making them ideal fields for
understanding the role of brown dwarf contamination of extragalactic samples.

####  2.1.1 JADES

JADES is an observing program that combines data from the NIRCam and NIRSpec
extragalactic Guaranteed Time Observations (GTO) teams. The final data set
includes NIRCam and MIRI imaging with NIRSpec spectroscopy across two well-
studied fields: GOODS-S (RA = 53.126 deg, DEC = -27.802 deg) and GOODS-N (RA =
189.229, DEC = +62.238 deg). For this study, we focus on the NIRCam
observations taken as of February 8 2023. The observations and reduction of
the JADES data we use in this study are fully described in Eisenstein et al.
(2023). Briefly, we use the JADES NIRCam observations taken as part of JWST
PID 1180 and 1181 (PI:Eisenstein), as well as PID 1210 and 1286 (PI:Ferruit).

In total, the JADES GOODS-S area is 67 square arcminutes, with 27 square
arcminutes for the JADES Deep program, and 40 square arcminutes for the JADES
Medium program. The filters used for the JADES Deep program in GOODS-S are
NIRCam F090W, F115W, F150W, F200W, F277W, F335M, F356W, F410M, and F444W
($\lambda=0.8-5.0\mu m$), and the JADES Medium area was observed with the same
filters, but without F335M. For the NIRCam parallel to the NIRSpec 1286
program, the F070W filter was included. The JADES GOODS-N area is 58 square
arcminutes, split into a deeper southeast (SE) portion (27.6 square
arcminutes) and a relatively shallower northwest (NW) portion (30.4 square
arcminutes). For the GOODS-N observations, the filters used were F090W, F115W,
F150W, F200W, F277W, F335M, F356W, F410M, and F444W, similar to JADES Deep in
GOODS-S.

We supplement the JADES observations with data from the publicly available
JWST Extragalactic Medium Survey (JEMS, Williams et al., 2023), which focuses
on the center of the JADES Deep region and uses NIRCam filters F182M, F210M,
F430M, F460M, and F480M. In addition, we include F444W observations from the
The First Reionization Epoch Spectroscopic COmplete Survey (FRESCO, PID 1895,
Oesch et al., 2023). As part of this NIRCam grism survey of the GOODS-S and
GOODS-N fields, NIRCam imaging was obtained in the F182M, F210M, and F444W
filters. We combine the F444W imaging from the FRESCO data with the JADES
imaging in the region where the surveys overlap. The 5$\sigma$ point-source
depths in a $0.2^{\prime\prime}$ diameter circular aperture vary across the
fields and filters between 28.8 and 30.6 AB Mag (see Eisenstein et al., 2023,
for more details).

At shorter wavelengths, to uncover potential proper motions, we also use
existing HST/ACS and WFC3 mosaics. We use the HST/ACS and HST/WFC3 mosiacs
from the Hubble Legacy Fields (HLF) v2.0 for GOODS-S and v2.5 for GOODS-N
($25^{\prime}\times 25^{\prime}$ for GOODS-S, and $20.5^{\prime}\times
20.5^{\prime}$ for GOODS-N, Illingworth et al., 2013; Whitaker et al., 2019).
We use data in the HST/ACS F435W, F606W, F775W, F814W, and F850LP filters to
aid in exploring galaxy fits, and HST/WFC3 F105W, F125W, F140W, and F160W for
exploring proper motions.

Details of the data reduction and flux extraction are described in Robertson
et al. (2023), Tacchella et al. (2023), and Eisenstein et al. (2023). To
select the brown dwarfs across the JADES survey, we use $0.2^{\prime\prime}$
diameter circular aperture magnitudes extracted from images that have been
convolved to match the F444W PSF to obtain accurate colors. We applied
aperture corrections to these magnitudes using empirical JWST/NIRCam point
spread functions assuming point source morphologies for our sources (see Ji et
al., 2023, for more details).

####  2.1.2 CEERS

To supplement our sample of low-mass stars, we additionally explore The Cosmic
Evolution Early Release Science (CEERS) Survey111https://ceers.github.io/
(Finkelstein et al., 2022) NIRCam observations. The CEERS Survey currently
targets 90.8 square arcminutes of the Extended Groth Strip (EGS) with the
NIRCam filters F115W, F150W, F200W, F277W, F356W, F410M, and F444W down to
$5\sigma$ point-source depths of 28.4 - 29.3 AB Mag in a $0.2^{\prime\prime}$
diameter circular aperture. We combine the JWST data with public HST/ACS data
covering the EGS in F606W and F814W (A. Koekemoer, private comm.). We used the
reduction process outlined above on the raw CEERS observations, and again,
chose the $0.2^{\prime\prime}$ diameter circular aperture magnitudes extracted
from F444W PSF-convolved images to select sources.

###  2.2 Selecting Brown Dwarf Candidates

The first of the brown dwarf candidates were found serendipitously as part of
the search for high-redshift galaxies presented in Hainline et al. (2023).
These sources were observed with red NIRCam F090W-F115W colors (similar to
F090W Lyman dropout galaxies), blue F115W - F277W colors, and then exceedingly
red F277W - F444W colors, producing a “V”-shaped SED. In addition, these
sources had point-like morphologies, often with diffraction spikes from the
NIRCam PSF observed at long wavelengths. When we fits these sources with
observed and simulated galaxy templates using the photometric redshift code
EAZY (Brammer et al., 2008) following the procedure outlined in Hainline et
al. (2023), the fits were generally very poor, with large $\chi^{2}$ results
for the best fits. The galaxy templates that we used included both star-
forming and quiescent sources, including those with very strong nebular
continuum and line emission, and we allowed EAZY to combine all of the
individual templates when fitting a given source. While galaxy templates could
not reproduce the colors of these sources, they were quite similar to the very
low temperature T dwarfs collected as part of the SpeX Prism Spectral
Libraries222https://cass.ucsd.edu/ ajb/browndwarfs/spexprism/index.html, as
well as the simulated brown dwarf SEDs from Marley et al. (2021) and Karalidi
et al. (2021).

To better understand these interesting brown dwarf candidates, we performed a
search across the JADES and CEERS footprints based on the colors of the
objects serendipitously discovered as well as the colors of brown dwarfs in
the literature. In Nonino et al. (2023), the authors selected a brown dwarf
candidate from the JWST GLASS survey with $m_{\mathrm{F444W,AB}}\leq 28$,
F356W-F444W $\geq 1.5$ and F115W-F444W $\geq 2$, criteria that were chosen
based on T and Y dwarf infrared colors presented in Meisner et al. (2020). For
our search, we extended this selection down to $m_{\mathrm{F444W,AB}}\leq
28.5$ due to the increased depth of the JADES and CEERS survey data, and in
addition, we explored two alternate color selection schemes: F277W-F444W $\geq
0.0$ combined with either F115W-F277W $\leq 1.0$ or F115W-F150W $\leq 1.0$.
These two selection criteria were designed to probe the “V”-shaped SED of
these brown dwarf candidates, and were chosen based on the positions of the
JADES galaxies and the colors of the Marley et al. (2021) and Karalidi et al.
(2021) model low-mass stars.

We find 134 unique sources are selected by any one of the three color
selection criteria in GOODS-S (2.0/arcmin2), 173 (3.0/arcmin2) in GOODS-N, and
355 (3.9/arcmin2) across the CEERS survey data. The increased density of
selected sources in GOODS-N and CEERS is due to the decreased observational
depth, as for shallower surveys, noise preferentially scatters sources into
the selection boxes. Additionally, we observe an increased number of hot
pixels in the GOODS-N and CEERS datasets, often seen in the NIRCam LW channel,
boosting the observed F444W flux.

Table 1: Brown Dwarf Candidate forcePho Fluxes (nJy)  Object ID | RA | DEC |  $r_{\mathrm{half}}$($\prime\prime$) | F090W | F115W | F150W | F200W | Object ID | F277W | F335M | F356W | F410M | F444W
---|---|---|---|---|---|---|---|---|---|---|---|---|---
JADES-GS-BD-1 |  153.026024 | -27.867171 | 0.001 | $37.06\pm 1.85$ | $267.43\pm 13.37$ | $139.33\pm 6.97$ | $92.14\pm 4.61$ |  | $51.18\pm 2.56$ | - | $131.92\pm 6.6$ | $427.53\pm 21.38$ | $327.93\pm 16.4$
JADES-GS-BD-2 |  153.036286 | -27.883739 | 0.001 | $18.83\pm 1.09$ | $115.93\pm 5.8$ | $67.78\pm 3.39$ | $41.41\pm 2.07$ |  | $23.72\pm 1.19$ | $19.04\pm 0.95$ | $65.16\pm 3.26$ | $178.97\pm 8.95$ | $129.29\pm 6.46$
JADES-GS-BD-3 |  153.056307 | -27.868408 | 0.002 | $0.35\pm 0.6$ | $11.36\pm 0.58$ | $4.81\pm 0.45$ | $2.27\pm 0.51$ |  | $-0.01\pm 0.01$ | - | $6.52\pm 0.59$ | $26.35\pm 1.74$ | $19.67\pm 1.06$
JADES-GS-BD-4 |  153.076738 | -27.889947 | 0.003 | $-1.46\pm 0.01$ | $1.91\pm 0.47$ | $1.76\pm 0.51$ | $-0.16\pm 0.01$ |  | $-0.23\pm 0.01$ | - | $3.59\pm 0.6$ | $21.73\pm 1.24$ | $24.04\pm 1.2$
JADES-GS-BD-5 |  153.084040 | -27.839348 | 0.001 | $0.09\pm 0.16$ | $2.99\pm 0.15$ | $1.91\pm 0.13$ | $1.25\pm 0.17$ |  | $8.77\pm 0.44$ | $2.17\pm 0.47$ | $60.25\pm 3.01$ | $482.67\pm 24.13$ | $624.61\pm 31.23$
JADES-GS-BD-6 |  153.103912 | -27.908507 | 0.002 | $3.24\pm 0.66$ | $25.58\pm 1.28$ | $13.02\pm 0.67$ | $5.66\pm 0.59$ |  | $4.25\pm 0.81$ | - | $15.69\pm 0.84$ | $55.39\pm 2.77$ | $43.55\pm 2.18$
JADES-GS-BD-7 |  153.105547 | -27.815154 | 0.001 | $12.98\pm 0.94$ | $118.67\pm 5.93$ | $58.75\pm 2.94$ | $33.5\pm 1.68$ |  | $20.3\pm 1.28$ | - | $66.83\pm 3.34$ | $242.62\pm 12.13$ | $209.54\pm 10.48$
JADES-GS-BD-8 |  153.133756 | -27.825521 | 0.003 | $-1.09\pm 0.01$ | $4.14\pm 0.39$ | $0.86\pm 0.44$ | $0.14\pm 0.39$ |  | $0.96\pm 0.54$ | $-1.23\pm 0.01$ | $6.52\pm 0.54$ | $47.2\pm 2.36$ | $49.2\pm 2.56$
JADES-GS-BD-9 |  153.161140 | -27.809163 | 0.001 | $1.4\pm 0.32$ | $17.11\pm 0.86$ | $9.1\pm 0.46$ | $3.63\pm 0.34$ |  | $3.0\pm 0.32$ | $1.72\pm 0.62$ | $13.81\pm 0.69$ | $53.26\pm 2.66$ | $44.22\pm 2.35$
JADES-GS-BD-10 |  153.161800 | -27.831613 | 0.001 | $305.8\pm 15.29$ | $2330.96\pm 116.55$ | $1313.29\pm 65.66$ | $934.8\pm 46.74$ |  | $559.6\pm 27.98$ | $344.54\pm 17.23$ | $1317.24\pm 65.86$ | $4082.01\pm 204.1$ | $2994.17\pm 149.71$
JADES-GS-BD-11 |  153.200319 | -27.764118 | 0.002 | $3.24\pm 0.87$ | $34.55\pm 1.73$ | $20.0\pm 1.0$ | $5.95\pm 0.71$ |  | $7.55\pm 0.47$ | $4.07\pm 1.13$ | $50.98\pm 2.55$ | $206.26\pm 10.31$ | $172.51\pm 8.63$
JADES-GN-BD-1 | 189.036189 |  -62.234159 | 0.003 | $5.21\pm 1.74$ | $40.12\pm 2.17$ | $16.42\pm 1.7$ | $8.83\pm 1.18$ |  | $6.95\pm 1.34$ | - | $33.03\pm 1.65$ | $185.23\pm 9.26$ | $183.02\pm 9.15$
JADES-GN-BD-2 | 189.118004 |  -62.268968 | 0.002 | $3.89\pm 1.25$ | $26.68\pm 1.33$ | $12.26\pm 1.08$ | $4.53\pm 0.99$ |  | $3.04\pm 1.13$ | $-2.09\pm 0.01$ | $18.03\pm 1.16$ | $86.01\pm 4.3$ | $69.02\pm 3.45$
JADES-GN-BD-3 | 189.204787 |  -62.245553 | 0.003 | $4.01\pm 1.48$ | $74.93\pm 3.75$ | $33.08\pm 1.65$ | $14.5\pm 1.23$ |  | $17.99\pm 1.22$ | $8.86\pm 1.96$ | $107.81\pm 5.39$ | $643.26\pm 32.16$ | $638.52\pm 31.93$
CEERS-EGS-BD-1 | 214.767936 |  -52.845299 | 0.004 | - | $8.97\pm 0.83$ | $3.53\pm 0.97$ | $0.59\pm 0.9$ |  | $0.4\pm 1.24$ | - | $7.22\pm 1.19$ | $31.04\pm 2.97$ | $25.53\pm 2.64$
CEERS-EGS-BD-2 | 214.823254 |  -52.874531 | 0.004 | - | $8.89\pm 0.88$ | $4.76\pm 0.98$ | $1.81\pm 0.83$ |  | $1.12\pm 1.0$ | - | $10.52\pm 1.28$ | $61.99\pm 3.18$ | $50.82\pm 2.54$
CEERS-EGS-BD-3 | 214.851340 |  -52.799294 | $<0.001$ | - | $289.62\pm 14.48$ | $138.55\pm 6.93$ | $78.11\pm 3.91$ |  | $61.89\pm 3.09$ | - | $143.33\pm 7.17$ | $478.62\pm 23.93$ | $389.8\pm 19.49$
CEERS-EGS-BD-4 | 214.910274 |  -52.860077 | 0.002 | - | $81.6\pm 4.08$ | $38.49\pm 1.92$ | $29.99\pm 1.5$ |  | $14.22\pm 0.74$ | - | $35.32\pm 1.8$ | $146.27\pm 7.31$ | $115.95\pm 5.8$
CEERS-EGS-BD-5 | 214.913696 |  -52.895919 | 0.002 | - | $17.05\pm 0.85$ | $8.55\pm 1.07$ | $6.54\pm 0.94$ |  | $2.7\pm 1.08$ | - | $8.89\pm 1.02$ | $34.09\pm 2.81$ | $22.37\pm 2.45$
CEERS-EGS-BD-6 | 214.940850 |  -52.907711 | 0.003 | - | $3.28\pm 0.64$ | $2.69\pm 0.92$ | $0.05\pm 0.68$ |  | $0.21\pm 1.29$ | - | $4.63\pm 0.93$ | $28.56\pm 3.13$ | $33.92\pm 2.56$
CEERS-EGS-BD-7 | 214.951604 |  -52.913456 | 0.003 | - | $13.67\pm 0.68$ | $7.38\pm 1.0$ | $3.79\pm 0.69$ |  | $2.9\pm 1.51$ | - | $8.6\pm 1.0$ | $60.33\pm 3.7$ | $59.95\pm 3.0$

After making our photometry and color cuts, we examined the resulting samples
of objects and rejected non-astrophysical sources, including hot pixels in the
F444W mosaic and diffraction spikes from bright stars in the footprint. We
also rejected sources with obvious extended morphologies, as those are likely
galaxies. To further aid in removing potential galaxies, our candidate
selection also required that the EAZY galaxy fit had $\chi^{2}>20$. One
source, CEERS-EGS-BD-5, has an EAZY galaxy fit with $\chi^{2}=18.31$, but we
included this source in our final sample as the EAZY fit is not consistent
with the non-detections in the ACS F606W and F814W filters. We rejected those
sources with red long-wavelength slopes similar to those described in Furtak
et al. (2022); Endsley et al. (2022); Barro et al. (2023); Labbé et al.
(2023); Labbe et al. (2023); Matthee et al. (2023); Hainline et al. (2023), as
these have colors inconsistent with brown dwarf models.

Our final sample consists of 11 sources in GOODS-S, 3 sources in GOODS-N, and
7 sources in CEERS, for a total of 21 brown dwarf candidates. For these
candidates, we re-fit the photometry using forcepho (BDJ in prep), a modeling
code that combines Sérsic profiles to measure the pixel-level fluxes for a
given source directly from the individual NIRCam exposures. The usage of
forcepho on NIRCam data is described in Robertson et al. (2023). We only fit
our candidates as it is not feasible to perform the computationally intensive
forcepho fits on the full JADES dataset in a reasonable timescale. The
forcepho photometry agrees with the photometry used to select the original
sources to within $3-26$% varying between the filters (the F410M fluxes are
the most discrepant, with the forcepho fluxes being systematically brighter
than those measured with a circular aperture), and none of the forcepho colors
for these sources are outside the selection criteria we used to initially
select them. We impose a SNR $<20$ limit on the forcepho uncertainties to
account for systematics.

In Wang et al. (2023), the authors present a brown dwarf candidate selected
from CEERS public data. In our independent reduction of the same dataset, we
extracted flux for this source and found that the photometric measurements
presented in their Table 1 are highly discrepant from our measurements. They
present an AB magnitude in the F277W filter for this source of 29.02, while we
measure a $0.2^{\prime\prime}$ diameter circular aperture magnitude in the
same filter of $27.53\pm 0.04$. A separate photometric catalog from the The
DAWN JWST Archive333https://dawn-cph.github.io/dja/ for the source includes a
$0.36^{\prime\prime}$ diameter circular aperture magnitude of $27.37\pm 0.05$.
In addition, the NIRCam colors of the source as presented in Wang et al.
(2023) are different from what we measure - we would not select this source
based on our color criteria previously described. We are confident, based on
the provided RA and DEC and the individual filter thumbnails presented in Wang
et al. (2023) for the source, that we were comparing to the same object within
our own reduction of the catalog. The EAZY fit to our measured fluxes for this
object results in a fit at a photometric redshift $z_{a}=5.9$ with
$\chi^{2}=2.4$. Given that in our own reduction of these data this source
would not be selected as a brown dwarf candidate, we do not include it in our
analysis.

Figure 1: Color criteria used to select brown dwarf candidates in this work.
In each panel, JADES sources with m${}_{F444W}<28.5$ are plotted in grey. Our
selected JADES and CEERS candidates discussed in this paper are shown with red
circles with black outlines, and the predicted low-mass star colors from the
Sonora Cholla models are plotted with small points colored by the Teff values
as shown in the colorbar. The black lines represent the color limits used for
selecting sources. Left: F356W - F444W color plotted against F115W - F444W
color, with dashed black lines showing the color criteria adopted by (Nonino
et al., 2023) for selecting their brown dwarf candidate in the JWST GLASS
data. These colors would limit the resulting Sonora Cholla brown dwarf models
to those with T${}_{\mathrm{eff}}<700$K. Middle: F277W - F444W color plotted
against F115W - F277W color, with the color limits used in selecting sources
represented with solid lines. Right: F277W - F444W color plotted against F115W
- F150W color, with the color limits used in selecting sources represented
with solid lines. Both the middle and right panels feature new color criteria
designed to recover the “V”-shaped SED seen in brown dwarf models from Marley
et al. (2021) and Karalidi et al. (2021).

In Figure 1, we plot the color selection regions that we used for selecting
our candidates. In each panel, we represent the colors for the JADES sources
measured using the $0.2^{\prime\prime}$ diameter circular aperture magnitudes
with a greyscale density plot. The final sample of JADES and CEERS candidates
is plotted with red points. We compare our candidates to the colors for the
“Sonora Cholla” brown dwarf model atmospheres from Karalidi et al. (2021). We
plot the solar metallicity Sonora Cholla points with colors shaded by the
model effective temperature. We find that most of our sources would not be
selected by the color criteria used in Nonino et al. (2023), as shown in the
left panel of Figure 1. These color limits would only select Sonora Cholla
models with effective temperature T${}_{\mathrm{eff}}<700$K (Y dwarfs). The
selection criteria depicted in the middle and right panels of Figure 1, on the
other hand, includes the majority of the brown dwarf Sonora Cholla model
points across the full range of modeled Teff values, and separates these
sources from the bulk of the JADES galaxies. Our candidates have colors that
nicely agree with the Sonora Cholla model colors, although one object, JADES-
GS-BD-5, lies outside of the F277W-F444W vs. F115W-F277W selection box by
virtue of its red F115W - F277W color. This source will be discussed further
in the next section.

To estimate the brown dwarf properties, including the distances to these
candidates, we fit the NIRCam forcepho fluxes for the sources with the
Karalidi et al. (2021) Sonora Cholla models. We do not fit the HST ACS or WFC3
fluxes because of the possibility of proper motion making it difficult to
assure that the measured flux corresponds to the correct source (see the next
section for a discussion of the sources with observed proper motion). The
models are provided with three free parameters: effective temperature (Teff),
surface gravity (log $g$), and eddy diffusion parameter (log $K_{zz}$). We
compared each provided model against the observed forcepho photometry, and
varied the overall normalization of the model for a fourth free parameter. The
adopted best fit was the one that resulted in the minimum $\chi^{2}$ for each
of our candidates.

We additionally fit the candidates using the ATMO 2020 solar metallicity brown
dwarf atmosphere and evolutionary models from Phillips et al. (2020). These
were generated using a 1D-2D atmospheric modeling code designed to study hot
Jupiters and brown dwarfs, and the ATMO 2020 release includes both atmospheric
and evolution models. For simplicity, we adopted the atmospheric chemical
equilibrium models with only two free parameters: effective temperature and
surface gravity. We performed a similar fit using these models as was done
with the Sonora Cholla models, where the best fit was the one corresponding to
the minimum $\chi^{2}$.

##  3 Results

###  3.1 Physical Properties

We present the full list of JADES and CEERS low-mass star candidates in Table
1, with the forcepho fluxes and effective radii. Our naming convention starts
with the survey name (JADES or CEERS), the region of the sky (GOODS-S,
GOODS-N, or EGS), “BD” indicating brown dwarf candidate, and a unique integer
based on the RA of the source. We plot the SEDs for the selected brown dwarf
candidates in Figures 2, 3, and 4 and indicate Teff, log $g$, log $K_{zz}$,
and minimum $\chi^{2}$ of the best-fitting Sonora Cholla model. In each panel,
we also provide the minimum $\chi^{2}$ derived from the EAZY galaxy fits to
the source, the best-fitting photometric redshift, and show the galaxy
template fit for each object.

Figure 2: Brown dwarf candidate SEDs measured using forcepho (red) with best-
fits from the Sonora-Cholla stellar atmospheric models (blue) overplotted. We
show the parameters for the model in the bottom right, and the $\chi^{2}$ of
the fit and derived distance based on the model in the upper left. We plot the
best-fitting EAZY fit in green, and provide the photometric redshift derived
from the fit and the resulting $\chi^{2}$. Beneath each SED plot are the
individual JWST NIRCam thumbnails ($2^{\prime\prime}\times 2^{\prime\prime}$)
for each source. This is continued in Figure 3 and 4.

Figure 3: Brown Dwarf Candidate SEDs (continued from Figure 2).

Figure 4: Brown Dwarf Candidate SEDs (continued from Figure 3).

To estimate the distances to the BD candidates based on the fits, we followed
the procedure detailed in Wang et al. (2023). The best-fit normalization of
the Sonora Cholla models to the observed photometry is equal to $(R/D)^{2}$,
where $R$ is the radius of the model star and $D$ is the distance to the star.
We used the “Sonora Bobcat” evolution models (Marley et al., 2021) to find the
brown dwarf radii corresponding to the best-fit Teff and log $g$ values for
both solar metallicity ([M/H] $=0$) and subsolar metallicity ([M/H] $=-0.5$)
models. Evolution models use energy loss rate estimates to explore how brown
dwarf radii and luminosity change over time. Because the (Marley et al., 2021)
Sonora Bobcat evolution models to do not include radii for stars with log $g$
= 5.5, we extrapolate the models to estimate those values. For the ATMO 2020
fits, we followed the same procedure and estimated the radius of each source
from the provided evolutionary models, which allowed us to calculate
distances.

As part of the Sonora Bobcat model data release, the authors provide observed
photometry at 10 pc for different values of Teff, log $g$, and stellar radii.
To demonstrate the uncertainty in distance estimates to these sources, we
additionally fit directly to these Sonora Bobcat model photometry, and we
derive a third distance estimate from the scaling to the observed photometry
for our candidates.

In Table 2, we provide the best-fitting parameters, the derived stellar radii,
and the distances for our candidates for the Sonora Cholla and ATMO 2020 fits.
We also provide the distances and parameters from fitting directly to the
Sonora Bobcat model photometry. Using the subsolar Sonora Bobcat models
results in distance measurements that are only 0.6% smaller. We do see a more
significant difference when comparing to the distances derived directly from
the Sonora Bobcat evolutionary models, which are, on average, 11% larger than
what we estimate using the Sonora Cholla models. Fits with the ATMO 2020
models result in distances that are 17% larger than those derived using the
solar metallicity Sonora Cholla models. At these best-fit effective
temperatures, adopting the Sonora Cholla fit values, fourteen of these these
sources are T dwarf candidates (700K to 1300K), and the remaining seven are Y
dwarf candidates ($<$700K).

In addition, in Table 1, we also provide the measured forcepho half-light
radii for these candidates. Each of the JADES and CEERS candidates in our
sample has a half-light radius less than 0.005′′, which strongly supports the
argument that they are unresolved. In addition, the image thumbnails shown in
Figures 2, 3, and 4 often have long wavelength morphologies with observable
diffraction features, indicating the sources are unresolved.

###  3.2 Proper Motions

Figure 5: Proper motion measurements between the HST WFC3 or Spitzer IRAC2 and
JWST NIRCam observations. We plot seven sources, and for each source we plot
the HST WFC3 or Spitzer IRAC2 thumbnail in the left panel and the JWST NIRCam
thumbnail in the right panel, with the filter being used shown in the bottom
left. We also plot a scale bar in the left panels, above the filter name. In
each panel, the centroid measured for the source from the HST or Spitzer
observations is shown with a blue circle, and the centroid for the source from
the JWST observations is shown in a red circle. The offset, in arcseconds, is
provided. For JADES-GS-BD-5, because the Spitzer IRAC2 image has a different
WCS than the JADES NIRCam mosaic, we measure the raw centroid and label it
with a square symbol, and the centroid corrected to the JADES WCS is plotted
with a circle, along with the corresponding separation.

The JADES regions, and portions of the EGS, have been targeted by deep
HST/WFC3 observations out to 1.6$\mu$m. These observations, which, in some
cases, date back to 2010, can be used to explore potential proper motion in
these sources, which would rule out them being extragalactic in origin. We
only detect 8 sources in the WFC3 data, five in GOODS-S, two in GOODS-N, and
one in CEERS, and we observed proper motion in only six. We do not observe a
proper motion for two sources, JADES-GS-BD-6 or JADES-GN-BD-2, although we
will discuss this latter source below.

Using the photutils centroids package, we calculated the center of the source
in both the HST and the JWST observations independently, and we plot the
thumbnails in Figure 5 with circles indicating the recovered centroid
positions. In addition, we provide the measured difference in position, which
ranges from 0.12′′ for JADES-GS-BD-1 to 0.75′′ for JADES-GN-BD-3. We queried
the MAST HST archive through astroquery package and obtained the HST WFC3
observation time of these brown dwarf candidates. We found that the mean time
difference between the HST and JWST observations were 9–11 years. If we use
those mean observed times for the individual sources, we derive proper motions
of $0.013^{\prime\prime}$/yr for JADES-GS-BD-1, $0.013^{\prime\prime}$/yr for
JADES-GS-BD-7, $0.017^{\prime\prime}$/yr for JADES-GS-BD-10,
$0.050^{\prime\prime}$/yr for JADES-GS-BD-11, $0.074^{\prime\prime}$/yr for
JADES-GN-BD-3, and $0.022^{\prime\prime}$/yr for CEERS-EGS-BD-3.

We additionally searched through the Spitzer Space Telescope archive to see if
any of our candidate brown dwarfs were detected in observations using IRAC
instrument taken as part of the GOODS Spitzer Legacy program (Dickinson et
al., 2003). We specifically looked at the images taken in the IRAC2
(4.5$\mu$m) channel, as, from the SEDs, our candidates are brightest at this
wavelength and therefore more likely to be detected. Only two sources were
detected, JADES-GN-BD-3 and JADES-GS-BD-5. As JADES-GN-BD-3 is also observed
in the HST WFC3 mosaic, we will use this measured separation for the proper
motion measurement owing to the resolution of HST. We should note that the
westward direction of the observed motion for JADES-GN-BD-3 is the same
between the HST and Spitzer observations.

For JADES-GS-BD-5, this source is too faint at $<2\mu m$ to be observed in the
WFC3 data. To measure the offset in the Spitzer IRAC2 observation, we used the
same method as was used to measure the HST WFC3 offsets, but we had to correct
for the fact that the IRAC2 world coordinate solution (WCS) is offset from the
Gaia-derived frame used in the JADES mosaics. To estimate this correction, we
found the offset for the centroid of a nearby bright star, and subtracted this
offset from the measured Spitzer centroid position. In Figure 5, we show
JADES-GS-BD-5 with both the measured Spitzer position of the star as well as
the corrected position, and derive a difference between the two positions of
0.62′′. Given the positional accuracy of the Spitzer IRAC instrument
($<0.3^{\prime\prime}$ as per the Spitzer IRAC
Knowledgebase444https://irsa.ipac.caltech.edu/docs/knowledgebase/spitzer_irac.html),
this is a $>2\sigma$ offset. The date range provided for this Spitzer
observation is August 12 2004 to August 18 2004, which would indicate a proper
motion for JADES-GS-BD-5 of $\sim 0.033^{\prime\prime}$/yr.

Finally, while we measured an HST centroid for JADES-GN-BD-2 that is similar
to the position of the JWST centroid, there appears to be a haze to the north
of the source in the HST image, which could potentially be a result of the
source moving in the individual images that were used in generating the HST
mosaic.

###  3.3 Discussion of Individual Sources

Because of the number of candidate sources in our sample, below, we will
discuss each source individually, highlighting any previous discussion of
these objects in the literature. For each source, we present an approximate
subdwarf classification based on the best-fit effective temperatures following
Burgasser et al. (2002). The true classification requires spectroscopic
follow-up of these sources.

Table 2: Derived Properties of the Brown Dwarf Candidates  | Sonora Cholla | Sonora Bobcat | ATMO 2020
---|---|---|---
Object ID | Teff (K) | log(g) | log(Kzz) | R (R⊙) | D (pc) | Teff (K) | log(g) | R (R⊙) | D (pc) | Teff (K) | log(g) | R (R⊙) | D (pc)
JADES-GS-BD-1 | 1150 | 5.5 | 2 | 0.077 | 749 | 1100 | 5.25 | 0.086 | 820 | 1100 | 5.5 | 0.125 | 1162
JADES-GS-BD-2 | 1200 | 5.5 | 4 | 0.078 | 1223 | 1200 | 5.25 | 0.087 | 1460 | 1200 | 5.5 | 0.079 | 1295
JADES-GS-BD-3 | 900 | 5.25 | 4 | 0.085 | 2274 | 1000 | 5.0 | 0.094 | 3480 | 900 | 5.5 | 0.078 | 2351
JADES-GS-BD-4 | 500 | 3.505 | 4 | 0.128 | 1181 | 525 | 5.25 | 0.081 | 1230 | 500 | 5.5 | 0.101 | 1364
JADES-GS-BD-5 | 500 | 5.5 | 2 | 0.073 | 251 | 375 | 4.0 | 0.114 | 190 | 350 | 5.0 | 0.104 | 160
JADES-GS-BD-6 | 1000 | 5.5 | 4 | 0.076 | 1633 | 1000 | 5.0 | 0.094 | 2290 | 900 | 5.5 | 0.078 | 1549
JADES-GS-BD-7 | 1000 | 5.5 | 2 | 0.076 | 770 | 1000 | 5.5 | 0.077 | 880 | 1000 | 5.5 | 0.13 | 1467
JADES-GS-BD-8 | 500 | 3.505 | 4 | 0.128 | 804 | 500 | 5.0 | 0.089 | 840 | 500 | 5.5 | 0.101 | 949
JADES-GS-BD-9 | 900 | 5.5 | 4 | 0.076 | 1429 | 900 | 5.0 | 0.093 | 2080 | 800 | 5.5 | 0.097 | 1710
JADES-GS-BD-10 | 1050 | 3.748 | 4 | 0.141 | 380 | 1100 | 5.25 | 0.086 | 260 | 1100 | 5.5 | 0.125 | 377
JADES-GS-BD-11 | 750 | 5.5 | 7 | 0.075 | 498 | 750 | 5.25 | 0.083 | 770 | 700 | 5.5 | 0.085 | 668
JADES-GN-BD-1 | 650 | 3.748 | 4 | 0.129 | 688 | 700 | 5.0 | 0.091 | 740 | 700 | 5.5 | 0.085 | 696
JADES-GN-BD-2 | 800 | 5.0 | 4 | 0.093 | 1137 | 850 | 5.5 | 0.076 | 1250 | 800 | 5.5 | 0.097 | 1407
JADES-GN-BD-3 | 750 | 5.5 | 2 | 0.075 | 335 | 600 | 5.25 | 0.082 | 290 | 600 | 5.5 | 0.1 | 354
CEERS-EGS-BD-1 | 800 | 5.25 | 4 | 0.084 | 1682 | 850 | 5.25 | 0.084 | 2330 | 800 | 5.5 | 0.097 | 2353
CEERS-EGS-BD-2 | 700 | 5.25 | 4 | 0.083 | 965 | 650 | 5.5 | 0.074 | 1020 | 600 | 5.5 | 0.1 | 1149
CEERS-EGS-BD-3 | 1100 | 5.5 | 4 | 0.077 | 642 | 1100 | 5.0 | 0.095 | 870 | 1000 | 5.5 | 0.13 | 984
CEERS-EGS-BD-4 | 1050 | 5.0 | 2 | 0.095 | 1391 | 1000 | 5.0 | 0.095 | 1350 | 1000 | 5.0 | 0.13 | 1813
CEERS-EGS-BD-5 | 1000 | 3.505 | 4 | 0.145 | 4222 | 1100 | 5.25 | 0.086 | 3100 | 1100 | 5.5 | 0.125 | 4503
CEERS-EGS-BD-6 | 600 | 5.0 | 2 | 0.09 | 1059 | 525 | 5.0 | 0.09 | 1130 | 500 | 5.0 | 0.101 | 1098
CEERS-EGS-BD-7 | 650 | 3.505 | 2 | 0.133 | 1290 | 650 | 5.0 | 0.09 | 1080 | 600 | 4.5 | 0.1 | 1006

####  3.3.1 JADES-GS-BD-1

This source is very bright, with fluxes above 100 nJy in five of the JADES
filters, and the Sonora Cholla fit accurately matches the observed fluxes. The
fit indicates that the source is at T${}_{\mathrm{eff}}=1150$K (T3-T5), and a
distance of $\sim 750$ pc. This source is unresolved, and has an observed
separation between the HST WFC3 F160W image and the JWST NIRCam F150W image of
$0.12^{\prime\prime}$. Given the HST detection, it is puzzling that this
source does not appear to have been written about: a search through the
literature for any sources within $5^{\prime\prime}$ of the WFC3 position of
this object did not result in any matches.

####  3.3.2 JADES-GS-BD-2

This source is a very bright candidate (F410M flux of $\sim 180$ nJy) detected
significantly at all of the JADES NIRCam wavelengths, with obvious diffraction
features in the 3 - 4$\mu$m images, suggesting that it is unresolved. The
larger $\chi^{2}$ of the Sonora Cholla fit is due in large part to the
mismatch in the F150W filter, while the resulting fit does accurately match
the observed fluxes, indicating that this is potentially an early T dwarf at
1.2 kpc. The EAZY galaxy template fit does not accurately predict the
relatively fainter F277W and F335M fluxes in this source, attributing the LW
emission to 3.3$\mu$m PAH emission.

####  3.3.3 JADES-GS-BD-3

This brown dwarf candidate has an excellent Sonora Cholla fit with
T${}_{\mathrm{eff}}=900$K (T5-T6) and a derived distance of 2.2 kpc. The
Sonora Bobcat fit has a slightly higher temperature (1000 K), and results in a
distance of almost 3.5 kpc, which would place it firmly in the Milky Way halo.
The extremely blue F115W - F200W color and the lack of an F277W detection make
it difficult to conclude that the source is extragalactic.

####  3.3.4 JADES-GS-BD-4

This brown dwarf candidate is very faint at wavelengths shorter than
3.5$\mu$m, with fluxes of only $\sim$2 nJy in F115W and F150W, and no
significant detections in F200W or F277W. The Sonora Cholla fit for this
source indicates that it is only a 500 K (Y dwarf) source at a distance of
almost 1.2 kpc, and this is one of three sources where the Sonora Cholla fit
results in such a low temperature. The EAZY redshift ($z_{a}=11.85$) is ruled
out due to the detection of F115W flux shortward of the best-fit Lyman break.
In the absence of detections at short wavelengths, this source would appear to
be a dusty red galaxy or even an F277W dropout at $z>20$. Faint Y-dwarfs
should be explored as possible alternate explanations for ultra high-redshift
JWST galaxy candidates.

####  3.3.5 JADES-GS-BD-5

This brown dwarf candidate has a Sonora Cholla fit with $\chi^{2}=1320$, but
this is driven by the small uncertainties on the fluxes, the blue F277W -
F335M color in this source (it is the only object in our sample not selected
by the F115W-F277W-F444W criteria shown in the middle panel of Figure 1), and
the relative lack of flux at $<1.6\mu$m. The best-fit results in a low
effective temperature (500 K, a Y dwarf) and positions the source at only 251
pc away, the closest among the entire sample. As discussed earlier in this
section, this source has an observed proper motion between the Spitzer IRAC2
and JWST NIRCam F444W observations of $0.62^{\prime\prime}$ after correcting
for the different reference frames. This best galaxy template EAZY fit returns
a photometric redshift of $z_{a}=17.84$, which is at odds with the observed
F115W, F150W, and F200W fluxes, and the $\chi^{2}$ is still significantly
worse than the Sonora Cholla fit. This could result from a limitation of the
cloud-free Sonora Cholla models, as any clouds, or differences in the
molecular absorption from the models, would significantly change the shape of
the SED. While the source could be a binary brown dwarf system (see Burgasser
et al., 2003a; Cruz et al., 2004; Burgasser et al., 2006; Dupuy et al., 2015;
Kiwy et al., 2022; Calissendorff et al., 2023, for examples in the
literature), this may not significantly effect the observed colors.

####  3.3.6 JADES-GS-BD-6

This source is quite well fit by a 1000 K Sonora Cholla model, indicating that
it is potentially a T5-T6 dwarf at a distance of 1.6 kpc. The EAZY galaxy
template fits attempts to replicate the very blue F115W - F277W color with a
Lyman break and the red F356W - F444W color with [OIII]$\lambda$5007 emission
boosting the F410M and F444W fluxes, but the resulting fit is very poor.

####  3.3.7 JADES-GS-BD-7

The Sonora Cholla fit for this candidate indicates that it is at 1000K (T5 -
T6), at a distance of $\sim$800 pc. The fit is significantly better than the
lowest $\chi^{2}$ EAZY fit, and the evidence for this source being a star is
strengthened by the $0.15^{\prime\prime}$ separation between the HST WFC3
F125W centroid and the JWST NIRCam F115W centroid. This source was discussed
previously in Oesch et al. (2012), Lorenzoni et al. (2013), and Finkelstein et
al. (2015). Oesch et al. (2012) removed the source from their list of $z\sim
7$ candidates as it had colors consistent with being a T-dwarf. It should be
noted that the proper motion between the HST and JWST observations for this
source was first discussed in Bunker et al. (2023) under the ID 10035328.

####  3.3.8 JADES-GS-BD-8

This source is at SNR $>10$ in the F115W, F356W, F410M and F444W filters, but
has SNR $<3$ in the other JADES NIRCam filters. The best Sonora Cholla fit
indicates the source could possibly be a 500K Y-dwarf at 804 pc, while the
best EAZY fit indicates the source could be at $z=12.46$, a redshift ruled out
by the F115W detection. The coverage at 1.15 $\mu$m is very important for
helping to confirm these faint Y-dwarfs.

####  3.3.9 JADES-GS-BD-9

This source is bright, with a point-like morphology and an excellent Sonora
Cholla fit with T${}_{\mathrm{eff}}=900$K (T6 - T7) indicating a distance of
1.4 kpc. The poor EAZY fit significantly overpredicts the 1.5 - 3.5$\mu$m flux
for this source.

####  3.3.10 JADES-GS-BD-10

This candidate is the brightest source in our sample, with an F410M flux of
$>4000$ nJy. The Sonora Cholla fit agrees with the observed photometry quite
well, and indicates that the source is potentially at 1050 K (T0 - T3), at a
distance of 380 pc. The EAZY fit is much worse, and while a $z_{a}=6.89$
galaxy solution is possible, it overpredicts the F277W and F335M fluxes. This
source has a proper motion of $0.017^{\prime\prime}$/yr, and so an
extragalactic origin is unlikely. It has been very well studied in the
literature owing to its brightness, and was first discovered in a search for
$z\sim 6$ galaxies in GOODS-S in Stanway et al. (2003), where the authors
suggest it is likely a star owing to its colors. Later, in Vanzella et al.
(2009), the authors remove the source from a list of high-redshift candidate
galaxies due to detection of proper motion.

####  3.3.11 JADES-GS-BD-11

This brown dwarf candidate is near a gap in the short-wavelength JADES mosaic,
as seen in the thumbnails in Figure 3. The forcepho flux measurements were not
affected by the position of the source, and this source is quite bright ($\sim
200$ nJy at 4$\mu$m). The best Sonora Cholla fit returns
T${}_{\mathrm{eff}}=750$K (T9) at $\sim$500 pc. This source has an observed
offset of $0.56^{\prime\prime}$ between the HST WFC3 F125W and JWST NIRCam
F150W positions. In addition, this source is GSDZ-2480745501 in Bouwens et al.
(2015), with an $z_{\tt EAZY}=6.69$, which would put the Lyman break at
0.93$\mu$m. The very red LW color (F335M - F444W = 4.07), and the observed
proper motion call this interpretation into doubt.

####  3.3.12 JADES-GN-BD-1

This candidate is very faint at short wavelengths, and both bright and
unresolved at longer wavelengths, with fluxes of around almost 200 nJy at
F410M and F444W. The best fit from the Sonora Cholla models suggests that the
candidate is only at 650 K (an early Y dwarf) at only $\sim 700$ pc away. The
EAZY galaxy template fit is very poor, with a photometric redshift of 11.88 in
disagreement with the significant F115W detection.

####  3.3.13 JADES-GN-BD-2

The Sonora Cholla best fit for this source is an 800K (T7 - T8 dwarf) model at
1 kpc, the furthest observed in GOODS-N. The EAZY fit, which cannot account
for the observed faint F200W, F277W, or F335M emission, attempts to fit the
source as an F090W dropout with strong line emission, and has a significantly
worse $\chi^{2}$.

####  3.3.14 JADES-GN-BD-3

This brown dwarf candidate in GOODS-N is detected in each of the JADES NIRCam
bands, with $\sim 700$ nJy fluxes in F410M and F444W. As a result, the
uncertainties on the fluxes are quite small, leading to a large $\chi^{2}$ for
the Sonora Cholla fit. However, from Figure 2, it can be seen that the fit
accurately matches the fluxes and the resulting parameters suggest that the
source could be nearby (334 pc) with T${}_{\mathrm{eff}}=750$ K (Y dwarf).
This source has a proper motion of $0.074^{\prime\prime}$/yr, the largest we
observe in our sample. This lends strong credibility to this source being
within our own galaxy.

A source at the HST location of this object was discussed in Bouwens et al.
(2015) and Finkelstein et al. (2015) as a $z\sim 7$ galaxy, likely due to the
HST WFC3 observations only probing out to $1.6\mu$m. An analysis by Endsley et
al. (2021) of the Spitzer observations for this source noted its red 3.6 - 4.5
$\mu$m color, but it was not included in their final sample of $z\sim 7$
[OIII]+H$\beta$ emitters.

####  3.3.15 CEERS-EGS-BD-1

The Sonora Cholla model for this source has a best-fit
T${}_{\mathrm{eff}}=800$ K (T7 - T8) at a distance of 1.7 kpc. The fit is
significantly better than the EAZY fit, which overpredicts both the F200W and
F277W non-detections and the F814W flux.

####  3.3.16 CEERS-EGS-BD-2

The brown dwarf candidate has an excellent fit ($\chi^{2}=4$) with
T${}_{\mathrm{eff}}=700$ K (T8 - L0) and a distance of 970 pc.

####  3.3.17 CEERS-EGS-BD-3

This candidate is the brightest among the CEERS sources, with F410M flux of
$\sim 480$ nJy. It is located north of a galaxy (separated by 1.4′′) at
$z_{\mathrm{spec}}=0.82$ (Momcheva et al., 2016). The Sonora Cholla fit
indicates that the object could be at T${}_{\mathrm{eff}}=1100$ K (T2 - T5)
and a distance of 642 pc. We also observe a proper motion of 0.022′′/yr for
the source, demonstrating that this source is likely not extragalactic in
nature. A search through the literature for sources detected at this HST-
derived position did not return any matches, potentially due to the proximity
to the nearby galaxy.

####  3.3.18 CEERS-EGS-BD-4

This brown dwarf candidate is the brightest across the CEERS footprint, with
an F444W flux of 116 nJy, and the Sonora Cholla fit is significantly better
than the EAZY fit ($\chi^{2}=237$). The best fit indicates a
T${}_{\mathrm{eff}}=1050$ K (T3 - T5), with a distance of 1.4 kpc. In this
source, and multiple other CEERS sources, the EAZY fit predicts a flux at
$<1\mu$m that is not observed in the HST ACS F814W mosaics.

####  3.3.19 CEERS-EGS-BD-5

This Sonora Cholla fit to this source has a $\chi^{2}=2.7$,
T${}_{\mathrm{eff}}=1000$ K (T4 - T6), and most importantly, a predicted
distance of 4.2 kpc, making it the farthest brown dwarf candidate in our
sample. The EAZY fit to the NIRCam data predicts an F814W detection that we do
not observe (the $0.2^{\prime\prime}$ diameter circular aperture flux for this
source is $-0.3\pm 1.8$ nJy, consistent with zero). If this source were a
galaxy, it would need to have a strong hot dust component coupled with a very
blue UV slope ($\beta=-3.75^{0.25}_{0.28}$). At this slope, it would be a
significant outlier among the JADES sample of F090W dropout galaxies assembled
by Topping et al. (2023).

####  3.3.20 CEERS-EGS-BD-6

This source is not significantly detected in F200W or F277W, and is only
detected at SNR $=5.1$ in F115W and SNR $=2.9$ in F150W. The best-fit indicate
a T${}_{\mathrm{eff}}=600$ K (Y dwarf) at 1.1 kpc. The EAZY fit, at
$z_{a}=12.0$, is not in agreement with the observed F115W flux.

####  3.3.21 CEERS-EGS-BD-7

The best-fit parameters for the Sonora Cholla fit to the photometry for this
source indicate a T${}_{\mathrm{eff}}=650$K (late T or early Y dwarf) at 1.3
kpc. The fit, however, underpredicts the observed F150W, F200W, and F277W
fluxes.

##  4 Discussion

In this section, we will discuss the implications of these brown dwarf
candidates by exploring their positions within the Milky Way and distances
from the Earth, as well as their observed colors with an eye towards how these
types of objects can be detected in current and future deep extragalactic
catalogs.

###  4.1 Position in the Milky Way

Figure 6: Positions of the JADES and CEERS brown dwarf candidates, with JADES
GOODS-S sources in shades of blue, JADES GOODS-N sources in shades of red, and
CEERS sources in shades of orange, plotted in three dimensional space with
respect to the Milky Way galaxy disk. The darker the shade, the closer the
source is to the viewpoint of the plot. The distances for these sources are
from fits using the Sonora Cholla models along with the solar metallicity
Sonora Bobcat models. We show the central plane of the Milky Way with a navy
blue line, and then the thin and thick disk with lighter blue lines. We also
plot the Sun as a yellow circle, and the spectroscopically-confirmed brown
dwarfs from the UNCOVER and GLASS surveys (Nonino et al., 2023; Langeroodi et
al., 2023; Burgasser et al., 2023) with purple points. For these sources, we
adopt the distances derived in Burgasser et al. (2023).

In Figure 6, we plot the positions of our brown dwarf candidates with respect
to the Sun and our galaxy. For this figure, we adopt the distances measured
using the Sonora Cholla models using the solar metallicity Sonora Bobcat
evolutionary models given in Table 2. We plot the GOODS-S sources with blue
points, the GOODS-N sources with red points, and the CEERS sources with orange
points. We include the Nonino et al. (2023) GLASS brown dwarf, as well as the
two UNCOVER brown dwarfs discussed in Langeroodi et al. (2023) and Burgasser
et al. (2023), with purple points. For these three sources, their distances
are taken from Burgasser et al. (2023). We also plot the plane of the Milky
Way with blue curves. The darkest blue curve represents the central plane of
the galaxy, with the thin disk (300 pc scale height) and thick disk (1.4 kpc
scale height) represented with lighter blue curves. The bulk of the JADES
candidates lie within the thick disk, although there are four that may be
outside the thick disk in the galactic halo (JADES-GS-BD-6, JADES-GS-BD-3,
CEERS-EGS-BD-5, and CEERS-EGS-BD-1). It should be cautioned that the distance
estimates from the fitting are highly uncertain.

Figure 7: Positions of the JADES brown dwarf candidates in galactic
coordinates. In the left panel we show the GOODS-S sources, in the middle
panel, we show the GOODS-N sources, and in the right panel, we show the CEERS
EGS sources. For those seven sources with measured proper motions, we plot an
arrow showing the direction between the previous HST or Spitzer observation
and the more current NIRCam observation. The length of the arrow is scaled
with the separation between the HST or Spitzer postion and the NIRCam
position. For the majority of the objects, their proper motion is along the
plane of the Milky Way, providing more evidence that these sources are within
the thick or thin disk of the galaxy.

To better explore the nature of these brown dwarf candidates, we also plot
their positions (GOODS-S sources in the left panel, GOODS-N sources in the
middle panel, and CEERS sources in the right panel) on the sky in galactic
coordinates in Figure 7. For those sources with observed proper motions, we
show an arrow indicating the direction of the motion with respect to the plane
of the galaxy. The length of the arrow for these sources is scaled by the
separation between the HST or Spitzer position and the NIRCam position. The
majority of the brown dwarf candidate sources with observed proper motions are
moving horizontally in this figure, i.e. along the plane of the galaxy. This
provides evidence that these sources are within the galaxy, which is supported
by their range of estimated distances (assuming the best-fit Teff and log(g)
from the Sonora Cholla models with the radii derived using the solar
metallicity Sonora Bobcat models) between $148-780$ pc.

Several authors have explored the number density of low-mass stars in the
Milky Way from both observations and simulations. Ryan & Reid (2016) provided
predicted number densities for brown dwarfs between spectral types M8-T8 for
thin and thick disk exponential models motivated by estimates of the ultracool
dwarf luminosity function. These authors predict a number density of T0-T5
dwarfs at all observed magnitudes (combining the thick and thin disk
estimates) in GOODS-S of $3.43\times 10^{-2}$ arcmin-1, in GOODS-N of
$3.32\times 10^{-2}$ arcmin-1, and in the EGS of $3.84\times 10^{-2}$
arcmin-1. Given the current JADES area of 67 arcmin2 for GOODS-S, 58 arcmin2
for GOODS-N, and 90.8 arcmin2 in the CEERS area in the EGS, the Ryan & Reid
(2016) prediction would be 2.3 T-dwarfs across the JADES GOODS-S footprint,
1.9 across GOODS-N, and 3.5 across CEERS. If we define the T0 - T5 dwarf
temperature range as T${}_{\mathrm{eff}}\sim 1000-1300$K (Burgasser et al.,
2002), then we find only five candidates in GOODS-S with best fits in this
temperature range, none in GOODS-N, and 3 in the CEERS EGS footprint. This is
in excellent agreement with the combined prediction across GOODS-S, GOODS-N,
and CEERS EGS from Ryan & Reid (2016).

An alternate method of comparing the observed number density of brown dwarf
candidates is adopted by Wang et al. (2023), where these authors compare the
likelihood of finding their source, CEERS-BD1, with the the observed space
density of early-L through early-Y dwarfs from Kirkpatrick et al. (2021).
Table 15 in Kirkpatrick et al. (2021) provides space densities as a function
of Teff complete only out to 20 pc. If we co-add the number densities of T0 to
Y1.5 sources from this table, these authors estimate a total of $14.92\times
10^{-3}$ arcmin-1. which, If we multiply this estimate by the total area of
the JADES GOODS-S and GOODS-N and CEERS EGS footprints (216 arcmin2), implies
only 3.2 brown dwarfs across this entire temperature range. The bulk of the
candidates in our sample have best-fit distances outside the thin disk probed
by the Kirkpatrick et al. (2021) sample, however. Indeed, these results and
others in the literature can be added to the growing number of low-mass
stellar candidates found in extragalactic deep fields as discussed in the
Introduction.

###  4.2 Comparing to High-Redshift Galaxy Colors

As discussed in Section 3, some of the brown dwarf candidates in our sample
have previously appeared in the literature as potential high-redshift galaxies
owing to them being selected as dropout galaxies at 0.9$\mu$m and the limited
HST WFC3 wavelength coverage in the near-IR. Following work exploring the use
of the 3 - 4$\mu$m slope in separating brown dwarf candidates from F090W
dropous in Hainline et al. (2020), in Figure 8 we plot the F090W - F115W
colors vs. the F335M - F444W colors for the JADES survey objects with
m${}_{F444W}<28.5$. Because the CEERS survey does not include coverage in
F090W or F335M, we do not plot any CEERS sources. We also plot the six brown
dwarf candidates in our sample with detections in F090W, F115W, F335M, and
F444W. As a subsample of the high-temperature Sonora Cholla models do not
extend to wavelengths that cover the entirety of the F090W filter, we instead
include the predictions from ATMO 2020, with points colored by the effective
temperature of the model. The F335M fluxes are quite low in our candidates,
which, if these were brown dwarfs, would be caused by molecular absorption by
CH4 at 3 - 3.6$\mu$m.

We additionally plot a sample of $z=8.0-8.5$ galaxy candidates from Hainline
et al. (2023). In this redshift range, these sources are are F090W Lyman
dropouts. Similar to the high-z sample, we measure F090W-F115W $>1$ for the
brown dwarf candidates (five of the six have F090W-F115W $>2$). The low-
temperature brown dwarf candidates differentiate themselves from the high-$z$
sample by having F335M - F444W $>2.0$. While ionized emission (e.g.
[OIII]$\lambda$5007) can provide extra flux in the F410M and F444W filters,
there are no galaxies in the $z=8.0-8.5$ JADES sample with F335M - F444W
colors similar to that of the low-mass brown dwarfs. At higher temperatures,
the predicted ATMO 2020 colors for late M and early Y dwarfs are similar to
the F090W dropouts, however.

Figure 8: F090W-F115W vs F335M-F444W color. In this plot, we show those JADES
sources with m${}_{F444W}<28.5$ in grey, but we plot the brown dwarf
candidates detected in F090W, F115W, F335M, and F444W with red circles, as
well as the ATMO 2020 model colors with points signifying their effective
temperature as shown in the colorbar. In addition, we also plot a sample of
F090W Lyman dropout galaxies from Hainline et al. (2023) with EAZY photometric
redshifts between $z=8-8.5$. A red F335M - F444W color is helpful for
distinguishing between F090W dropout galaxies at $z\sim 8$ and low-temperature
brown dwarfs, although at higher temperatures, the ATMO models predict that
late M and early L dwarfs would have similar colors to the dropout galaxies.

In Labbe et al. (2023), the authors selected sources red LW colors from the
JWST Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of
Reionization survey (Bezanson et al., 2022, UNCOVER). A number of the members
of their sample have colors similar to the brown dwarf candidates selected
here. The authors fit these sources with both galaxy stellar light as well as
reprocessed light from dust heated by accretion onto a supermassive black
hole. While these fits can account for the red F335M - F444W colors in these
sources, they have difficulties fitting the extremely blue F115W - F277W
colors at the same time. The alternate explanation, that a subsample of their
sources are unresolved brown dwarf candidates, was recently confirmed by
independent reductions of spectroscopy for two of their sources in Langeroodi
et al. (2023) and Burgasser et al. (2023). Multiple authors have looked at
similar samples of extremely red sources in extragalactic datasets (e.g. Barro
et al., 2023; Pérez-González et al., 2023; Franco et al., 2023, among others),
and we have outlined color selection criteria here which may be used to find
the brown dwarfs from among those samples.

If we assume that the candidates in our sample are F090W dropouts at
$z=8.0-8.5$, we can measure a potential UV slope for the sources by performing
a linear fitting to the F115W, F150W, and F200W fluxes. The resulting slopes,
which range between $\beta=-3.62$ (JADES-GS-BD-5) and $\beta=-6.75$ (JADES-GS-
BD-8), have a median $\beta=-4.81$. This is significantly bluer than the
assembled F090W dropouts selected across the JADES survey in Topping et al.
(2023), which have a median $\beta=-2.32^{+0.03}_{-0.02}$. This provides more
evidence that these sources are not extragalactic in origin.

###  4.3 Brown Dwarf Selection in Other Extragalactic Programs

The JADES depth and filter selection, along with the large number of candidate
brown dwarfs in our sample, allows us to predict number counts of sources
found in current and future extragalactic JWST programs. In particular, we can
compare our sources to the survey depths for the COSMOS-Web (Casey et al.,
2022) and PRIMER (Dunlop et al., 2021) programs which will feature NIRCam
photometric coverage across large areas. For COSMOS-Web, the total area will
be 1929 square arcmin with 5$\sigma$ point-source depths of 26.9 - 28.3 across
four NIRCam filters (F115W, F150W, F277W, and F444W). PRIMER will survey 378
square arcminutes with eight filters (F090W, F115W, F150W, F200W, F277W,
F356W, F410M, and F444W) down to 5$\sigma$ point-source depths of 27.6 - 29.5.
Because of these extended areas, these surveys should uncover large numbers of
these relatively rare brown dwarf candidates similar to those discussed in
this paper.

Figure 9: Brown dwarf candidate SEDs plotted along with the 5$\sigma$ point
source depths for the COSMOS Web (blue shaded area) and Primer (green shaded
area) surveys. We highlight those sources that would be detected above the
COSMOS Web limits in F115W, F150W, F277W, and F444W with black heavy circles.
While our sources are bright enough to be detected in F444W and F410M (for
PRIMER), at $<3\mu$m the majority are too faint, making characterizing their
properties difficult in these large area programs.

In Figure 9, we plot the SEDs for our sources and indicate the 5$\sigma$
point-source depths for both the PRIMER and COSMOS-Web filters to demonstrate
which of the sources in our sample would be found in these surveys. For
COSMOS-Web, we use the values estimated for the deepest portion (4 exposures)
of the full survey as measured using $0.15^{\prime\prime}$ radius apertures
and provided in Casey et al. (2022). For PRIMER, we use the measurements for
the current data using $0.16^{\prime\prime}$ radius apertures and provided in
Morishita et al. (2023), but we note that this is shallower than the future
depths for the completed survey.

Because of how our sources were chosen with $m_{\mathrm{F444W}}<28.5$, each of
the sources would be significantly detected in that filter for both PRIMER and
COSMOS-Web. However, for the majority of our sources, they are not bright
enough at short wavelengths to be observed in these shallow, large area
surveys, so the actual selection of these sources would be difficult. When we
compare to the COSMOS-Web depths and filters, we do find that six of the
sources in our sample have F115W, F150W, and F277W fluxes brighter than the
5$\sigma$ flux limits, and five of these sources have best-fit
T${}_{\mathrm{eff}}>1000$K. The remaining object, JADES-GN-BD-3, has
T${}_{\mathrm{eff}}=750$K, demonstrating that bright sources with these low
effective temperatures, while rare, are still accessible to this survey. We
highlight these sources with thick outlined points in Figure 9. One additional
source, CEERS-EGS-BD-4, is just fainter than the F277W flux limit for COMOS-
Web. Across the JADES+CEERS footprints we find only $2.7\times 10^{-2}$
sources arcmin-2, which which would result in a total of 52 brown dwarfs
bright enough to be detected in all four filters across the COSMOS-Web survey
footprint, although this is a rough estimate given the limited number of
sources we find. For the current depths of the PRIMER survey the number of
sources that satisfy the detection threshold in F115W, F150W, and F277W from
our sample is only four, although two additional sources would be detected in
F115W and F150W, but not F277W. For the PRIMER depths, then, our sample
predicts $1.8\times 10^{-2}$ sources arcmin-2, and a total of 6.8 brown dwarfs
bright enough to be detected in F115W, F150W, F277W, and F444W, although this
is at the current depths, which will increase with future observations.

While all of our sources will still be found at 4$\mu$m at the COSMOS-Web and
PRIMER survey depths, to characterize the SEDs of the cooler Y-dwarfs requires
deeper observational data targeting primarily the 1 - 2$\mu$m wavelength
range. With such large areas probed, the COSMOS-Web and PRIMER surveys still
represent an incredible opportunity for understanding statistical samples of
low-mass T-dwarf stars and rare bright Y-dwarfs at hundreds of parsec or even
kiloparsec distances.

##  5 Conclusions

We have searched the first year of deep NIRCam JADES and CEERS extragalactic
photometric data to hunt for low-mass brown dwarf candidates. These sources
are vital for understanding the low-mass end of the initial mass function,
especially for the types of low-metallicity stars that are found in the Milky
Way thick disk and halo. Our primary conclusions are:

  1. 1.

Using color selection and visual inspection, we found a sample of 21 sources
across the combined 215.8 square arcminutes (0.09 candidates arcmin-2). These
sources were chosen by targeting the blue F115W - F277W and red F277W - F444W
colors observed in late L, T, and Y subdwarfs.

  2. 2.

After fitting these sources to the Karalidi et al. (2021) Sonora Cholla
models, we find that these objects range in temperature between
T${}_{\mathrm{eff}}=500-1200$K, and are at distances of $0.1-4.2$ kpc. While
the majority of the sources are in the thick disk, we do find four sources
that are potentially in the Milky Way halo. These sources are poorly fit with
galaxy template models. We find similar results when fitting with the Phillips
et al. (2020) ATMO 2020 models, albeit at larger distances.

  3. 3.

By comparing these sources to existing HST/WFC3 and Spitzer infrared data, we
estimate proper motions for seven of the candidates of
$0.013^{\prime\prime}$/yr - $0.074^{\prime\prime}$/yr, providing evidence that
these sources are not extragalactic in nature. The directions of these proper
motions are roughly in line with the axis of the Milky Way.

  4. 4.

We present a set of color criteria which can help separate these brown dwarf
candidates from NIRCam F090W dropouts at $z\sim 8-8.5$, which do not have the
very red F335M - F444W colors seen in subdwarfs.

  5. 5.

We use our sample to explore the temperatures of brown dwarfs that might be
observed in the PRIMER and COSMOS Web surveys, which probe much larger areas
than JADES and CEERS at shallower depths. Based on comparing to the fluxes of
our brown dwarf candidates, we find that sources at T${}_{\mathrm{eff}}<1000$K
are too faint at $<3\mu$m to be seen in these surveys, and will appear only in
the F410M and F444W filters. Based on the fluxes of our candidates, we would
expect $\sim 50$ sources across the COSMOS Web survey area bright enough to be
detected across all four observational filters.

These sources provide an exciting opportunity for follow-up observations with
targeted JWST NIRSpec spectroscopy, considering their positions within the
Milky Way. A JWST NIRSpec + MIRI spectrum at 1 - 21$\mu$m of a
T${}_{\mathrm{eff}}=467$K Y-dwarf was recently presented in Beiler et al.
(2023). This source presented a challenge to theoretical models, demonstrating
the importance of increasing the number of these sources explored
spectroscopically. Similarly, NIRSpec observations for three brown dwarfs
found in the UNCOVER data (Langeroodi et al., 2023; Burgasser et al., 2023)
were used to understand the atmospheres of these distant sources. Both sets of
authors found evidence which suggests these sources are in the thick disk or
halo. The existence of sources with these colors should be considered when
selecting samples of high-redshift galaxies with signatures of black hole
accretion.

This study demonstrates the importance of the NIRCam wavelength range of 2 -
5$\mu$m in selecting very faint, low-mass brown dwarfs. While extragalactic
deep fields are designed to point outside the plane of our galaxy to minimize
stellar contamination, the usage of JWST deep observations to collect these
sources has implications for the population of objects outside of the thick
disk, which may be among the most metal-poor and oldest stars at these
temperatures in the galaxy. We expect that the large bounty of brown dwarf
candidates found over the next several years will shed light on the history of
star and planet formation in our galaxy.

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 NAS5-03127 for JWST. These observations are associated with PID 1063,
1345, 1180, 1181, 1210, 1286, 1963, 1837, 1895, and 2738. Additionally, this
work made use of the lux supercomputer at UC Santa Cruz which is funded by NSF
MRI grant AST1828315, as well as the High Performance Computing (HPC)
resources at the University of Arizona which is funded by the Office of
Research Discovery and Innovation (ORDI), Chief Information Officer (CIO), and
University Information Technology Services (UITS). We acknowledge support from
the NIRCam Science Team contract to the University of Arizona, NAS5-02015.
Funding for this research was provided by the Johns Hopkins University,
Institute for Data Intensive Engineering and Science (IDIES). W.B.
acknowledges support by the Science and Technology Facilities Council (STFC),
ERC Advanced Grant 695671 ”QUENCH”. REH acknowledges acknowledges support from
the National Science Foundation Graduate Research Fellowship Program under
Grant No. DGE-1746060. The research of CCW is supported by NOIRLab, which is
managed by the Association of Universities for Research in Astronomy (AURA)
under a cooperative agreement with the National Science Foundation. The
authors are extremely grateful to the CEERS team lead by PI Steve Finkelstein
and the FRESCO team led by PI Pascal Oesch for their work developing their
observing programs and the zero-exclusive-access period on both. The authors
also acknowledge Anton Koekemoer for the usage of CEERS HST data.

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