44 research outputs found
A galaxy group candidate at z approximate to 3.7 in the COSMOS field
We report a galaxy group candidate HPC1001 at z approximate to 3.7 in the COSMOS field. This structure was selected as a high galaxy overdensity at z > 3 in the COSMOS2020 catalog. It contains ten candidate members, of which eight are assembled in a 10 '' x 10 '' area with the highest sky density among known protoclusters and groups at z > 3. Four out of ten sources were also detected at 1.2mm with Atacama Large Millimeter Array continuum observations. Photometric redshifts, measured by four independent methods, fall within a narrow range of 3.5 < z < 3.9 and with a weighted average of z = 3.65 +/- 0.07. The integrated far-IR-to-radio spectral energy distribution yields a total UV and IR star formation rate SFR approximate to 900 M-circle dot yr(-1). We also estimated a halo mass of similar to 10(13) M-circle dot for the structure, which at this redshift is consistent with potential cold gas inflow. Remarkably, the most massive member has a specific star formation rate and dust to stellar mass ratio of M-dust/M-* that are both significantly lower than that of star-forming galaxies at this redshift, suggesting that HPC1001 could be a z approximate to 3.7 galaxy group in maturing phase. If confirmed, this would be the earliest structure in maturing phase to date, and an ideal laboratory to study the formation of the earliest quiescent galaxies as well as cold gas accretion in dense environments.Non peer reviewe
Massive galaxy formation caught in action at z~5 with JWST
We report the discovery of a compact group of galaxies, CGG-z5, at z~5.2 in
the EGS field covered by the JWST/CEERS survey. CGG-z5 was selected as the
highest overdensity of galaxies at z>2 in recent JWST public surveys and it
consists of six candidate members lying within a projected area of
(1020~kpc). All group members are HST/F435W and
HST/F606W dropouts while securely detected in the JWST/NIRCam bands, yielding a
narrow range of robust photometric redshifts . The most massive
galaxy in the group has a stellar mass log, while
the rest are low-mass satellites (log). While
several group members were already detected in the HST and IRAC bands, the low
stellar masses and the compactness of the structure required the sensitivity
and resolution of JWST for its identification. To assess the nature and
evolutionary path of CGG-z5, we searched for similar compact structures in the
\textsc{Eagle} simulations and followed their evolution with time. We find that
all the identified structures merge into a single galaxy by z=3 and form a
massive galaxy (log) at z~1. This implies that CGG-z5
could be a "proto-massive galaxy" captured during a short-lived phase of
massive galaxy formation.Comment: A&A Letter in pres
COSMOS2020: Exploring the dawn of quenching for massive galaxies at 3 < z < 5 with a new colour selection method
We select and characterise a sample of massive
(log(MM) quiescent galaxies (QGs) at in the
latest COSMOS2020 catalogue. QGs are selected using a new rest-frame colour
selection method, based on their probability of belonging to the quiescent
group defined by a Gaussian Mixture Model (GMM) trained on rest-frame colours
() of similarly massive galaxies at . We calculate the
quiescent probability threshold above which a galaxy is classified as quiescent
using simulated galaxies from the SHARK semi-analytical model. We find that at
in SHARK, the GMM/ method out-performs classical rest-frame
selection and is a viable alternative. We select galaxies as quiescent
based on their probability in COSMOS2020 at , and compare the selected
sample to both and selected samples. We find that although the
new selection matches and in number, the overlap between colour
selections is only , implying that rest-frame colour commonly used
at lower redshifts selections cannot be equivalently used at . We compute
median rest-frame SEDs for our sample and find the median quiescent galaxy at
has a strong Balmer/4000 Angstrom break, and residual flux
indicating recent quenching. We find the number densities of the entire
quiescent population (including post-starbursts) more than doubles from
Mpc at to
Mpc at , confirming that the onset of massive galaxy quenching
occurs as early as .Comment: 19 pages, 10 figures + appendix. Accepted for publication in AJ. Both
the GMM model and code to calculate quiescent probabilities from rest frame
flux densities are made available online at
https://github.com/kmlgould/GMM-quiescen
Cosmic Vine: A z=3.44 Large-Scale Structure Hosting Massive Quiescent Galaxies
We report the discovery of a large-scale structure at z=3.44 revealed by JWST
data in the EGS field. This structure, dubbed "Cosmic Vine", consists of 20
galaxies with spectroscopic redshifts at and six galaxy
overdensities with consistent photometric redshifts, making up a vine-like
structure extending over a ~4x0.2 pMpc^2 area. The two most massive galaxies
(M*~10^10.9 Msun) of the Cosmic Vine are found to be quiescent with
bulge-dominated morphologies (). Comparisons with simulations suggest
that the Cosmic Vine would form a cluster with halo mass >10^14 Msun at z=0,
and the two massive galaxies are likely forming the brightest cluster galaxies
(BCGs). The results unambiguously reveal that massive quiescent galaxies can
form in growing large-scale structures at z>3, thus disfavoring the
environmental quenching mechanisms that require a virialized cluster core.
Instead, as suggested by the interacting and bulge-dominated morphologies, the
two galaxies are likely quenched by merger-triggered starburst or AGN feedback
before falling into a cluster core. Moreover, we found that the observed
specific star formation rates of massive quiescent galaxies in z>3 dense
environments are two orders of magnitude lower than that of the BCGs in the
TNG300 simulation. This discrepancy potentially poses a challenge to the models
of massive cluster galaxy formation. Future studies comparing a large sample
with dedicated cluster simulations are required to solve the problem.Comment: Submitted to A&
COSMOS2020: Identification of High-z Protocluster Candidates in COSMOS
We conduct a systematic search for protocluster candidates at in
the COSMOS field using the recently released COSMOS2020 source catalog. We
select galaxies using a number of selection criteria to obtain a sample of
galaxies that have a high probability of being inside a given redshift bin. We
then apply overdensity analysis to the bins using two density estimators, a
Weighted Adaptive Kernel Estimator and a Weighted Voronoi Tessellation
Estimator. We have found 15 significant () candidate galaxy
overdensities across the redshift range . The majority of the
galaxies appear to be on the galaxy main sequence at their respective epochs.
We use multiple stellar-mass-to-halo-mass conversion methods to obtain a range
of dark matter halo mass estimates for the overdensities in the range of
, at the respective redshifts of the
overdensities. The number and the masses of the halos associated with our
protocluster candidates are consistent with what is expected from the area of a
COSMOS-like survey in a standard CDM cosmology. Through comparison
with simulation, we expect that all the overdensities at will evolve
into a Virgo-/Coma-like clusters at present (i.e., with masses ). Compared to other overdensities identified at
via narrow-band selection techniques, the overdensities presented
appear to have higher stellar masses and star-formation rates.
We compare the evolution in the total star-formation rate and stellar mass
content of the protocluster candidates across the redshift range
and find agreement with the total average star-formation rate from simulations.Comment: 52 pages, 32 figues, 18 tables, main text is 30 pages, appendix is 22
pages, to be published in Ap
JWST and ALMA discern the assembly of structural and obscured components in a high-redshift starburst galaxy
We present observations and analysis of the starburst, PACS-819, at z=1.45
( M), using high-resolution (;
0.8 kpc) ALMA and multi-wavelength JWST images from the COSMOS-Web program.
Dissimilar to HST/ACS images in the rest-frame UV, the redder NIRCam and MIRI
images reveal a smooth central mass concentration and spiral-like features,
atypical for such an intense starburst. Through dynamical modeling of the CO
J=5--4 emission with ALMA, PACS-819 is rotation-dominated thus has a disk-like
nature. However, kinematic anomalies in CO and asymmetric features in the bluer
JWST bands (e.g., F150W) support a more disturbed nature likely due to
interactions. The JWST imaging further enables us to map the distribution of
stellar mass and dust attenuation, thus clarifying the relationships between
different structural components, not discernable in the previous HST images.
The CO J = 5 -- 4 and FIR dust continuum emission are co-spatial with a
heavily-obscured starbursting core (<1 kpc) which is partially surrounded by
much less obscured star-forming structures including a prominent arc, possibly
a tidally-distorted dwarf galaxy, and a clump, either a sign of an ongoing
violent disk instability or a recently accreted low-mass satellite. With
spatially-resolved maps, we find a high molecular gas fraction in the central
area reaching (/) and short depletion times
( 120 Myrs) across the entire system. These
observations provide insights into the complex nature of starbursts in the
distant universe and underscore the wealth of complementary information from
high-resolution observations with both ALMA and JWST.Comment: 18 pages, 12 figures, Submitted to Ap
Massive Optically Dark Galaxies Unveiled by JWST Challenge Galaxy Formation Models
Over the past decade, the existence of a substantial population of optically
invisible, massive galaxies at has been implied from mid-infrared
to millimeter observations. With the unprecedented sensitivity of the JWST,
such extremely massive galaxy candidates have immediately been identified even
at , in much larger numbers than expected. These discoveries raised a hot
debate. If confirmed, early, high-mass galaxies challenge the current models of
galaxy formation. However, the lack of spectroscopic confirmations leads to
uncertain stellar mass () estimates, and the possible presence of
active galactic nuclei (AGN) adds further uncertainty. Here, we present the
first sample of 36 dust-obscured galaxies with robust spectroscopic redshifts
at from the JWST FRESCO survey. The three most extreme
sources at (1 billion years after the Big Bang) are so massive
(log ) that they would require, on average,
about 50% of the baryons in their halos to be converted into stars -- two to
three times higher than even the most efficient galaxies at later times. The
extended emission of these galaxies suggests limited contribution by AGN. This
population of ultra-massive galaxies accounts for 20% of the total cosmic star
formation rate density at , suggesting a substantial proportion of
extremely efficient star formation in the early Universe.Comment: Submitted to Nature. 22 pages, 4 main figures, 7 supplementary
figures, 3 supplementary tables. Comments are welcom
COSMOS-Web: Intrinsically Luminous z10 Galaxy Candidates Test Early Stellar Mass Assembly
We report the discovery of 15 exceptionally luminous
candidate galaxies discovered in the first 0.28 deg of JWST/NIRCam imaging
from the COSMOS-Web Survey. These sources span rest-frame UV magnitudes of
, and thus constitute the most intrinsically luminous
candidates identified by JWST to-date. Selected via NIRCam imaging
with Hubble ACS/F814W, deep ground-based observations corroborate their
detection and help significantly constrain their photometric redshifts. We
analyze their spectral energy distributions using multiple open-source codes
and evaluate the probability of low-redshift solutions; we conclude that 12/15
(80%) are likely genuine sources and 3/15 (20%) likely
low-redshift contaminants. Three of our candidates push the limits of
early stellar mass assembly: they have estimated stellar masses
, implying an effective stellar baryon fraction of
, where . The assembly of such stellar reservoirs is made
possible due to rapid, burst-driven star formation on timescales 100\,Myr
where the star-formation rate may far outpace the growth of the underlying dark
matter halos. This is supported by the similar volume densities inferred for
galaxies relative to
-- both about Mpc -- implying they live in halos of comparable
mass. At such high redshifts, the duty cycle for starbursts would be of order
unity, which could cause the observed change in the shape of the UVLF from a
double powerlaw to Schechter at . Spectroscopic redshift
confirmation and ensuing constraints of their masses will be critical to
understanding how, and if, such early massive galaxies push the limits of
galaxy formation in CDM.Comment: 30 pages, 9 figures; ApJ submitte
Uncovering a Massive z~7.65 Galaxy Hosting a Heavily Obscured Radio-Loud QSO Candidate in COSMOS-Web
In this letter, we report the discovery of the highest redshift, heavily
obscured, radio-loud QSO candidate selected using JWST NIRCam/MIRI, mid-IR,
sub-mm, and radio imaging in the COSMOS-Web field. Using multi-frequency radio
observations and mid-IR photometry, we identify a powerful, radio-loud (RL),
growing supermassive black hole (SMBH) with significant spectral steepening of
the radio SED ( mJy, ,
, ). In conjunction
with ALMA, deep ground-based observations, ancillary space-based data, and the
unprecedented resolution and sensitivity of JWST, we find no evidence of QSO
contribution to the UV/optical/NIR data and thus infer heavy amounts of
obscuration (N cm). Using the wealth of deep UV
to sub-mm photometric data, we report a singular solution photo-z of
= 7.65 and estimate an extremely massive
host-galaxy (). This
source represents the furthest known obscured RL QSO candidate, and its level
of obscuration aligns with the most representative but observationally scarce
population of QSOs at these epochs.Comment: Submitted to ApJL, Comments welcom
Unveiling the distant Universe: Characterizing Galaxies in the first epoch of COSMOS-Web
We report the identification of 15 galaxy candidates at using the
initial COSMOS-Web JWST observations over 77 arcmin through four NIRCam
filters (F115W, F150W, F277W, F444W) with an overlap with MIRI (F770W) of 8.7
arcmin. We fit the sample using several publicly-available SED fitting and
photometric redshift codes and determine their redshifts between and
(), UV-magnitudes between M =
21.2 and 19.5 (with M) and rest-frame
UV slopes (). These galaxies are, on average, more
luminous than most candidates discovered by JWST so far in the
literature, while exhibiting similar blue colors in their rest-frame UV. The
rest-frame UV slopes derived from SED-fitting are blue ([2.0,
2.7]) without reaching extremely blue values as reported in other recent
studies at these redshifts. The blue color is consistent with models that
suggest the underlying stellar population is not yet fully enriched in metals
like similarly luminous galaxies in the lower redshift Universe. The derived
stellar masses with MM are not in tension with the standard
CDM model and our measurement of the volume density of such UV
luminous galaxies aligns well with previously measured values presented in the
literature at . Our sample of galaxies, although compact, are
significantly resolved.Comment: Submitted to Ap