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 $1.5''\times3''$ (10$\times$20~kpc$^2$). 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 $5.0<z<5.3$. The most massive galaxy in the group has a stellar mass log$(M_{*}/M_{\odot})\approx9.8$, while the rest are low-mass satellites (log$(M_{*}/M_{\odot})\approx8.4-9.2$). 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$(M_{*}/M_{\odot})>11$) 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

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 $3.43<z<3.45$ 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 ($B/T>70\%$). 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: 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(M$_{*}/$M$_{\odot})>10.6$) quiescent galaxies (QGs) at $3<z<5$ 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 ($NUV-U, U-V, V-J$) of similarly massive galaxies at $2<z<3$. 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 $z\geq3$ in SHARK, the GMM/$NUVU-VJ$ method out-performs classical rest-frame $UVJ$ selection and is a viable alternative. We select galaxies as quiescent based on their probability in COSMOS2020 at $3<z<5$, and compare the selected sample to both $UVJ$ and $NUVrJ$ selected samples. We find that although the new selection matches $UVJ$ and $NUVrJ$ in number, the overlap between colour selections is only $\sim50-80\%$, implying that rest-frame colour commonly used at lower redshifts selections cannot be equivalently used at $z>3$. We compute median rest-frame SEDs for our sample and find the median quiescent galaxy at $3<z<5$ has a strong Balmer/4000 Angstrom break, and residual $NUV$ flux indicating recent quenching. We find the number densities of the entire quiescent population (including post-starbursts) more than doubles from $3.5\pm2.2\times10^{-6}$ Mpc$^{-3}$ at $4<z<5$ to $1.4\pm0.4\times10^{-5}$ Mpc$^{-3}$ at $3<z<4$, confirming that the onset of massive galaxy quenching occurs as early as $3<z<5$.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

COSMOS2020: Identification of High-z Protocluster Candidates in COSMOS

We conduct a systematic search for protocluster candidates at $z \geq 6$ 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 ($>4\sigma$) candidate galaxy overdensities across the redshift range $6\le z\le7.7$. 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 $\sim10^{11-13}\,M_{\rm \odot}$, 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 $\Lambda$CDM cosmology. Through comparison with simulation, we expect that all the overdensities at $z\simeq6$ will evolve into a Virgo-/Coma-like clusters at present (i.e., with masses $\sim 10^{14}-10^{15}\,M_{\rm \odot}$). Compared to other overdensities identified at $z \geq 6$ via narrow-band selection techniques, the overdensities presented appear to have $\sim10\times$ 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 $6\le z\le7.7$ 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 ($M_*=10^{10.7}$ M$_{ \odot}$), using high-resolution ($0^{\prime \prime}.1$; 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 $\sim3$ ($M_{\text{gas}}$/$M_*$) and short depletion times ($M_{\text{gas}}/SFR\sim$ 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 $z\gtrsim3$ 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 $z>7$, 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 ($M_{\star}$) 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 $z_{\rm spec}=5-9$ from the JWST FRESCO survey. The three most extreme sources at $z\sim5-6$ ($\sim$1 billion years after the Big Bang) are so massive (log$M_{\star}/M_{\odot}$ $\gtrsim11.0$) 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 $z\sim5-6$, 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 z≳\gtrsim10 Galaxy Candidates Test Early Stellar Mass Assembly

We report the discovery of 15 exceptionally luminous $10\lesssim z\lesssim14$ candidate galaxies discovered in the first 0.28 deg$^2$ of JWST/NIRCam imaging from the COSMOS-Web Survey. These sources span rest-frame UV magnitudes of $-20.5>M_{\rm UV}>-22$, and thus constitute the most intrinsically luminous $z\gtrsim10$ 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 $z\gtrsim10$ sources and 3/15 (20%) likely low-redshift contaminants. Three of our $z\sim12$ candidates push the limits of early stellar mass assembly: they have estimated stellar masses $\sim5\times10^{9}\,M_\odot$, implying an effective stellar baryon fraction of $\epsilon_{\star}\sim0.2-0.5$, where $\epsilon_{\star}\equiv M_{\star}/(f_{b}M_{halo})$. 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 $M_\star\sim10^{10}\,M_\odot$ galaxies relative to $M_\star\sim10^{9}\,M_\odot$ -- both about $10^{-6}$ Mpc$^{-3}$ -- 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 $z\approx8$. 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 $\Lambda$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 ($f_{1.32 \mathrm{GHz}} \sim 2$ mJy, $q_{24\mu m} = -1.1$, $\alpha_{1.32-3\mathrm{GHz}}=-1.2$, $\Delta \alpha = -0.4$). 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$_{\mathrm{H}} > 10^{23}$ cm$^{-2}$). Using the wealth of deep UV to sub-mm photometric data, we report a singular solution photo-z of $z_\mathrm{phot}$ = 7.65$^{+0.4}_{-0.3}$ and estimate an extremely massive host-galaxy ($\log M_{\star} = 11.92 \pm 0.06\,\mathrm{M}_{\odot}$). 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 z≥9z\ge9 Galaxies in the first epoch of COSMOS-Web

We report the identification of 15 galaxy candidates at $z\ge9$ using the initial COSMOS-Web JWST observations over 77 arcmin$^2$ through four NIRCam filters (F115W, F150W, F277W, F444W) with an overlap with MIRI (F770W) of 8.7 arcmin$^2$. We fit the sample using several publicly-available SED fitting and photometric redshift codes and determine their redshifts between $z=9.3$ and $z=10.9$ ($\langle z\rangle=10.0$), UV-magnitudes between M$_{\rm UV}$ = $-$21.2 and $-$19.5 (with $\langle$M$_{\rm UV}\rangle=-20.2$) and rest-frame UV slopes ($\langle \beta\rangle=-2.4$). These galaxies are, on average, more luminous than most $z\ge9$ 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 ($\beta\sim$[$-$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 $\langle \log_{\rm 10} ($M$_\star/$M$_\odot)\rangle\approx8-9$ are not in tension with the standard $\Lambda$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 $z\sim9-10$. Our sample of galaxies, although compact, are significantly resolved.Comment: Submitted to Ap