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Galaxy groups with $M_{tot} \lesssim 10^{14}$ $M_\odot$ and up to a few tens of members are the most common galaxy environment, marking the transition between field and massive clusters. Identifying groups plays a crucial role in understanding structure formation and galaxy evolution. Modern deep surveys allow us to build well-characterized samples of groups up to the regime where structures were taking shape. We aimed to build the largest deep catalog of galaxy groups to date over the COSMOS-Web field effective area of 0.45 deg$^2$, leveraging the deep high quality data of the new COSMOS-Web photometric catalog resulted from the James Webb Space Telescope observations of the COSMOS-Web field. We performed the group search with the AMICO algorithm, a linear matched filter based on an analytical model for the group signal. AMICO has already been tested in wide and deep field surveys, including COSMOS data up to $z=2$. In this work, we tested the algorithm performances at even higher redshift and searched for protocluster cores at $z>2$. We compiled a list of known protoclusters in COSMOS at $2 \leq z \leq 3.7$, matched them with our detections and studied the clustering of the detected cores. We estimated purity and completeness of our sample by creating data-driven mocks with the SinFoniA code and linked signal-to-noise to purity. We detected 1678 groups in the COSMOS-Web field up to $z=3.7$, including lists of members extending nearly two magnitudes deeper than the previous AMICO-COSMOS catalog. 756 groups were detected with purity of 80\%. More than 500 groups have their redshift confirmed by assigning spectroscopic counterparts. This group catalog offers a unique opportunity to explore galaxy evolution in different environments spanning $\sim$12 Gyr and to study groups, from the least rich population to the formation of the most massive clusters., Comment: 17 pages, 14 figures, 2 tables (including appendices), submitted to Astronomy & Astrophysics (A&A)

We present the properties of two candidate massive ($M_\star\sim10^{11}M_\odot$) and dusty ($A_{\rm v}>2.5$ mag) galaxies at $z=5-7$ in the first 0.28 deg$^2$ of the COSMOS-Web survey. One object is spectroscopically confirmed at $z_{\rm spec}=5.051$, while the other has a robust $z_{\rm phot}=6.7\pm0.3$. Thanks to their extremely red colors ($F277W-F444W\sim1.7$ mag), these galaxies satisfy the nominal color-selection for the widely-studied ``little red dot" (LRD) population with the exception of their spatially-resolved morphologies. The morphology of our targets allows us to conclude that their red continuum is dominated by highly obscured stellar emission and not by reddened nuclear activity. Using a variety of SED-fitting tools and star formation histories, we estimate the stellar masses to be $\log(M_\star)=11.32^{+0.07}_{-0.15}$ $M_\odot$ and $\log(M_\star)=11.2^{+0.1}_{-0.2}$ $M_\odot$, respectively, with a red continuum emission dominated by a recent episode of star formation. We then compare their number density to the halo mass function to infer stellar baryon fractions of $\epsilon_\star\sim0.25$ and $\epsilon_\star\sim0.5$. Both are significantly higher than what is commonly observed in lower-z galaxies or more dust-obscured galaxies at similar redshifts. With very bright ultra-high-z Lyman-Break Galaxies and some non-AGN dominated LRDs, such ``extended" LRDs represent another population that may require very efficient star formation at early times., Comment: 11 pages, 4 figures, 1 table. Accepted for publication in ApJL

Photometric redshifts are widely used in studies of dusty star-forming galaxies (DSFGs), but catastrophic photo-$z$ failure can undermine all redshift-dependent results. Here we report the spectroscopic redshift confirmation of COSBO-7, a strongly lensed DSFG in the COSMOS-PRIMER field. Recently, a photometric redshift solution of $z\gtrsim7.0$ was reported for COSBO-7 based on ten bands of {\it James Webb} Space Telescope (JWST) NIRCam and MIRI imaging data. This $z$ value was favored by four independent spectral energy distribution (SED) fitting codes, and the result provided an appealing candidate for the most distant massive DSFG known to date. This photo-$z$ solution was also supported by a single line detection in Atacama Large Millimeter Array (ALMA) Band 3 consistent with CO(7-6) at $z=7.46$. However, our new ALMA observations robustly detect two lines in Band 6 identified as CO(7-6) and [CI](2-1) at $z_{\rm spec}=2.625$, and thus the Band 3 line as CO(3-2). These three robust line detections decidedly place COSBO-7 at $z=2.625$, refuting the photo-$z$ solution. We derive physical parameters by fitting near-infrared(NIR)-to-millimeter(mm) photometry and lens modeling, revealing that COSBO-7 is a main sequence galaxy. We examine possible reasons for this photo-$z$ failure and attribute it to (1) the likely underestimation of photometric uncertainties at 0.9\,$\mu$m and 1.15 \,$\mu$m; and (2) the lack of photometry at wavelengths beyond 20\,$\mu$m. Notably, we recover a bona fide $z_{\rm phot}\sim 2.3$ by including the existing MIPS $24\,\mu$m photometry, demonstrating the critical importance of mid-infrared (MIR) data in bolstering photo-$z$ measurements. This work highlights a common challenge in modeling the SEDs of DSFGs, and provides a cautionary tale regarding the reliability of photometric redshifts as well as pseudo-spectroscopic redshifts based on single line detection., Comment: Published in A&A

The onset of galaxy formation is thought to be initiated by the infall of neutral, pristine gas onto the first protogalactic halos. However, direct constraints on the abundance of neutral atomic hydrogen (HI) in galaxies have been difficult to obtain at early cosmic times. Here we present spectroscopic observations with JWST of three galaxies at redshifts $z=8.8 - 11.4$, about $400-600$ Myr after the Big Bang, that show strong damped Lyman-$\alpha$ absorption ($N_{\rm HI} > 10^{22}$ cm$^{-2}$) from HI in their local surroundings, an order of magnitude in excess of the Lyman-$\alpha$ absorption caused by the neutral intergalactic medium at these redshifts. Consequently, these early galaxies cannot be contributing significantly to reionization, at least at their current evolutionary stages. Simulations of galaxy formation show that such massive gas reservoirs surrounding young galaxies so early in the history of the universe is a signature of galaxy formation in progress., Comment: Submitted

We present a statistical study of 180 dust continuum sources identified in 33 massive cluster fields by the ALMA Lensing Cluster Survey (ALCS) over a total of 133 arcmin$^{2}$ area, homogeneously observed at 1.2 mm. ALCS enables us to detect extremely faint mm sources by lensing magnification, including near-infrared (NIR) dark objects showing no counterparts in existing {\it Hubble Space Telescope} and {\it Spitzer} images. The dust continuum sources belong to a blind sample ($N=141$) with S/N $\gtrsim$ 5.0 (a purity of $>$ 0.99) or a secondary sample ($N=39$) with S/N= $4.0-5.0$ screened by priors. With the blind sample, we securely derive 1.2-mm number counts down to $\sim7$ $\mu$Jy, and find that the total integrated 1.2mm flux is 20.7$^{+8.5}_{-6.5}$ Jy deg$^{-2}$, resolving $\simeq$ 80 % of the cosmic infrared background light. The resolved fraction varies by a factor of $0.6-1.1$ due to the completeness correction depending on the spatial size of the mm emission. We also derive infrared (IR) luminosity functions (LFs) at $z=0.6-7.5$ with the $1/V_{\rm max}$ method, finding the redshift evolution of IR LFs characterized by positive luminosity and negative density evolution. The total (=UV+IR) cosmic star-formation rate density (SFRD) at $z>4$ is estimated to be $161^{+25}_{-21}$ % of the established measurements, which were almost exclusively based on optical$-$NIR surveys. Although our general understanding of the cosmic SFRD is unlikely to change beyond a factor of 2, these results add to the weight of evidence for an additional ($\approx 60$ %) SFRD component contributed by the faint-mm population, including NIR dark objects., Comment: ApJS in press, 34 pages, 8 Tables, and 18 Figures (+38 pages, 5 Tables, and 6 Figures in Appendix)

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 $5.011$) 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: Published in A&A Letter

Galaxies throughout the last 12 Gyr of cosmic time follow a single, universal relation that connects their star-formation rates (SFRs), stellar masses ($M_\star$) and chemical abundances. Deviation from these fundamental scaling relations would imply a drastic change in the processes that regulate galaxy evolution. Observations have hinted at the possibility that this relation may be broken in the very early universe. However, until recently, chemical abundances of galaxies could be only measured reliably as far back as redshift $z=3.3$. With JWST, we can now characterize the SFR, $M_\star$, and chemical abundance of galaxies during the first few hundred million years after the Big Bang, at redshifts $z=7-10$. Here we show that galaxies at this epoch follow unique SFR-$M_\star$--main-sequence and mass-metallicity scaling relations, but their chemical abundance is a factor of three lower than expected from the fundamental-metallicity relation of later galaxies. These findings suggest that galaxies at this time are still intimately connected with the intergalactic medium and subject to continuous infall of pristine gas which effectively dilutes their metal abundances., Comment: Resubmitted version, taking reviewers comments into account

We present the survey design, implementation, and outlook for COSMOS-Web, a 255 hour treasury program conducted by the James Webb Space Telescope in its first cycle of observations. COSMOS-Web is a contiguous 0.54 deg$^2$ NIRCam imaging survey in four filters (F115W, F150W, F277W, and F444W) that will reach 5$\sigma$ point source depths ranging $\sim$27.5-28.2 magnitudes. In parallel, we will obtain 0.19 deg$^2$ of MIRI imaging in one filter (F770W) reaching 5$\sigma$ point source depths of $\sim$25.3-26.0 magnitudes. COSMOS-Web will build on the rich heritage of multiwavelength observations and data products available in the COSMOS field. The design of COSMOS-Web is motivated by three primary science goals: (1) to discover thousands of galaxies in the Epoch of Reionization ($64$ and place constraints on the formation of the Universe's most massive galaxies ($M_\star>10^{10}$\,M$_\odot$), and (3) directly measure the evolution of the stellar mass to halo mass relation using weak gravitational lensing out to $z\sim2.5$ and measure its variance with galaxies' star formation histories and morphologies. In addition, we anticipate COSMOS-Web's legacy value to reach far beyond these scientific goals, touching many other areas of astrophysics, such as the identification of the first direct collapse black hole candidates, ultracool sub-dwarf stars in the Galactic halo, and possibly the identification of $z>10$ pair-instability supernovae. In this paper we provide an overview of the survey's key measurements, specifications, goals, and prospects for new discovery., Comment: 46 pages, 16 figures, ApJ accepted

We report a galaxy group candidate HPC1001 at $z\approx3.7$ 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 $10''\times10''$ area with the highest sky density among known protoclusters and groups at $z>3$. Four out of ten sources were also detected at 1.2$~$mm with Atacama Large Millimeter Array continuum observations. Photometric redshifts, measured by four independent methods, fall within a narrow range of $3.5

We present the characteristics of 2mm-selected sources from the largest Atacama Large Millimeter and submillimeter Array (ALMA) blank-field contiguous survey conducted to-date, the Mapping Obscuration to Reionization with ALMA (MORA) survey covering 184arcmin$^2$ at 2mm. Twelve of the thirteen detections above 5$\sigma$ are attributed to emission from galaxies, eleven of which are dominated by cold dust emission. These sources have a median redshift of $\langle z_{\rm 2mm}\rangle=3.6^{+0.4}_{-0.3}$ primarily based on optical/near-infrared (OIR) photometric redshifts with some spectroscopic redshifts, with 77$\pm$11% of sources at $z>3$ and 38$\pm$12% of sources at $z>4$. This implies that 2mm selection is an efficient method for identifying the highest redshift dusty star-forming galaxies (DSFGs). Lower redshift DSFGs ($z<3$) are far more numerous than those at $z>3$ yet likely to drop out at 2mm. MORA shows that DSFGs with star-formation rates in excess of 300M$_\odot$ yr$^{-1}$ and relative rarity of $\sim$10$^{-5}$ Mpc$^{-3}$ contribute $\sim$30% to the integrated star-formation rate density between $32$. Analysis of MORA sources' spectral energy distributions hint at steeper empirically-measured dust emissivity indices than typical literature studies, with $\langle\beta\rangle=2.2^{+0.5}_{-0.4}$. The MORA survey represents an important step in taking census of obscured star-formation in the Universe's first few billion years, but larger area 2mm surveys are needed to more fully characterize this rare population and push to the detection of the Universe's first dusty galaxies., Comment: 42 pages, 14 figures, accepted for publication in ApJ

When the light from a distant object passes very near to a foreground galaxy or cluster, gravitational lensing can cause it to appear as multiple images on the sky. If the source is variable, it can be used to constrain the cosmic expansion rate and dark energy models. Achieving these cosmological goals requires many lensed transients with precise time delay measurements. Lensed supernovae (SN) are attractive for this purpose because they have relatively simple photometric behavior, with well-understood light curve shapes and colours $-$ in contrast to the stochastic variation of quasars. Here we report the discovery of a multiply-imaged supernova, AT2016jka ("SN Requiem"). It appeared in an evolved galaxy at $z=1.95$, gravitationally lensed by a foreground galaxy cluster. It is likely a Type Ia supernova $-$ the explosion of a low-mass stellar remnant, whose light curve can be used to measure cosmic distances. In archival Hubble Space Telescope imaging, three lensed images of the supernova are detected with relative time delays of $<$200 days. We predict a fourth image will appear close to the cluster core in the year 2037$\pm$2. Observation of the fourth image could provide a time delay precision of $\approx$7 days, $<1\%$ of the extraordinary 20 year baseline. The SN classification and the predicted reappearance time could be improved with further lens modelling and a comprehensive analysis of systematic uncertainties., Comment: Accepted for publication in a peer-reviewed journal. Main text = 6 pages, 3 figures, 1 table; Full document = 28 pages, 12 figures with Methods, Supplemental Info and references. v2: reformatted; minor corrections in SI

We present bright [CII] 158 $\mu$m line detections from a strongly magnified and multiply-imaged ($\mu\sim20-160$) sub-$L^{*}$ ($M_{\rm UV}$ = $-19.75^{+0.55}_{-0.44}$) Lyman-break galaxy (LBG) at $z=6.0719\pm0.0004$ from the ALMA Lensing Cluster Survey (ALCS). Emission lines are identified at 268.7 GHz at $\geq$ 8$\sigma$ exactly at positions of two multiple images of the LBG behind the massive galaxy cluster RXCJ0600$-$2007. Our lens models, updated with the latest spectroscopy from VLT/MUSE, indicate that a sub region of the LBG crosses the caustic and is lensed into a long ($\sim6''$) arc with a local magnification of $\mu\sim 160$, for which the [CII] line is also significantly detected. The source-plane reconstruction resolves the interstellar medium (ISM) structure, showing that the [CII] line is co-spatial with the rest-frame UV continuum at the scale of $\sim$300 pc. The [CII] line properties suggest that the LBG is a rotation-dominated system whose velocity gradient explains a slight difference of redshifts between the whole LBG and its sub region. The star formation rate (SFR)-$L_{\rm [CII]}$ relations from the sub to the whole regions of the LBG are consistent with those of local galaxies. We evaluate the lower limit of the faint-end of the [CII] luminosity function at $z=6$, and find that it is consistent with predictions from semi analytical models and from the local SFR-$L_{\rm [CII]}$ relation with a SFR function at $z=6$. These results imply that the local SFR-$L_{\rm [CII]}$ relation is universal for a wide range of scales including the spatially resolved ISM, the whole region of galaxy, and the cosmic scale, even in the epoch of reionization., Comment: 27 pages, 10 figures, 6 tables, ApJ in press

Extensive surveys with the \textit{Hubble Space Telescope} (HST) over the past decade, targeting some of the most massive clusters in the sky, have uncovered dozens of galaxy-cluster strong lenses. The massive cluster strong-lens scale is typically $\theta_{E}\sim10\arcsec$ to $\sim30-35\arcsec$, with only a handful of clusters known with Einstein radii $\theta_{E}\sim40\arcsec$ or above (for $z_{source}=2$, nominally). Here we report another very large cluster lens, RXC J0032.1+1808 ($z=0.3956$), the second richest cluster in the redMapper cluster catalog and the 85th most massive cluster in the Planck Sunyaev-Zel'dovich catalog. With our Light-Traces-Mass and fully parametric (dPIEeNFW) approaches, we construct strong lensing models based on 18 multiple images of 5 background galaxies newly identified in the \textit{Hubble} data mainly from the \textit{Reionization Lensing Cluster Survey} (RELICS), in addition to a known sextuply imaged system in this cluster. Furthermore, we compare these models to Lenstool and GLAFIC models that were produced independently as part of the RELICS program. All models reveal a large effective Einstein radius of $\theta_{E}\simeq40\arcsec$ ($z_{source}=2$), owing to the obvious concentration of substructures near the cluster center. Although RXC J0032.1+1808 has a very large critical area and high lensing strength, only three magnified high-redshift candidates are found within the field targeted by RELICS. Nevertheless, we expect many more high-redshift candidates will be seen in wider and deeper observations with \textit{Hubble} or \emph{JWST}. Finally, the comparison between several algorithms demonstrates that the total error budget is largely dominated by systematic uncertainties., Comment: 23 pages, accepted for publication in ApJ

Two decades of effort have been poured into both single-dish and interferometric millimeter-wave surveys of the sky to infer the volume density of dusty star-forming galaxies (DSFGs, with SFR>100M$_\odot$ yr$^{-1}$) over cosmic time. Though obscured galaxies dominate cosmic star-formation near its peak at $z\sim2$, the contribution of such heavily obscured galaxies to cosmic star-formation is unknown beyond $z\sim2.5$ in contrast to the well-studied population of Lyman-break galaxies (LBGs) studied through deep, space- and ground-based pencil beam surveys in the near-infrared. Unlocking the volume density of DSFGs beyond $z>3$, particularly within the first 1 Gyr after the Big Bang is critical to resolving key open questions about early Universe galaxy formation: (1) What is the integrated star-formation rate density of the Universe in the first few Gyr and how is it distributed among low-mass galaxies (e.g. Lyman-break galaxies) and high-mass galaxies (e.g. DSFGs and quasar host galaxies)? (2) How and where do the first massive galaxies assemble? (3) What can the most extreme DSFGs teach us about the mechanisms of dust production (e.g. supernovae, AGB stars, grain growth in the ISM) <1 Gyr after the Big Bang? We summarize the types of observations needed in the next decade to address these questions., Comment: Concept Paper submitted to Astro2020

Large surveys of galaxy clusters with the Hubble and Spitzer Space Telescopes, including CLASH and the Frontier Fields, have demonstrated the power of strong gravitational lensing to efficiently deliver large samples of high-redshift galaxies. We extend this strategy through a wider, shallower survey named RELICS, the Reionization Lensing Cluster Survey. This survey, described here, was designed primarily to deliver the best and brightest high-redshift candidates from the first billion years after the Big Bang. RELICS observed 41 massive galaxy clusters with Hubble and Spitzer at 0.4-1.7um and 3.0-5.0um, respectively. We selected 21 clusters based on Planck PSZ2 mass estimates and the other 20 based on observed or inferred lensing strength. Our 188-orbit Hubble Treasury Program obtained the first high-resolution near-infrared images of these clusters to efficiently search for lensed high-redshift galaxies. We observed 46 WFC3/IR pointings (~200 arcmin^2) with two orbits divided among four filters (F105W, F125W, F140W, and F160W) and ACS imaging as needed to achieve single-orbit depth in each of three filters (F435W, F606W, and F814W). As previously reported by Salmon et al., we discovered 322 z ~ 6 - 10 candidates, including the brightest known at z ~ 6, and the most distant spatially-resolved lensed arc known at z ~ 10. Spitzer IRAC imaging (945 hours awarded, plus 100 archival) has crucially enabled us to distinguish z ~ 10 candidates from z ~ 2 interlopers. For each cluster, two HST observing epochs were staggered by about a month, enabling us to discover 11 supernovae, including 3 lensed supernovae, which we followed up with 20 orbits from our program. We delivered reduced HST images and catalogs of all clusters to the public via MAST and reduced Spitzer images via IRSA. We have also begun delivering lens models of all clusters, to be completed before the JWST GO call for proposals., Comment: 29 pages, 6 figures, submitted to ApJ. For reduced images, catalogs, lens models, and more, see relics.stsci.edu

Strong gravitational lensing (SL) is a powerful means to map the distribution of dark matter. In this work, we perform a SL analysis of the prominent X-ray cluster RXJ0152.7-1357 (z=0.83, also known as CL 0152.7-1357) in \textit{Hubble Space Telescope} images, taken in the framework of the Reionization Lensing Cluster Survey (RELICS). On top of a previously known $z=3.93$ galaxy multiply imaged by RXJ0152.7-1357, for which we identify an additional multiple image, guided by a light-traces-mass approach we identify seven new sets of multiply imaged background sources lensed by this cluster, spanning the redshift range [1.79-3.93]. A total of 25 multiple images are seen over a small area of ~0.4 $arcmin^2$, allowing us to put relatively high-resolution constraints on the inner matter distribution. Although modestly massive, the high degree of substructure together with its very elongated shape make RXJ0152.7-1357 a very efficient lens for its size. This cluster also comprises the third-largest sample of z~6-7 candidates in the RELICS survey. Finally, we present a comparison of our resulting mass distribution and magnification estimates with those from a Lenstool model. These models are made publicly available through the MAST archive., Comment: 15 Pages, 7 Figures, 4 Tables Accepted for publication in ApJ

We present a strong-lensing analysis of four massive galaxy clusters imaged with the Hubble Space Telescope in the Reionization Lensing Cluster Survey. We use a Light-Traces-Mass technique to uncover sets of multiply images and constrain the mass distribution of the clusters. These mass models are the first published for Abell S295 and MACS J0159.8-0849, and are improvements over previous models for Abell 697 and MACS J0025.4-1222. Our analysis for MACS J0025.4-1222 and Abell S295 shows a bimodal mass distribution supporting the merger scenarios proposed for these clusters. The updated model for MACS J0025.4-1222 suggests a substantially smaller critical area than previously estimated. For MACS J0159.8-0849 and Abell 697 we find a single peak and relatively regular morphology, revealing fairly relaxed clusters. Despite being less prominent lenses, three of these clusters seem to have lensing strengths, i.e. cumulative area above certain magnification, similar to the Hubble Frontier Fields clusters (e.g., A($\mu>5$) $\sim 1-3$ arcmin$^2$, A($\mu>10$) $\sim 0.5-1.5$ arcmin$^2$), which in part can be attributed to their merging configurations. We make our lens models publicly available through the Mikulski Archive for Space Telescopes. Finally, using Gemini-N/GMOS spectroscopic observations we detect a single emission line from a high-redshift $J_{125}\simeq25.7$ galaxy candidate lensed by Abell 697. While we cannot rule out a lower-redshift solution, we interpret the line as Ly$\alpha$ at $z=5.800\pm 0.001$, in agreement with its photometric redshift and dropout nature. Within this scenario we measure a Ly$\alpha$ rest-frame equivalent width of $52\pm22$ \AA, and an observed Gaussian width of $117\pm 15$ km/s., Comment: 23 pages, 16 figures; V2, accepted for publication in ApJ

Strong gravitational lensing by galaxy clusters has become a powerful tool for probing the high-redshift Universe, magnifying distant and faint background galaxies. Reliable strong lensing (SL) models are crucial for determining the intrinsic properties of distant, magnified sources and for constructing their luminosity function. We present here the first SL analysis of MACS J0308.9+2645 and PLCK G171.9-40.7, two massive galaxy clusters imaged with the Hubble Space Telescope in the framework of the Reionization Lensing Cluster Survey (RELICS). We use the Light-Traces-Mass modeling technique to uncover sets of multiply imaged galaxies and constrain the mass distribution of the clusters. Our SL analysis reveals that both clusters have particularly large Einstein radii ($\theta_E>30"$ for a source redshift of $z_s=2$), providing fairly large areas with high magnifications, useful for high-redshift galaxy searches ($\sim2$ arcmin$^{2}$ with $\mu>5$ to $\sim1$ arcmin$^{2}$ with $\mu>10$, similar to a typical \textit{Hubble Frontier Fields} cluster). We also find that MACS J0308.9+2645 hosts a promising, apparently bright (J$\sim23.2-24.6$ AB), multiply imaged high-redshift candidate at $z\sim6.4$. These images are amongst the brightest high-redshift candidates found in RELICS. Our mass models, including magnification maps, are made publicly available for the community through the Mikulski Archive for Space Telescopes., Comment: 14 pages, 6 figures, 4 tables, accepted for publication in ApJ

Context. A possible correlation between CO luminosity (L_CO ) and its line width (FWHM) has been suggested and denied in the literature. Such claims were often based on a small, or heterogeneous sample of galaxies, and thus inconclusive. Aims. We aim to prove or dis-prove the L_CO -FWHM correlation. Methods. We compile a large sample of submm galaxies at z>2 from the literature, and investigate the L_CO-FWHM relation. Results. After carefully evaluating the selection effects and uncertainties such as inclination and magnification via gravitational lensing, we show that there exist a weak but significant correlation between L_CO and FWHM. We also discuss a feasibility to measure the cosmological distance using the correlation., Comment: 4 pages, 3 figures. Accepted for publication in A&A. A related video is at https://youtu.be/scWMFurOJi8

We present stringent constraints on the average mid-, far-infrared and radio emissions of $\sim$14200 quiescent galaxies (QGs), identified out to $z=3$ in the COSMOS field via their rest-frame NUV$-$r and r$-$J colors, and with stellar masses $M_{\star}=10^{9.8-12.2} \,M_{\odot} $. Stacking in deep Spitzer (MIPS $24\,\mu$m), Herschel (PACS and SPIRE), and VLA (1.4 GHz) maps reveals extremely low dust-obscured star formation rates for QGs (SFR $<0.1-3\,M_{\odot}$yr$^{-1}$ at $z \leqslant 2$ and $<6-18\,M_{\odot}$yr$^{-1}$ at $z > 2$), consistent with the low unobscured SFRs ($<0.01-1.2\,M_{\odot}$yr$^{-1}$) inferred from modeling their ultraviolet-to-near-infrared photometry. The average SFRs of QGs are $>10\times$ below those of star-forming galaxies (SFGs) within the $M_{\star}$- and $z$-ranges considered. The stacked 1.4 GHz signals (S/N $> 5$) are, if attributed solely to star formation, in excess of the total (obscured plus unobscured) SFR limits, suggestive of a widespread presence of low-luminosity active galactic nuclei (AGN) among QGs. Our results reaffirm the existence of a significant population QGs out to $z = 3$, thus corroborating the need for powerful quenching mechanism(s) to terminate star formation in galaxies at earlier epochs., Comment: 7 pages, 3 color figures, 2 tables (including a landscape table). Submitted to ApJL. Comments welcome

We measure the merger fraction of massive galaxies using the UltraVISTA/COSMOS $Ks$-band selected catalog, complemented with the deeper, higher resolution 3DHST+CANDELS catalog selected in the HST/WFC3 $H$-band, presenting the largest mass-complete photometric merger sample up to $z\sim3$. We find that selecting mergers using the $H_{160}$-band flux ratio leads to an increasing merger fraction with redshift, while selecting mergers using the stellar mass ratio causes a diminishing redshift dependence. Defining major and minor mergers as having stellar mass ratios of 1:1 - 4:1 and 4:1 - 10:1 respectively, the results imply $\sim$1 major and $\lesssim$1 minor merger for an average massive (log$(M_{\star}/M_{\odot}) \geqslant 10.8$) galaxy during $z=0.1-2.5$. There may be an additional $\sim 0.5(0.3)$ major (minor) merger if we use the $H$-band flux ratio selection. The observed amount of major merging alone is sufficient to explain the observed number density evolution for the very massive (log$(M_{\star}/M_{\odot}) \geqslant 11.1$) galaxies. We argue that these very massive galaxies can put on a maximum of $6\%$ of stellar mass in addition to major and minor merging, so that their number density evolution remains consistent with observations. The observed number of major and minor mergers can increase the size of a massive quiescent galaxy by a factor of two at most. This amount of merging is enough to bring the compact quiescent galaxies formed at $z>2$ to lie at $1\sigma$ below the mean of the stellar mass-size relation as measured in some works (e.g. Newman et al. 2012), but additional mechanisms are needed to fully explain the evolution, and to be consistent with works suggesting stronger evolution (e.g. van der Wel et al. 2014)., Comment: Submitted to Astrophysical Journal. 25 pages, 12 figures, 6 tables

RXJ0848.6+4453 (Lynx W) at redshift 1.27 is part of the Lynx Supercluster of galaxies. Our analysis of stellar populations and star formation history in the cluster covers 24 members and is based on deep optical spectroscopy from Gemini North and imaging data from HST. Focusing on the 13 bulge-dominated galaxies for which we can determine central velocity dispersions, we find that these show a smaller evolution of sizes and velocity dispersions than reported for field galaxies and galaxies in poorer clusters. The galaxies in RXJ0848.6+4453 populate the Fundamental Plane similar to that found for lower redshift clusters with a zero point offset corresponding to an epoch of last star formation at z_form= 1.95+-0.2. The spectra of the galaxies in RXJ0848.6+4453 are dominated by young stellar populations at all galaxy masses and in many cases show emission indicating low level on-going star formation. The average age of the young stellar populations (estimated from H-zeta) is consistent with a major star formation episode 1-2 Gyr prior, which in turn agrees with z_form=1.95. Galaxies dominated by young stellar populations are distributed throughout the cluster. We speculate that low level star formation has not yet been fully quenched in the center of this cluster may be because the cluster is significantly poorer than other clusters previously studied at similar redshifts, which appear to have very little on-going star formation in their centers., Comment: Accepted for publication in Astronomical Journal. High-resolution figures available from the first author by request

We present deep, near-infrared HST/WFC3 grism spectroscopy and imaging for a sample of 14 galaxies at z~2 selected from a mass-complete photometric catalog in the COSMOS field. By combining the grism observations with photometry in 30 bands, we derive accurate constraints on their redshifts, stellar masses, ages, dust extinction and formation redshifts. We show that the slope and scatter of the z~2 mass-size relation of quiescent galaxies is consistent with the local relation, and confirm previous findings that the sizes for a given mass are smaller by a factor of two to three. Finally, we show that the observed evolution of the mass-size relation of quiescent galaxies between z=2 and 0 can be explained by quenching of increasingly larger star-forming galaxies, at a rate dictated by the increase in the number density of quiescent galaxies with decreasing redshift. However, we find that the scatter in the mass-size relation should increase in the quenching-driven scenario in contrast to what is seen in the data. This suggests that merging is not needed to explain the evolution of the median mass-size relation of massive galaxies, but may still be required to tighten its scatter, and explain the size growth of individual z=2 galaxies quiescent galaxies., Comment: 16 pages, 8 figures, accepted for publication in the Astrophysical Journal

We present improved constraints on the formation time scale of massive cluster galaxies based on rest-frame optical spectra of galaxies in a forming cluster located at z=2.16. The spectra are obtained with MOIRCS on the Subaru telescope with an integration time of ~7 hours. We achieve accurate redshift measurements by fitting SEDs using the spectra and broad-band photometry simultaneously, allowing us to identify probable cluster members. Clusters at low redshifts are dominated by quiescent galaxies, but we find that quiescent galaxies and star forming galaxies co-exist in this z=2 system. Interestingly, the quiescent galaxies form a weak red sequence in the process of forming. By stacking the spectra of star forming galaxies, we observe strong emission lines such as [OII] and [OIII] and we obtain a tentative hint of AGN activities in these galaxies. On the other hand, the stacked spectrum of the quiescent galaxies reveals a clear 4000A break with a possible CaII H+K absorption feature and strong emission lines such as [OII] are absent in the spectrum, confirming the quiescent nature of these galaxies. We then perform detailed spectral analyses of the stacked spectrum, which suggest that these massive quiescent galaxies formed at redshifts between 3 and 4 on a time scale of <~0.5 Gyr. This short formation time scale is not reproduced in recent numerical simulations. We discuss possible mechanisms for how these galaxies form 10^11 Msun stellar mass on a short time scale and become red and quiescent by z=2., Comment: 17 pages, 14 figures, accepted for publication in ApJ

Quiescent massive galaxies at z~2 are thought to be the progenitors of present-day massive ellipticals. Observations revealed them to be extraordinarily compact. The determination of stellar ages, star formation rates and dust properties via spectroscopic measurements has up to now only been feasible for the most luminous and massive specimens (~3x M*). Here we present a spectroscopic study of two near-infrared selected galaxies which are close to the characteristic stellar mass M* (~0.9x M* and ~1.3x M*) and whose observed brightness has been boosted by the gravitational lensing effect. We measure the redshifts of the two galaxies to be z=1.71\pm0.02 and z=2.15\pm0.01. By fitting stellar population synthesis models to their spectro-photometric SEDs we determine their ages to be 2.4^{+0.8}_{-0.6} Gyr and 1.7\pm0.3 Gyr, respectively, which implies that the two galaxies have higher mass-to-light ratios than most quiescent z~2 galaxies in other studies. We find no direct evidence for active star-formation or AGN activity in either of the two galaxies, based on the non-detection of emission lines. Based on the derived redshifts and stellar ages we estimate the formation redshifts to be z=4.3^{+3.4}_{-1.2} and z=4.3^{+1.0}_{-0.6}, respectively. We use the increased spatial resolution due to the gravitational lensing to derive constraints on the morphology. Fitting Sersic profiles to the de-lensed images of the two galaxies confirms their compactness, with one of them being spheroid-like, and the other providing the first confirmation of a passive lenticular galaxy at a spectroscopically derived redshift z~2., Comment: accepted for publication in ApJ

We quantify the presence of Active Galactic nuclei (AGN) in a mass-complete (M_* >5e10 M_sun) sample of 123 star-forming and quiescent galaxies at 1.5 < z < 2.5, using X-ray data from the 4 Ms Chandra Deep Field-South (CDF-S) survey. 41+/-7% of the galaxies are detected directly in X-rays, 22+/-5% with rest-frame 0.5-8 keV luminosities consistent with hosting luminous AGN (L_0.5-8keV > 3e42 ergs/s). The latter fraction is similar for star-forming and quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that perhaps luminous AGN are triggered by external effects such as mergers. We detect significant mean X-ray signals in stacked images for both the individually non-detected star-forming and quiescent galaxies, with spectra consistent with star formation only and/or a low luminosity AGN in both cases. Comparing star formation rates inferred from the 2-10 keV luminosities to those from rest-frame IR+UV emission, we find evidence for an X-ray excess indicative of low-luminosity AGN. Among the quiescent galaxies, the excess suggests that as many as 70-100% of these contain low- or high-luminosity AGN, while the corresponding fraction is lower among star-forming galaxies (43-65%). The ubiquitous presence of AGN in massive, quiescent z ~ 2 galaxies that we find provides observational support for the importance of AGN in impeding star formation during galaxy evolution., Comment: 9 pages, 3 figures, 3 tables; Accepted for publication in ApJ. Minor editing changes and a few references added. Matches published version

In order to investigate the structure and dynamics of the recently discovered massive (M_* > 10^11 M_sun) compact z~2 galaxies, cosmological hydrodynamical/N-body simulations of a proto-cluster region have been undertaken. At z=2, the highest resolution simulation contains ~5800 resolved galaxies, of which 509, 27 and 5 have M_* > 10^10 M_sun, > 10^11 M_sun and > 4x10^11 M_sun, respectively. Effective radii and characteristic stellar densities have been determined for all galaxies. At z=2, for the definitely well resolved mass range of M_* > 10^11 Msun, the mass-size relation is consistent with observational findings for the most compact z~2 galaxies. The very high velocity dispersion recently measured for a compact z~2 galaxy (~510 km/s; van Dokkum et al 2009) can be matched at about the 1-sigma level, although a somewhat larger mass than the estimated M_* ~ 2 x 10^11 M_sun is indicated. For the above mass range, the galaxies have an average axial ratio = 0.64 +/- 0.02 with a dispersion of 0.1, an average rotation to 1D velocity dispersion ratio = 0.46 +/- 0.06 with a dispersion of 0.3, and a maximum value of v/sigma ~ 1.1. Rotation and velocity anisotropy both contribute in flattening the compact galaxies. Some of the observed compact galaxies appear flatter than any of the simulated galaxies. Finally, it is found that the massive compact galaxies are strongly baryon dominated in their inner parts, with typical dark matter mass fractions of order only 20% inside of r=2R_eff., Comment: 10 pages, 8 figures, submitted to ApJ