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Abstract The cluster environment has been shown to affect the molecular gas content of cluster members, yet a complete understanding of this often subtle effect has been hindered due to a lack of detections over the full parameter space of galaxy star formation rates (SFRs) and stellar masses. Here, we stack CO(2–1) spectra ofz ∼ 1.6 cluster galaxies to explore the average molecular gas fractions of galaxies both at lower mass (log(M_*/M_⊙) ∼ 9.6) and further below the star-forming main sequence (SFMS; ΔMS ∼ −0.9) than other literature studies; this translates to a 3σgas mass limit of  ∼7 × 10⁹M_⊙for stacked galaxies below the SFMS. We divide our sample of 54z ∼ 1.6 cluster galaxies, derived from the Spitzer Adaptation of the Red-Sequence Cluster Survey, into nine groupings, for which we recover detections in 8. The average gas content of the full cluster galaxy population is similar to coeval field galaxies matched in stellar mass and SFR. However, when further split by CO-undetected and CO-detected, we find that galaxies below the SFMS have statistically different gas fractions from the field scaling relations, spanning deficiencies to enhancements from 2σbelow to 3σabove the expected field gas fractions, respectively. These differences betweenz= 1.6 cluster and field galaxies below the SFMS are likely due to environmental processes, though further investigation of spatially resolved properties and more robust field scaling relation calibration in this parameter space are required. 

ABSTRACT High-redshift ($$z\sim 1$$) galaxy clusters are the domain where environmental quenching mechanisms are expected to emerge as important factors in the evolution of the quiescent galaxy population. Uncovering these initially subtle effects requires exploring multiple dependencies of quenching across the cluster environment, and through time. We analyse the stellar mass functions (SMFs) of 17 galaxy clusters within the GOGREEN and GCLASS surveys in the range $0.8< z<1.5$, and with $$\log {(M/{\rm {M_\odot }})}>9.5$$. The data are fit simultaneously with a Bayesian model that allows the Schechter function parameters of the quiescent and star-forming populations to vary smoothly with cluster-centric radius and redshift. The model also fits the radial galaxy number density profile of each population, allowing the global quenched fraction to be parametrized as a function of redshift and cluster velocity dispersion. We find the star-forming SMF to not depend on radius or redshift. For the quiescent population however, there is $$\sim 2\sigma$$ evidence for a radial dependence. Outside the cluster core ($$R>0.3\, R_{\rm 200}$$), the quenched fraction above $$\log {(M/{\rm {M_\odot }})}=9.5$$ is $$\sim 40{\rm\,\,per\, cent}$$, and the quiescent SMF is similar in shape to the star-forming field. In contrast, the cluster core has an elevated quenched fraction ($$\sim 70{\rm \,\,per\, cent}$$), and a quiescent SMF similar in shape to the quiescent field population. We explore contributions of ‘early mass-quenching’ and mass-independent ‘environmental-quenching’ models in each of these radial regimes. The core is well described primarily by early mass-quenching, which we interpret as accelerated quenching of massive galaxies in protoclusters, possibly through merger-driven feedback mechanisms. The non-core is better described through mass-independent environmental-quenching of the infalling field population. 

Abstract Despite the ubiquity of clumpy star-forming galaxies at high-redshift, the origin of clumps are still largely unconstrained due to the limited observations that can validate the mechanisms for clump formation. We postulate that if clumps form due to the accretion of metal-poor gas that leads to violent disk instability, clumpy galaxies should have lower gas-phase metallicities compared to nonclumpy galaxies. In this work, we obtain the near-infrared spectrum for 42 clumpy and nonclumpy star-forming galaxies of similar masses, star formation rates, and colors atz ≈ 0.7 using the Gemini Near-Infrared Spectrograph (GNIRS) and infer their gas-phase metallicity from the [Nii]λ6584 and Hαline ratio. We find that clumpy galaxies have lower metallicities compared to nonclumpy galaxies, with an offset in the weighted average metallicity of 0.07 ± 0.02 dex. We also find an offset of 0.06 ± 0.02 dex between clumpy and nonclumpy galaxies in a comparable sample of 23 star-forming galaxies atz ≈ 1.5 using existing data from the FMOS-COSMOS survey. Similarly, lower [Nii]λ6584/Hαratios are typically found in galaxies that have more of their UV_restluminosity originating from clumps, suggesting that clumpier galaxies are more metal-poor. We also derive the intrinsic velocity dispersion and line-of-sight rotational velocity for galaxies from the GNIRS sample. The majority of galaxies haveσ₀/v_c ≈ 0.2, with no significant difference between clumpy and nonclumpy galaxies. Our result indicates that clump formation may be related to the inflow of metal-poor gas; however, the process that forms them does not necessarily require significant, long-term kinematic instability in the disk. 

We present spectroscopic confirmation of an ultra-massive galaxy (UMG) with $log\left(M_{\star }/M_{\odot }\right)=10.98\pm 0.07$at $z_{\mathrm{s}\mathrm{p}\mathrm{e}\mathrm{c}}=4.8947$in the Extended Groth Strip (EGS), based on deep observations of Ly $\alpha$emission with Keck/DEIMOS. The ultra-massive galaxy (UMG-28740) is the most massive member in one of the most significant overdensities in the EGS, with four additional photometric members with $log\left(M_{\star }/M_{\odot }\right)>10.5$within $R_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{j}}\sim 1$cMpc. Spectral energy distribution (SED) fitting using a large suite of star formation histories and two sets of high-quality photometry from ground- and space-based facilities consistently estimates the mass of this object to be $log\left(M_{\star }/M_{\odot }\right)\sim 11$with a small standard deviation between measurements ( $\sigma =0.07$). While the best-fit SED models agree on stellar mass, we find discrepancies in the estimated star formation rate for UMG-28740, resulting in either a star-forming or quiescent system. $𝐽𝑊𝑆𝑇$/NIRCam photometry of UMG-28740 strongly favors a quiescent scenario, demonstrating the need for high-quality mid-IR observations. Assuming the galaxy to be quiescent, UMG-28740 formed the bulk of its stars at $z>10$and is quenching at $z\sim 8$, resulting in a high star formation efficiency at high redshift ( $ϵ\sim 0.2$at $z\sim 5$and $ϵ\gtrsim 1$at $z\gtrsim 8$). As the most massive galaxy in its protocluster environment, UMG-28740 is a unique example of the impossibly early galaxy problem. 

Abstract The measured ages of massive, quiescent galaxies atz∼ 3–4 imply that massive galaxies quench as early asz∼ 6. While the number of spectroscopic confirmations of quiescent galaxies atz< 3 has increased over the years, there are only a handful atz> 3.5. We report spectroscopic redshifts of one secure (z= 3.757) and two tentative (z= 3.336 andz= 4.673) massive ( $\log \left(M_{*}/M_{\odot }\right)>10.3$) quiescent galaxies with 11 hr of Keck/MOSFIREK-band observations. Our candidates were selected from the FLAMINGOS-2 Extragalactic Near-InfraredK-band Split (FENIKS) survey, which uses deep Gemini/Flamingos-2K_bK_rimaging optimized for increased sensitivity to the characteristic red colors of galaxies atz> 3 with a strong Balmer/4000 Å break. The rest-frameUVJand (ugi)_scolors of three out of four quiescent candidates are consistent with 1–2 Gyr old stellar populations. This places these galaxies as the oldest objects at these redshifts, and challenges the notion that quiescent galaxies atz> 3 are all recently quenched, post-starburst galaxies. Our spectroscopy shows that the other quiescent-galaxy candidate is a broad-line active galactic nucleus (z= 3.594) with strong, redshifted Hβ+ [OIII] emission with a velocity offset > 1000 km s⁻¹, indicative of a powerful outflow. The star formation history of our highest redshift candidate suggests that its progenitor was already in place byz∼ 7–11, reaching ∼10¹¹M_⊙byz≃ 8. These observations reveal the limit of what is possible with deep near-infrared photometry and targeted spectroscopy from the ground and demonstrate that secure spectroscopic confirmation of quiescent galaxies atz> 4 is feasible only with JWST. 

Abstract We examine the quiescent fractions of massive galaxies in sixz≳ 3 spectroscopically confirmed protoclusters in the COSMOS field, one of which is newly confirmed and presented here. We report the spectroscopic confirmation of MAGAZ3NE J100143+023021 at $z={3.122}_{-0.004}^{+0.007}$by the Massive Ancient Galaxies Atz> 3 NEar-infrared (MAGAZ3NE) survey. MAGAZ3NE J100143+023021 contains a total of 79 protocluster members (28 spectroscopic and 51 photometric). Three spectroscopically confirmed members are star-forming ultramassive galaxies (UMGs; $\log (M_{\star }/M_{\odot })$> 11), the most massive of which has $\log (M_{\star }/M_{\odot })$ $={11.15}_{-0.06}^{+0.05}$. Combining Keck/MOSFIRE spectroscopy and the COSMOS2020 photometric catalog, we use a weighted Gaussian kernel density estimator to map the protocluster and measure its total mass ${2.25}_{-0.65}^{+1.55}\times {10}^{14}{M}_{\odot }$in the dense “core” region. For each of the six COSMOS protoclusters, we compare the quiescent fraction to the status of the central UMG as star-forming or quiescent. We observe that galaxies in these protoclusters appear to obey galactic conformity: Elevated quiescent fractions are found in protoclusters withUVJ-quiescent UMGs and low quiescent fractions are found in protoclusters containingUVJstar-frming UMGs. This correlation of star formation/quiescence in UMGs and the massive galaxies nearby in these protoclusters is the first evidence for the existence of galactic conformity atz> 3. Despite disagreements over mechanisms behind conformity at low redshifts, its presence at these early cosmic times would provide strong constraints on the physics proposed to drive galactic conformity. 

Abstract We present rest-frame optical spectra from Keck/MOSFIRE and Keck/NIRES of 16 candidate ultramassive galaxies targeted as part of the Massive Ancient Galaxies atz> 3 Near-Infrared Survey (MAGAZ3NE). These candidates were selected to have photometric redshifts 3 ≲z_phot<4, photometric stellar masses $\log \left(M_{\ast }/M_{\odot }\right)$> 11.7, and well-sampled photometric spectral energy distributions (SEDs) from the UltraVISTA and VIDEO surveys. In contrast to previous spectroscopic observations of blue star-forming and poststarburst ultramassive galaxies, candidates in this sample have very red SEDs implying significant dust attenuation, old stellar ages, and/or active galactic nuclei (AGN). Of these galaxies, eight are revealed to be heavily dust-obscured 2.0 <z< 2.7 galaxies with strong emission lines, some showing broad features indicative of AGN, three are Type I AGN hosts atz> 3, one is az∼ 1.2 dusty galaxy, and four galaxies do not have a confirmed spectroscopic redshift. In fact, none of the sample has ∣z_spec−z_phot∣ < 0.5, suggesting difficulties for photometric redshift programs in fitting similarly red SEDs. The prevalence of these red interloper galaxies suggests that the number densities of high-mass galaxies are overestimated atz≳ 3 in large photometric surveys, helping to resolve the “impossibly early galaxy problem” and leading to much better agreement with cosmological galaxy simulations. A more complete spectroscopic survey of ultramassive galaxies is required to pin down the uncertainties on their number densities in the early Universe. 

Abstract We present individual star formation histories (SFHs) of ∼3000 massive galaxies (log(M_*/M_⊙) > 10.5) from the Large Early Galaxy Astrophysics Census spectroscopic survey at a lookback time of ∼7 billion yr and quantify the population trends leveraging 20 hr deep-integrated spectra of these ∼1800 star-forming and ∼1200 quiescent galaxies at 0.6 <z< 1.0. Essentially all galaxies at this epoch contain stars of age <3 Gyr, in contrast with older massive galaxies today, facilitating better recovery of previous generations of star formation at cosmic noon and earlier. We conduct spectrophotometric analysis using parametric and nonparametric Bayesian stellar population synthesis modeling tools—BagpipesandProspector—to constrain the median SFHs of this mass complete sample and characterize population trends. A consistent picture arises for the late-time stellar mass growth when quantified ast₅₀andt₉₀, corresponding to the age of the Universe when galaxies formed 50% and 90% of their total stellar mass, although the two methods disagree at the earliest formation times (e.g.,t₁₀). Our results reveal trends in both stellar mass and stellar velocity dispersion as in the local Universe—low-mass galaxies with shallower potential wells grow their stellar masses later in cosmic history compared to high-mass galaxies. Unlike local quiescent galaxies, the median duration of late-time star formation (τ_SF,late=t₉₀–t₅₀) does not consistently depend on the stellar mass. This census sets a benchmark for future deep spectrophotometric studies of the more distant Universe. 

Abstract We present the construction of a deep multiwavelength point-spread-function-matched photometric catalog in the Ultra-Deep Survey (UDS) field following the final UKIDSS UDS release. The catalog includes photometry in 24 filters, from the MegaCam-uS0.38μm band to the Spitzer-IRAC 8μm band, over ∼0.9 deg²and with a 5σdepth of 25.3 AB in theK-band detection image. The catalog, containing ≈188,564 (136,235) galaxies at 0.2 <z< 8.0 with stellar mass $\log \left(M_{*}/M_{\odot }\right)>8$andK-band total magnitudeK< 25.2 (24.3) AB, enables a range of extragalactic studies. We also provide photometric redshifts, corresponding redshift probability distributions, and rest-frame absolute magnitudes and colors derived using the template-fitting codeeazy-py. Photometric redshift errors are less than 3%−4% atz< 4 across the full brightness range in theKband and stellar mass range $8<\log \left(M_{*}/M_{\odot }\right)<12$. Stellar population properties (e.g., stellar mass, star formation rate, dust extinction) are derived from the modeling of the spectral energy distributions using the codesFASTand Dense Basis. 

ABSTRACT We have identified 189 candidate z > 1.3 protoclusters and clusters in the LSST Deep Drilling Fields. This sample will enable the measurement of the metal enrichment and star formation history of clusters during their early assembly period through the direct measurement of the rate of supernovae identified through the LSST. The protocluster sample was selected from galaxy overdensities in a Spitzer/IRAC colour-selected sample using criteria that were optimized for protocluster purity using a realistic light-cone. Our tests reveal that $$60\!-\!80~{{\ \rm per\ cent}}$$ of the identified candidates are likely to be genuine protoclusters or clusters, which is corroborated by a ∼4σ stacked X-ray signal from these structures. We provide photometric redshift estimates for 47 candidates which exhibit strong peaks in the photo-z distribution of their candidate members. However, the lack of a photo-z peak does not mean a candidate is not genuine, since we find a stacked X-ray signal of similar significance from both the candidates that exhibit photo-z peaks and those that do not. Tests on the light-cone reveal that our pursuit of a pure sample of protoclusters results in that sample being highly incomplete ($$\sim 4~{{\ \rm per\ cent}}$$) and heavily biased towards larger, richer, more massive, and more centrally concentrated protoclusters than the total protocluster population. Most ($$\sim 75~{{\ \rm per\ cent}}$$) of the selected protoclusters are likely to have a maximum collapsed halo mass of between 1013 and 1014 M⊙, with only $$\sim 25~{{\ \rm per\ cent}}$$ likely to be collapsed clusters above 1014 M⊙. However, the aforementioned bias ensures our sample is $$\sim 50~{{\ \rm per\ cent}}$$ complete for structures that have already collapsed into clusters more massive than 1014 M⊙.