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We report the discovery of post-starburst ultra-diffuse galaxies (UDGs), identified through spectroscopic analysis with KCWI at the Keck II Telescope. Our analysis is based on a sample of 44 candidate UDGs selected from the Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) program. Our measured spectroscopic redshifts reveal \\(\\sim 85\\%\\) of the entire KCWI sample exhibit large physical sizes (\\(R_{e} \\gtrsim 1~{\\rm kpc}\\)) and low surface brightnesses (\\(24 \\lesssim \\mu_{0,g} \\lesssim 25\\) mag arcsec\\(^{-2}\\)) which categorize them as UDGs. We find \\(20\\%\\) of the confirmed UDG population contain post-starburst (or K+A) features, characterized by minimal to no emission in H\\(\\beta\\) indicative of quenched star formation and a predominant presence of spectral A-type stars. In surveying the local environments of the post-starburst UDGs, we find that nearly half are isolated systems, which is unusual given that isolated UDGs are most commonly found to be star-forming. Two of these systems reside \\(2-3~R_{\\rm vir}\\) away from potential nearby massive hosts (\\(M_{\\star} >10^{10}~\\mathrm{M}_{\\odot}\\)), indicating the absence of environmental influence. These post-starburst UDGs may represent systems experiencing star formation feedback such that a recent burst may lead to (at least temporary) quenching. Overall, our results highlight the potentially diverse quenching pathways of UDGs in the local Universe.

Not all galaxies at Cosmic Noon evolve in the same way. It remains unclear how the local environment -- especially the extreme overdensities of protoclusters -- affects stellar mass assembly at high redshift. The stellar mass function (SMF) encodes these processes; comparing SMFs across environments reveals differences in evolutionary history. We present the SMF of the Hyperion proto-supercluster at \\(z2.5\\), one of the largest and most massive protostructures known. This dataset provides the most statistically robust SMF of a single protostructure at \\(z>2\\). By comparing the SMF of overdense peaks within Hyperion to the coeval field, we ask: how early, and how strongly, does a dense environment favor massive galaxies? Using COSMOS2020 photometry with ground-based and new HST grism spectroscopy, we construct a 3D overdensity map that assigns galaxies to peaks, outskirts, or the field. We perform 100 Monte Carlo realizations to propagate redshift and mass uncertainties, and derive SMFs normalized to the field. The peaks show a clear excess of massive galaxies: number densities at \\((M_*/M_) 11\\) are ~10x higher than the field, while those at \\((M_*/M_) 9.5\\) are enhanced by only ~3.5x. By contrast, the outskirts and Hyperion as a whole mirror the field. Environmental effects on stellar mass growth are thus evident by \\(z 2.5\\). The densest regions already host galaxies with accelerated growth, while the global SMF masks this signal. Protostructures therefore begin shaping the high-mass end of the SMF well before cluster quenching, and may drive the elevated star formation at Cosmic Noon.

Observations suggest that satellite quenching plays a major role in the build-up of passive, low-mass galaxies at late cosmic times. Studies of low-mass satellites, however, are limited by the ability to robustly characterize the local environment and star-formation activity of faint systems. In an effort to overcome the limitations of existing data sets, we utilize deep photometry in Stripe 82 of the Sloan Digital Sky Survey, in conjunction with a neural network classification scheme, to study the suppression of star formation in low-mass satellite galaxies in the local Universe. Using a statistically-driven approach, we are able to push beyond the limits of existing spectroscopic data sets, measuring the satellite quenched fraction down to satellite stellar masses of \\({\\sim}10^7~{\\rm M}_{\\odot}\\) in group environments (\\({M}_{\\rm{halo}} = 10^{13-14}~h^{-1}~{\\rm M}_{\\odot}\\)). At high satellite stellar masses (\\(\\gtrsim 10^{10}~{\\rm M}_{\\odot}\\)), our analysis successfully reproduces existing measurements of the quenched fraction based on spectroscopic samples. Pushing to lower masses, we find that the fraction of passive satellites increases, potentially signaling a change in the dominant quenching mechanism at \\({M}_{\\star} \\sim 10^{9}~{\\rm M}_{\\odot}\\). Similar to the results of previous studies of the Local Group, this increase in the quenched fraction at low satellite masses may correspond to an increase in the efficacy of ram-pressure stripping as a quenching mechanism in groups.

We present results from a Keck/DEIMOS survey to study satellite quenching in group environments at \\(z \\sim 0.8\\) within the Extended Groth Strip (EGS). We target \\(11\\) groups in the EGS with extended X-ray emission. We obtain high-quality spectroscopic redshifts for group member candidates, extending to depths over an order of magnitude fainter than existing DEEP2/DEEP3 spectroscopy. This depth enables the first spectroscopic measurement of the satellite quiescent fraction down to stellar masses of \\(\\sim 10^{9.5}~{\\rm M}_{\\odot}\\) at this redshift. By combining an infall-based environmental quenching model, constrained by the observed quiescent fractions, with infall histories of simulated groups from the IllustrisTNG100-1-Dark simulation, we estimate environmental quenching timescales (\\(\\tau_{\\mathrm{quench}}\\)) for the observed group population. At high stellar masses (\\({M}_{\\star}=10^{10.5}~{\\rm M}_{\\odot}\\)) we find that \\(\\tau_{\\mathrm{quench}} = 2.4\\substack{+0.2 \\\ -0.2}\\) Gyr, which is consistent with previous estimates at this epoch. At lower stellar masses (\\({M}_{\\star}=10^{9.5}~{\\rm M}_{\\odot}\\)), we find that \\(\\tau_{\\mathrm{quench}}=3.1\\substack{+0.5 \\\ -0.4}\\) Gyr, which is shorter than prior estimates from photometry-based investigations. These timescales are consistent with satellite quenching via starvation, provided the hot gas envelope of infalling satellites is not stripped away. We find that the evolution in the quenching timescale between \\(0 \\lt z \\lt 1\\) aligns with the evolution in the dynamical time of the host halo and the total cold gas depletion time. This suggests that the doubling of the quenching timescale in groups since \\(z\\sim1\\) could be related to the dynamical evolution of groups or a decrease in quenching efficiency via starvation with decreasing redshift.

We compare satellite quenched fractions across three cosmological simulation suites (FIREbox, the FIRE-2 zoom-ins, and IllustrisTNG50) and observational datasets from SAGA, ELVES, and the combined satellite population of the Milky Way and M31. To enable consistent comparisons, we select Milky Way-mass hosts with \\(M_{\\rm halo} = 10^{11.9}\\) - \\(10^{12.2} \\, M_{\\odot}\\) and satellites with stellar masses of \\(10^{7}\\) - \\(10^{10}\\, M_{\\odot}\\), applying uniform projected apertures and a common quenching definition. All three simulations reproduce the strong observed trend that lower-mass satellites are more likely to be quenched, closely matching the stellar-mass dependence seen in SAGA, ELVES, and the MW+M31 system. This agreement indicates that the mass dependence of satellite quenching is a robust outcome of contemporary galaxy formation models. Radial trends, however, show meaningful differences. SAGA and ELVES exhibit gently declining quenched fractions with projected distance, reflecting strong environmental quenching at small radii. TNG50 most closely matches this behavior, FIREbox, remains consistent with with a nearly flat trend within uncertainties, and the FIRE-2 zoom-ins show suppressed inner quenched fractions driven almost entirely by their paired MW-M31 hosts, which lack high-mass satellites and show strong radial segregation between star-forming and quenched systems. This environmental imprint suggests that host environment and assembly history can influence satellite quenching outcomes and may contribute to diversity across simulations. Overall, while the simulations consistently recover the stellar-mass dependence of quenching their radial trends vary, highlighting the influence of host-halo conditions and motivating deeper exploration of how host environments shape satellite quenching.

We use the TNG-Cluster simulation to investigate how stellar mass and star formation rate (SFR) incompleteness affect the identification of density peaks within galaxy protoclusters at different redshifts. Our analysis focuses on a sample of \\(352\\) protoclusters, defined as the progenitor populations of galaxies that reside within the virialized region of \\(z=0\\) clusters with \\(M_200^z=010^14.3-15.5~ M_\\). For comparison, we define our \"baseline\" protocluster population as galaxies with \\(M_> 10^8.5~ M_\\) at any redshift. We find that \\(M_\\)-limited (\\(M_ > 10^9.5~ M_\\)) and SFR-limited (\\(SFR > 10~ M_ yr^-1\\)) subpopulations recover the baseline highest galaxy density peak in roughly \\(60\\%\\) of cases within an accuracy of \\(1.0\\) pMpc (corresponding to an angular scale of \\( 2-2.5\\) arcmin) at \\(z > 2\\). This recovery fraction drops to \\(40-50\\%\\) when restricting to galaxies with \\(M_ > 10^10.0~ M_\\). We find that the baseline highest galaxy density peaks typically coincide with the highest dark matter and stellar mass density peaks, with separations less than \\(0.5\\) pMpc in \\(60-75\\%\\) of cases at \\(z>2\\). This agreement drops to \\(45-50\\%\\) when restricting to galaxies with \\(M_ > 10^10.0~ M_\\). These results indicate that identifying the densest regions of protoclusters -- i.e., the core -- is highly sensitive to stellar mass and SFR completeness limits. Nevertheless, at \\(z>2\\) we find that the baseline highest galaxy density peaks are generally sites of enhanced star formation and accelerated mass growth relative to the remainder of the protocluster, consistent with some observational studies.

High-redshift (\\(z\\sim1\\)) 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 between \\(0.89.5\\). 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 parameterised 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 \\(\\sim2\\sigma\\) evidence for a radial dependence. Outside the cluster core (\\(R>0.3\\,R_{\\rm200}\\)), the quenched fraction above \\(\\log{(M/{\\rm{M_\\odot}})}=9.5\\) is \\(\\sim40{\\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 (\\(\\sim70{\\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.

Understanding the processes that transform star-forming galaxies into quiescent ones is key to unraveling the role of environment in galaxy evolution. We present measurements of the luminosity functions (LFs) and stellar mass functions (SMFs) of passive red-sequence galaxies in four galaxy clusters at \\(0.8 < z < 1.3\\), selected using deep VLT observations complemented with data from the GCLASS and GOGREEN surveys. We find a significant enhancement in the abundance of faint/low-mass passive galaxies in both the LFs and SMFs of all four clusters compared to the field. This is further evidenced by a shallower low-mass slope in the composite passive cluster SMF, which yields a Schechter parameter \\(\\alpha = -0.54^{+\\,0.03}_{-0.03}\\), compared to \\(\\alpha = 0.12^{+\\,0.01}_{-0.01}\\) for the field. Our findings indicate that quenching processes that act in clusters are enhanced compared to the field, suggesting that environmental quenching mechanisms may already be active by \\(z\\sim1\\). To reproduce the observed passive cluster SMF, we estimate that \\(25\\pm5\\%\\) of the star-forming field population that falls into the cluster must have been quenched. Our results largely support traditional quenching models but highlight the need for deeper studies of larger cluster samples to better understand the role of environmental quenching in the distant Universe.

Recent observations have shown that the environmental quenching of galaxies at z ~ 1 is qualitatively different to that in the local Universe. However, the physical origin of these differences has not yet been elucidated. In addition, while low-redshift comparisons between observed environmental trends and the predictions of cosmological hydrodynamical simulations are now routine, there have been relatively few comparisons at higher redshifts to date. Here we confront three state-of-the-art suites of simulations (BAHAMAS+MACSIS, EAGLE+Hydrangea, IllustrisTNG) with state-of-the-art observations of the field and cluster environments from the COSMOS/UltraVISTA and GOGREEN surveys, respectively, at z ~ 1 to assess the realism of the simulations and gain insight into the evolution of environmental quenching. We show that while the simulations generally reproduce the stellar content and the stellar mass functions of quiescent and star-forming galaxies in the field, all the simulations struggle to capture the observed quenching of satellites in the cluster environment, in that they are overly efficient at quenching low-mass satellites. Furthermore, two of the suites do not sufficiently quench the highest-mass galaxies in clusters, perhaps a result of insufficient feedback from AGN. The origin of the discrepancy at low stellar masses (Mstar <~ 1E10 Msun), which is present in all the simulations in spite of large differences in resolution, feedback implementations, and hydrodynamical solvers, is unclear. The next generation of simulations, which will push to significantly higher resolution and also include explicit modelling of the cold interstellar medium, may help to shed light on the low-mass tension.

Not all galaxies at Cosmic Noon evolve in the same way. It remains unclear how the local environment -- especially the extreme overdensities of protoclusters -- affects stellar mass assembly at high redshift. The stellar mass function (SMF) encodes these processes; comparing SMFs across environments reveals differences in evolutionary history. We present the SMF of the Hyperion proto-supercluster at \\(z\\sim2.5\\), one of the largest and most massive protostructures known. This dataset provides the most statistically robust SMF of a single protostructure at \\(z>2\\). By comparing the SMF of overdense peaks within Hyperion to the coeval field, we ask: how early, and how strongly, does a dense environment favor massive galaxies? Using COSMOS2020 photometry with ground-based and new HST grism spectroscopy, we construct a 3D overdensity map that assigns galaxies to peaks, outskirts, or the field. We perform 100 Monte Carlo realizations to propagate redshift and mass uncertainties, and derive SMFs normalized to the field. The peaks show a clear excess of massive galaxies: number densities at \\(\\log(M_*/M_\\odot)\\sim 11\\) are ~10x higher than the field, while those at \\(\\log(M_*/M_\\odot)\\sim 9.5\\) are enhanced by only ~3.5x. By contrast, the outskirts and Hyperion as a whole mirror the field. Environmental effects on stellar mass growth are thus evident by \\(z\\sim 2.5\\). The densest regions already host galaxies with accelerated growth, while the global SMF masks this signal. Protostructures therefore begin shaping the high-mass end of the SMF well before cluster quenching, and may drive the elevated star formation at Cosmic Noon.