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Abstract We present optical follow-up observations for candidate clusters in the Clusters Hiding in Plain Sight survey, which is designed to find new galaxy clusters with extreme central galaxies that were misidentified as bright isolated sources in the ROSAT All-Sky Survey catalog. We identify 11 cluster candidates around X-ray, radio, and mid-IR-bright sources, including six well-known clusters, two false associations of foreground and background clusters, and three new candidates, which are observed further with Chandra. Of the three new candidates, we confirm two newly discovered galaxy clusters: CHIPS 1356-3421 and CHIPS 1911+4455. Both clusters are luminous enough to be detected in the ROSAT All-Sky Survey data if not because of their bright central cores. CHIPS 1911+4455 is similar in many ways to the Phoenix cluster, but with a highly disturbed X-ray morphology on large scales. We find the occurrence rate for clusters that would appear to be X-ray-bright point sources in the ROSAT All-Sky Survey (and any surveys with similar angular resolution) to be 2% ± 1%, and the occurrence rate of clusters with runaway cooling in their cores to be <1%, consistent with predictions of chaotic cold accretion. With the number of new groups and clusters predicted to be found with eROSITA, the population of clusters that appear to be point sources (due to a central QSO or a dense cool core) could be around 2000. Finally, this survey demonstrates that the Phoenix cluster is likely the strongest cool core at z < 0.7—anything more extreme would have been found in this survey. 

Abstract Using stellar population synthesis models to infer star formation histories (SFHs), we analyze photometry and spectroscopy of a large sample of quiescent galaxies that are members of Sunyaev–Zel’dovich (SZ)-selected galaxy clusters across a wide range of redshifts. We calculate stellar masses and mass-weighted ages for 837 quiescent cluster members at 0.3 < z < 1.4 using rest-frame optical spectra and the Python-based Prospector framework, from 61 clusters in the SPT-GMOS Spectroscopic Survey (0.3 < z < 0.9) and three clusters in the SPT Hi-z cluster sample (1.25 < z < 1.4). We analyze spectra of subpopulations divided into bins of redshift, stellar mass, cluster mass, and velocity-radius phase-space location, as well as by creating composite spectra of quiescent member galaxies. We find that quiescent galaxies in our data set sample a diversity of SFHs, with a median formation redshift (corresponding to the lookback time from the redshift of observation to when a galaxy forms 50% of its mass, t 50 ) of z = 2.8 ± 0.5, which is similar to or marginally higher than that of massive quiescent field and cluster galaxy studies. We also report median age–stellar mass relations for the full sample (age of the universe at t 50 (Gyr) = 2.52 (±0.04)–1.66 (±0.12) log 10 ( M /10 11 M ⊙ )) and recover downsizing trends across stellar mass; we find that massive galaxies in our cluster sample form on aggregate ∼0.75 Gyr earlier than lower-mass galaxies. We also find marginally steeper age–mass relations at high redshifts, and report a bigger difference in formation redshifts across stellar mass for fixed environment, relative to formation redshifts across environment for fixed stellar mass. 

Abstract SPT0311-58, a system of two interacting galaxies in the Epoch of Reionization, exists in one of the rarest, most massive dark matter halos theoretically possible in that era. Studying the interstellar medium (ISM) in these galaxies can illuminate the process of galaxy formation in the early Universe. In this work, we explore the multiphase ISM in this system, using ALMA observations of the [Cii] 158, [Oi] 146, [Nii] 122, and [Oiii] 88 fine-structure lines and dust continuum. We find wide variations in line ratios between the eastern and western galaxies, as well as across the western galaxy. Continuum colors indicate that SPT0311-58 E has a higher ionization parameter ( $\log U\approx -2.8$) than SPT0311-58 W ( $\log U\approx -3.1$). The ratio of [Oiii] 88–[Nii] 122 and the ionization parameter constraints combine to demonstrate near-solar metallicity in these objects just 800 Myr after the Big Bang. 

Abstract The SPT 0311–58 system atz= 6.900 is an extremely massive structure within the reionization epoch and offers a chance to understand the formation of galaxies at an extreme peak in the primordial density field. We present 70 mas Atacama Large Millimeter/submillimeter Array observations of the dust continuum and [Cii] 158μm emission in the central pair of galaxies and reach physical resolutions of ∼100–350 pc, among the most detailed views of any reionization-era system to date. The observations resolve the source into at least a dozen kiloparsec-size clumps. The global kinematics and high turbulent velocity dispersion within the galaxies present a striking contrast to recent claims of dynamically cold thin-disk kinematics in some dusty galaxies just 800 Myr later atz∼ 4. We speculate that both gravitational interactions and fragmentation from massive parent disks have likely played a role in the overall dynamics and formation of clumps in the system. Each clump individually is comparable in mass to other 6 <z< 8 galaxies identified in rest-UV/optical deep field surveys, but with star formation rates elevated by a factor of ~3-5. Internally, the clumps themselves bear close resemblance to greatly scaled-up versions of virialized cloud-scale structures identified in low-redshift galaxies. Our observations are qualitatively similar to the chaotic and clumpy assembly within massive halos seen in simulations of high-redshift galaxies.