This submission is the HST complement to an accepted Chandra Cycle 27 proposal. -2084772747 Assigned ID: 6914 ----- Attempting Submission 1 (Wed Aug 27 14:58:03 GMT 2025) ----- 0 ----- Attempting Submission 1 (Mon Sep 29 17:32:26 GMT 2025) ----- Multiwavelength Observations of the Highest-Redshift Dynamically Relaxed Galaxy Cluster We propose joint Chandra, XMM-Newton, and HST observations of ACT-CL J0123.5-0428, a z=1.5 galaxy cluster newly identified as the highest redshift relaxed cool-core cluster known. Deeper X-ray observations from Chandra and XMM, paired with wideband WFC3/IR imaging from Hubble, will allow us to exquisitely characterize the (thermo)dynamics and chemistry of the ICM out to large radius - including a precise gas mass profile for cosmological studies - and constrain the morphologies and stellar histories of member galaxies over the same spatial scale. In combination, these observations will provide a greatly enhanced picture of feedback processes in the early Universe and significantly extend the redshift range probed by studies of ICM/galaxy evolution and cosmology in relaxed systems. This program consists of 10 total HST orbits using the WFC3 to observe a high redshift (z=1.5) galaxy cluster. This system will be observed for 5 total orbits using the F105W IR filter, and 5 total orbits using the F140W IR filter. This program contains 5 visits to the object. Each visit will consist of 2 orbits, with one orbit corresponding to each of the above filters. For each visit, the center of the cluster (roughly the position of the BCG) will be used as the coordinates of the target. The following is the Team Expertise section submitted to the Chandra Cycle 27 TAC: Anthony Flores, Adam Mantz, Steven Allen, Glenn Morris, Michael McDonald, Taweewat Somboonpanyakul, Haley Stueber, and Abigail Pan collectively have extensive experience using ACIS, XMM, and Hubble data to study clusters of galaxies across a wide range in redshift and mass. Within our team, we have performed, and in some cases pioneered, exactly the kinds of astrophysical and cosmological studies that are proposed here, in particular on the study of dynamically relaxed/cool-core galaxy clusters, the evolution of the intracluster medium, galaxy cluster cosmology (cluster counts and fgas), and multiwavelength characterizations of BCGs and other cluster member galaxies. Additionally, the new X-ray and O/IR data awarded through this proposal will be complemented with ground-based spectroscopy, available to the proposal team via Stanford University's institutional access to the Magellan telescopes. An incomplete bibliography of relevant papers follows. Allen S. W., Evrard A. E., Mantz A. B., 2011, Cosmological Parameters from Observations of Galaxy Clusters, ARA&A, 49, 409 Calzadilla M. S. et al., 2023, SPT-CL J2215-3537: A Massive Starburst at the Center of the Most Distant Relaxed Galaxy Cluster, ApJ, 947, 44 Flores A. M. et al., 2021, The history of metal enrichment traced by X-ray observations of highredshift galaxy clusters, MNRAS, 507, 5195 Mantz A. B. et al., 2015a, Weighing the giants - IV. Cosmology and neutrino mass, MNRAS, 446, 2205 Mantz A. B., Allen S. W., Morris R. G., Schmidt R. W., von der Linden A., Urban O., 2015b, Cosmology and astrophysics from relaxed galaxy clusters - I. Sample selection, MNRAS, 449, 199 Mantz A. B., Allen S. W., Morris R. G., 2016a, Cosmology and astrophysics from relaxed galaxy clusters - V. Consistency with cold dark matter structure formation, MNRAS, 462, 681 Mantz A. B. et al., 2016b, Weighing the giants- V. Galaxy cluster scaling relations, MNRAS, 463, 3582 Mantz A. B. et al., 2022, Cosmological constraints from gas mass fractions of massive, relaxed galaxy clusters, MNRAS, 510, 131 McDonald M. et al., 2014, The Redshift Evolution of the Mean Temperature, Pressure, and Entropy Profiles in 80 SPT-Selected Galaxy Clusters, ApJ, 794, 67 McDonald M. et al., 2016, The Evolution of the Intracluster Medium Metallicity in Sunyaev Zeldovich-selected Galaxy Clusters at 0z1.5, ApJ, 826, 124 McDonald M. et al., 2019, Anatomy of a Cooling Flow: The Feedback Response to Pure Cooling in the Core of the Phoenix Cluster, ApJ, 885, 63 Somboonpanyakul T. et al., 2022, The Evolution of AGN Activity in Brightest Cluster Galaxies, AJ, 163, 146 Galaxies 6 false false false false false false true SECONDARY CHANDRA 27800035 false SUPPORTED CLUSTER OF GALAXIES HIGH REDSHIFT CLUSTER YES J2000 J2000 21 (z band), 18.5 (CATWISE W1) WFC3/IR Imaging No No F105W WFC3/IR Imaging No No F140W POS-TARG DITHER 1.716 LINE 1.095 false false 74.653 4 18.528 WFC3-IR-DITHER-BOX-MIN 1 false /private/var/folders/cq/xb355zb945zds27cqf5wm8fw0000gn/T/OPVisit11059712244154338345.xml The scheduling data was generated by CASM 33.1. 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