0 Assigned ID: 7872 ----- Attempting Submission 1 (Thu Sep 17 20:26:13 GMT 2026) ----- 0 ----- Attempting Submission 1 (Thu Oct 15 19:59:45 GMT 2026) ----- Follow-up of New, Relaxed Cool-Core Clusters We propose observations of 4 massive, relaxed, cool-core galaxy clusters at redshifts 0.35-0.62, selected through a new effort to identify such systems in archival and snapshot data. The new data will enhance cluster cosmology with new, precise measurements of the gas-mass fraction at the redshifts where dark energy is dominant and specifically where DESI data suggest a departure from the cosmological constant model. They will also constrain the gas thermodynamics, enrichment, and total mass profiles, key to evolutionary studies of cool cores and cluster astrophysics. We expect these targets to be the vanguard of a an expanded sample of highly relaxed clusters, ultimately double the current sample's size, enabling a new generation of studies of cosmology, feedback and other astrophysics. We propose imaging of the center of 1 galaxy cluster at redshift 0.35 in the F475W and F814W filters using WFC3/UV. The field of view will reach ~400 kpc (~0.75 r2500), easily covering the central region of radius <30'' where strong lensing is expected. The HST exposure time calculator indicates that, for an elliptical galaxy spectral template (model 2) with 1'' radius source regions and standard background levels, we will reach limiting magnitudes (S/N=5) of 25 and 27 in F475W and F814W, respectively, with 1 orbit in each filter. This depth enables firm identifications of the red sequence, and will provide comparable sensitivity to other clusters with archival data in the our sample. Adam Mantz, Steve Allen, Glenn Morris and Michael McDonald and Taweewat Somboonpanyakul all have extensive experience using ACIS 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 kind of studies that are proposed, as well as published on a wide range of cluster astrophysics and cosmology. Galaxies 6 false false false false false false false true SECONDARY CHANDRA 28800073 false SUPPORTED CLUSTER OF GALAXIES BCM RICH CLUSTER J2000 J2000 WFC3/UVIS Imaging No No F475W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 1 false /Users/amantz/data/proposals/Chandra/ao28/fgas_followup/OP-cache/hst_phase2-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18487 :visit-id "01" :proposal-version -1 :trans-version "20260828" :date "15-Oct-2026 12:55:25.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18487 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18487 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2594 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2974 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18487 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2594 :al-overhead-at-end 116 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2974 :pcs-gap 0 ) true 2 0