18487( 2) - 16-Oct-2026 13:39:57 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18487 Version: 2 Check-in Time: 16-Oct-2026 13:39:57 STScI Edit Number: 0 Title Follow-up of New, Relaxed Cool-Core Clusters ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Adam Mantz Stanford University Y CoI: Prof. Steven W. Allen Stanford University N CoI: Dr. R. Glenn Morris Stanford University N CoI: Dr. Taweewat Somboonpanyakul Chulalongkorn University N CoI: Prof. Michael A. McDonald Massachusetts Institute of Technology N CoI: Dr. Anthony M Flores Rutgers the State University of New Jersey N CoI: Haley Rose Stueber Stanford University N CoI: Abby Y Pan Stanford University N CoI: Dr. Artem Poliszczuk Stanford University N CoI: Gia Ancone Stanford University N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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. ------------------------------------------------------------------------------------ 18487( 2) - 16-Oct-2026 13:39:57 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 ACT-CLJ0728. CLUSTER OF GALAXIES, RA=07H28M35.3280S, J2000 V = 18.5 5+1407 BCM, RICH CLUSTER DEC=+14D07'42.24" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18487( 2) - 16-Oct-2026 13:39:57 - [ 3] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.145 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18487( 2) - 16-Oct-2026 13:39:57 - [ 4] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 ACT-CLJ0728 WFC3/UVI ACCUM UVIS F475W 1 1140 S PATTERN 1 1-2 .5+1407 S see below (2428 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ACT-CLJ0728 WFC3/UVI ACCUM UVIS F814W 1 1140 S .5+1407 S see below (2428 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ ACT-CLJ0728.5+1407 1 WFC3/UVI ACCUM UVIS F475W 1 none none none 1 1219.0 S S ACT-CLJ0728.5+1407 1 WFC3/UVI ACCUM UVIS F475W 2 none none none 2 1209.0 S S ACT-CLJ0728.5+1407 2 WFC3/UVI ACCUM UVIS F814W 1 none none none 1 1219.0 S S ACT-CLJ0728.5+1407 2 WFC3/UVI ACCUM UVIS F814W 2 none none none 2 1209.0 S S 18487( 2) - 16-Oct-2026 13:39:57 - [ 5] Summary Form for Proposal 18487 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1-2 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F475W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ACT-CLJ0728.5+1407 ------------------------------------------------------------------------------------------------------------------------------------