18032( 3) - 03-Dec-2025 18:52:15 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18032 Version: 3 Check-in Time: 03-Dec-2025 18:52:15 STScI Edit Number: 0 Title The Mysterious Death of a Lonely Dwarf: Who Quenched Cetus? ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Paul Bennet Space Telescope Science Institute Y CoI: Dr. Ekta Patel Villanova University N CoI: Dr. Sangmo Tony Sohn Space Telescope Science Institute N CoI: Dr. Roeland P. van der Marel Space Telescope Science Institute N CoI: Dr. Laura L. Watkins Space Telescope Science Institute - ESA - N JWST CoI: Dr. Kevin Andrew McKinnon University of Toronto N CoI: Dr. Mark Fardal Eureka Scientific Inc. N CoI: Dr. Eduardo Vitral University of Edinburgh, Institute for N Astronomy CoI: Jack Thomas Warfield The University of Virginia N CoI: Dr. Nitya Kallivayalil The University of Virginia N CoI: Dr. Gurtina Besla University of Arizona N ------------------------------------------------------------------------------------ Abstract Cetus dSph (Cetus) is a classical isolated dwarf galaxy in the LG, separated from the Milky Way (MW) and M31 by ~750 kpc and ~680 kpc, respectively. Despite this apparent isolation, Cetus is a quenched dwarf galaxy with no HI reservoir or recent star formation. This is contrary to the distance-morphology relation where quenched galaxies are typically closer to a massive host and star-forming galaxies are more isolated. Traditionally, the quenched nature of Cetus has been explained by labeling it a backsplash galaxy, i.e. a galaxy, typically a dwarf, that has previously interacted with a massive galaxy but is now significantly outside of its virial radius due to a highly elliptical orbit. Significantly, this explanation is not compatible with recent kinematic studies, which suggest that Cetus may be on first infall with no previous interaction with the MW or M31. If on first infall, Cetus would defy our current understanding of how galaxies quench; there are no analogs of quenched, Cetus-mass galaxies in simulations that have not encountered a more massive companion. However, existing kinematic data for Cetus has significant measurement uncertainty, making its first infall orbit classification highly uncertain. We propose a new epoch of imaging for the Cetus dSph with HST to measure the proper motions (PM) of Cetus to sufficient accuracy to definitively constrain its past orbit and interaction history. Given the clear theoretical challenge of a first infall orbit to galaxy quenching models, it is critical to constrain the orbit of Cetus to advance or verify our understanding of how galaxies quench. ------------------------------------------------------------------------------------ Observations Description ------------------------ We will observe Cetus using ACS/WFC, with coordinated parallel observations using WFC3/UVIS. PM measurement requires at least two epochs of data separated by a significant period of time. Cetus has deep ACS/WFC and parallel WFC3/UVIS 18032( 3) - 03-Dec-2025 18:52:15 - [ 2] imaging data in both F814W and F475W in the HST archive from 2006 (GO-10505), this gives a time baseline of 20 years compared to Cycle 33. To measure PMs, it is sufficient to observe only in one filter. We will image Cetus for 8 orbits divided into half orbit exposures in F814W. ------------------------------------------------------------------------------------ 18032( 3) - 03-Dec-2025 18:52:15 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 CETUS-DSPH GALAXY, DWARF RA=00H26M9.7000S, J2000 V = 13.2 SPHEROIDAL DEC=-11D04'35.00" Reference Frame: Other: NED Comments: This object was generated by the targetselector and retrieved from the NED database. ------------------------------------------------------------------------------------------------------------------------------------ 18032( 3) - 03-Dec-2025 18:52:15 - [ 4] Visit: 01 Visit Requirements: ORIENT 266.015D TO 266.015 D; GROUP 01, 02, 03, 04 WITHIN 30D On Hold Comments: Additional Comments: A 2 orbit visit for Cetus dSph in F814W. 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 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0, 0; PAR 2 see below WITH 1 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.2228, 0.1158; see below PAR 4 WITH 3 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1128 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.0998, 0.2303; see below PAR 6 WITH 5 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.3225, 0.3460; see below PAR 8 WITH 7 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 1 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S CETUS-DSPH 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 1188.0 S S CETUS-DSPH 3 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S 18032( 3) - 03-Dec-2025 18:52:15 - [ 5] 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 1128.0 S S CETUS-DSPH 5 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S CETUS-DSPH 7 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 8 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S 18032( 3) - 03-Dec-2025 18:52:15 - [ 6] Visit: 02 Visit Requirements: ORIENT 266.015D TO 266.015 D; GROUP 02, 01, 03, 04 WITHIN 30D On Hold Comments: Additional Comments: A 2 orbit visit for Cetus dSph in F814W. 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 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.4075, 0.0306; see below PAR 2 WITH 1 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.6304, 0.1464; see below PAR 4 WITH 3 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1128 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.5073, 0.2609; see below PAR 6 WITH 5 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.7302, 0.3767; see below PAR 8 WITH 7 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 1 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S CETUS-DSPH 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 1188.0 S S CETUS-DSPH 3 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S 18032( 3) - 03-Dec-2025 18:52:15 - [ 7] 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 1128.0 S S CETUS-DSPH 5 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S CETUS-DSPH 7 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 8 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S 18032( 3) - 03-Dec-2025 18:52:15 - [ 8] Visit: 03 Visit Requirements: ORIENT 266.015D TO 266.015 D; GROUP 03, 01, 02, 04 WITHIN 30D On Hold Comments: Additional Comments: A 2 orbit visit for Cetus dSph in F814W. 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 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.0018, 0.4085; see below PAR 2 WITH 1 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.2246, 0.5243; see below PAR 4 WITH 3 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1128 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.1017, 0.6388; see below PAR 6 WITH 5 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.3244, 0.7545; see below PAR 8 WITH 7 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 1 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S CETUS-DSPH 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 1188.0 S S CETUS-DSPH 3 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S 18032( 3) - 03-Dec-2025 18:52:15 - [ 9] 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 1128.0 S S CETUS-DSPH 5 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S CETUS-DSPH 7 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 8 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S 18032( 3) - 03-Dec-2025 18:52:15 - [ 10] Visit: 04 Visit Requirements: ORIENT 266.015D TO 266.015 D; GROUP 04, 01, 02, 03 WITHIN 30D On Hold Comments: Additional Comments: A 2 orbit visit for Cetus dSph in F814W. 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 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.4093, 0.4392; see below PAR 2 WITH 1 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1188 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.6322, 0.5550; see below PAR 4 WITH 3 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1128 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.5091, 0.6695; see below PAR 6 WITH 5 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 CETUS-DSPH ACS/WFC ACCUM WFCENTER F814W 1 1100 S POS TARG 0.7320, 0.7852; see below PAR 8 WITH 7 (1129 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 CETUS-DSPH WFC3/UVI ACCUM UVIS F814W 1 1100 S S see below (1179 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 1 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S CETUS-DSPH 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 1188.0 S S CETUS-DSPH 3 ACS/WFC ACCUM WFCENTER F814W none none none none 1 1108.0 S 18032( 3) - 03-Dec-2025 18:52:15 - [ 11] 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-DSPH 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 1128.0 S S CETUS-DSPH 5 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S CETUS-DSPH 7 ACS/WFC ACCUM WFCENTER F814W none none none none 2 1129.0 S CETUS-DSPH 8 WFC3/UVI ACCUM UVIS F814W none none none none 2 1179.0 S S 18032( 3) - 03-Dec-2025 18:52:15 - [ 12] Summary Form for Proposal 18032 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS, WFCENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GROUP 01, 02, 03, 04 WITHIN 30D, GROUP 02, 01, 03, 04 WITHIN 30D, GROUP 03, 01, 02, 04 WITHIN 30D, GROUP 04, 01, 02, 03 WITHIN 30D, ORIENT 266.015D TO 266.015 D, PAR 2 WITH 1, PAR 4 WITH 3, PAR 6 WITH 5, PAR 8 WITH 7, POS TARG 0, 0, POS TARG 0.0018, 0.4085, POS TARG 0.0998, 0.2303, POS TARG 0.1017, 0.6388, POS TARG 0.2228, 0.1158, POS TARG 0.2246, 0.5243, POS TARG 0.3225, 0.3460, POS TARG 0.3244, 0.7545, POS TARG 0.4075, 0.0306, POS TARG 0.4093, 0.4392, POS TARG 0.5073, 0.2609, POS TARG 0.5091, 0.6695, POS TARG 0.6304, 0.1464, POS TARG 0.6322, 0.5550, POS TARG 0.7302, 0.3767, POS TARG 0.7320, 0.7852 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names CETUS-DSPH ------------------------------------------------------------------------------------------------------------------------------------