Third Submission. Minor changes to PDF. 600261954 Assigned ID: 6334 ----- Attempting Submission 1 (Thu Apr 10 01:02:38 GMT 2025) ----- HST Phase I Proposal 6334 successfully submitted. Receipt: # 6334-1 ----- Attempting Submission 2 (Thu Apr 10 19:26:42 GMT 2025) ----- HST Phase I Proposal 6334 successfully submitted. Receipt: # 6334-2 ----- Attempting Submission 3 (Thu Apr 10 23:48:09 GMT 2025) ----- Phase II, submission two, now with visits linked so they execute in 30 days in accordance with approved change request 93400. This also fixes the orbital overruns introduced by the bug in APT 2025.3.3 356157498 ----- Attempting Submission 1 (Sat Aug 09 19:46:34 GMT 2025) ----- ----- Attempting Submission 2 (Tue Dec 02 21:04:55 GMT 2025) ----- The Mysterious Death of a Lonely Dwarf: Who Quenched Cetus? 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. 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 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. All investigators are members of the High-resolution Space Telescope PROper MOtion (HSTPROMO) group, led by co-I van der Marel, which has published numerous papers reporting proper motion measurements of dwarf galaxies, globular clusters, individual stars along stellar streams, and distant Milky Way (MW) halo stars. PI Dr Paul Bennet is an expert in satellites inside and outside of the LG. He has had previous experience leading successful HST programs, e.g GO-15858 (Bennet et al. 2020). Dr Bennet has also succesfully led multiple HST programs related to the photometry and astrometry of dwarf galaxies using ACS (e.g. GO-15858, GO-17174 and GO-17501) including looking at orbital histories, dwarf galaxy quenching and the links between them. Dr Bennet will lead and perform the majority of the work for this project. Co-PI Dr Ekta Patel is an expert in orbital modelling and mass modelling in the Local Group. Dr Patel will contribute to the orbital history of Phoenix based on the PMs found in this program and contribute to the MW mass estimation. Co-I Dr Tony Sohn is an expert in measuring HST proper motions, his past measurements include M31, bulk proper motions of dwarf spheroidal galaxies in the Milky Way halo (Leo I, Draco, and Sculptor), individual proper motions of stars along stellar streams such as Sagittarius, Orphan, and Lethe, distant Milky Way globular clusters, and M31 satellites NGC 147 and 185. Dr Sohn will contribute to the PM measurements part of this work. Co-1, Dr K. McKinnon is an expert in rigorous statistical combinations of HST images with Gaia data, which enables significantly improved measurements of astrometry than either dataset can measure alone. Co-Is Dr. Gurtina Besla, Dr Mark Fardal, Dr Roeland van der Marel, Dr Nitya Kallivayalil, Dr Eduardo Vitral, Dr Laura Watkins and Mr Jack Warfield are experts in HST kinematic analysis, dynamical mass estimation, and dynamical modeling of Local Group galaxies including both equilibrium and N-body modeling. They will contribute to the orbital calculations and interpretation of the results. See list of refereed publications by the HSTPROMO group at http://www.stsci.edu/~marel/hstpromo.html#Papers Stellar Populations and the Interstellar Medium Galaxies 6 false false false false false false SUPPORTED GALAXY DWARF SPHEROIDAL This object was generated by the targetselector and retrieved from the NED database. J2000 J2000 ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F814W /private/var/folders/w_/jnjpw_kd2_dgbfb8dt29vldc0002m5/T/OPVisit5225053108086018480.xml Timing link suitability includes constraints ([Group (01 02 03 04) Within 30.0 days]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. A 2 orbit visit for Cetus dSph in F814W. 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