Third submission Second submission Initial submission -317376417 Assigned ID: 7226 ----- Attempting Submission 1 (Wed Apr 15 22:06:44 GMT 2026) ----- HST Phase I Proposal 7226 successfully submitted. Receipt: # 7226-1 ----- Attempting Submission 2 (Thu Apr 16 23:17:39 GMT 2026) ----- HST Phase I Proposal 7226 successfully submitted. Receipt: # 7226-2 ----- Attempting Submission 3 (Thu Apr 16 23:29:42 GMT 2026) ----- July 29, 2026 – Initial submission 1510509520 ----- Attempting Submission 1 (Wed Jul 29 18:56:43 GMT 2026) ----- Classical dwarf spheroidal galaxies (dSphs) are premier laboratories for testing dark matter (DM) physics, yet their mass profiles remain limited by the mass-anisotropy degeneracy in line-of-sight velocities. To break this degeneracy, internal proper motions are essential to yield 3D kinematics. We propose a GO-Archival Cycle 34 ACS/WFC program combining new observations with archival 2006 and 2011 epochs to study Leo Is internal kinematics. Leo I is dynamically extreme, combining three properties that connect the core-cusp debate to orbital archaeology and black-hole demographics. First, its unusual orbit, including a recent first-infall pericenter passage, provides a clean testbed for how environmental factors shape dwarf-galaxy evolution. Second, recent dynamical analyses suggest that Leo I may host one of the highest central DM densities among classical Milky Way dSphs, making it a critical test of Cold Dark Matter cusps versus tidal compression. Third, it lies at the center of a controversy regarding a claimed 3.3 million solar mass central black hole, an extraordinary mass that challenges standard scaling relations. By matching the footprint of archival HST observations from 2006 and 2011, our new data will extend the baseline to 21 years and deliver the sub-milliarcsecond proper-motion (PM) precision needed to map Leo Is internal 3D velocity dispersion and measure its bulk PM. We will perform dynamical modeling of the inferred kinematics to break the mass-anisotropy degeneracy. This will allow us to determine the origin of Leo Is extreme DM density, constrain the black-hole interpretation, and provide a lasting kinematic legacy for Local Group astrophysics. The primary goal of this program is to measure the internal proper motion kinematics of the Leo I dwarf spheroidal galaxy using multi-epoch HST imaging data. We will observe a single primary ACS/WFC field + a single parallel WFC3/UVIS field for 8 orbits with the same telescope pointing and orientation as older epoch observations. The telescope orientation conditions are added as special requirements for each visit to match the filed coverages between the pre-existing data. Individual exposures will be sub-pixel dithered and will last half an orbit. PI 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. He will be leading the proper motion measurements. Co-PI Vitral is an expert in velocity anisotropy analysis and dynamical mass estimation, and has particularly published papers studying the internal kinematics and dynamics of MW dwarf galaxies and globular clusters. Co-I Bennet is an expert in satellites inside and outside of the LG, and has previous experience leading successful HST programs. Co-I van der Marel is an expert in dynamical modeling of Local Group galaxies. He leads the HSTPROMO collaboration which for the past two decades has worked to improve our dynamical understanding of stars, clusters, and galaxies in the nearby Universe through measurement and interpretation of their proper motions. Co-I Watkins is an expert in velocity anisotropy analysis and dynamical mass estimation, and has particularly published papers studying the internal kinematics and dynamics of MW globular clusters. Co-I Patel is an expert in the dynamical modelling of Local Group galaxies, and has used HST-based proper motion measurements to calculate the orbits of Andromeda, the LMC/SMC, and nearly twenty of the Milky Way's dwarf satellite galaxies. Co-Is Bellini and Libralato are experts of data reduction, proper-motion measurements and internal kinematicsof globular clusters. Co-I 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-I Fardal is an expert in HST kinematic analysis and dynamical modeling of Local Group galaxies. Stellar Populations and the Interstellar Medium Galaxies 6 false false false false true false false false false false SUPPORTED GALAXY DWARF SPHEROIDAL J2000 J2000 ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F814W 1785168109769 /Users/tsohn/Library/CloudStorage/Dropbox/HST Phase II/18411-LeoI/OP-cache/18411-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. 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