18411( 2) - 29-Jul-2026 19:25:43 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18411 Version: 2 Check-in Time: 29-Jul-2026 19:25:43 STScI Edit Number: 0 Title Proper Motion Constraints on Dark Matter in Dwarf Galaxies: The Extreme Case of Leo I ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Sangmo Tony Sohn Space Telescope Science Institute Y CoI: Dr. Eduardo Vitral University of Edinburgh, Institute for N Astronomy CoI: Dr. Paul Bennet 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. Ekta Patel Villanova University N CoI: Dr. Andrea Bellini Space Telescope Science Institute N CoI: Dr. Mattia Libralato INAF - Osservatorio Astronomico di Padova N CoI: Dr. Kevin Andrew McKinnon University of Toronto N CoI: Dr. Mark Fardal Eureka Scientific Inc. N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 18411( 2) - 29-Jul-2026 19:25:43 - [ 2] 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. ------------------------------------------------------------------------------------ 18411( 2) - 29-Jul-2026 19:25:43 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-I GALAXY, DWARF RA=10H08M22.5000S, J2000 V = 12 SPHEROIDAL DEC=+12D18'5.90" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18411( 2) - 29-Jul-2026 19:25:43 - [ 4] Visit: 01 Visit Requirements: ORIENT 315D TO 315 D 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 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S PAR 2 WITH 1 (1108 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.2228, 0.1158; (1108 S) PAR 4 WITH 3 ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.0998, 0.2303; (1108 S) PAR 6 WITH 5 ------------------------------------------------------------------------------------------------------------------------------------ 6 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.3225, 0.3460; (1108 S) PAR 8 WITH 7 ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 9 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.4075, 0.0306; (1108 S) PAR 10 WITH 9 ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 11 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.6304, 0.1464; (1108 S) PAR 12 WITH 11 ------------------------------------------------------------------------------------------------------------------------------------ 12 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 13 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.5073, 0.2609; (1108 S) PAR 14 WITH 13 ------------------------------------------------------------------------------------------------------------------------------------ 14 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 18411( 2) - 29-Jul-2026 19:25:43 - [ 5] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 15 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.7302, 0.3767; (1108 S) PAR 16 WITH 15 ------------------------------------------------------------------------------------------------------------------------------------ 16 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F814W none none none none 1 N/A ANY 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S LEO-I 3 ACS/WFC ACCUM WFCENTER F814W none none none none 1 N/A ANY 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S LEO-I 5 ACS/WFC ACCUM WFCENTER F814W none none none none 2 N/A ANY 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 N/A S LEO-I 7 ACS/WFC ACCUM WFCENTER F814W none none none none 2 N/A ANY 8 WFC3/UVI ACCUM UVIS F814W none none none none 2 N/A S LEO-I 9 ACS/WFC ACCUM WFCENTER F814W none none none none 3 N/A ANY 10 WFC3/UVI ACCUM UVIS F814W none none none none 3 N/A S LEO-I 11 ACS/WFC ACCUM WFCENTER F814W none none none none 3 N/A ANY 12 WFC3/UVI ACCUM UVIS F814W none none none none 3 N/A S LEO-I 13 ACS/WFC ACCUM WFCENTER F814W none none none none 4 N/A ANY 14 WFC3/UVI ACCUM UVIS F814W none none none none 4 N/A S LEO-I 15 ACS/WFC ACCUM WFCENTER F814W none none none none 4 N/A ANY 16 WFC3/UVI ACCUM UVIS F814W none none none none 4 N/A S 18411( 2) - 29-Jul-2026 19:25:43 - [ 6] Visit: 02 Visit Requirements: ORIENT 315D TO 315 D 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 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.0018, 0.4085; (1108 S) PAR 2 WITH 1 ------------------------------------------------------------------------------------------------------------------------------------ 2 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.2246, 0.5243; (1108 S) PAR 4 WITH 3 ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.1017, 0.6388; (1108 S) PAR 6 WITH 5 ------------------------------------------------------------------------------------------------------------------------------------ 6 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 7 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.3244, 0.7545; (1108 S) PAR 8 WITH 7 ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 9 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.4093, 0.4392; (1108 S) PAR 10 WITH 9 ------------------------------------------------------------------------------------------------------------------------------------ 10 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 11 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.6322, 0.5550; (1108 S) PAR 12 WITH 11 ------------------------------------------------------------------------------------------------------------------------------------ 12 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 13 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.5091, 0.6695; (1108 S) PAR 14 WITH 13 ------------------------------------------------------------------------------------------------------------------------------------ 14 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 S) ------------------------------------------------------------------------------------------------------------------------------------ 18411( 2) - 29-Jul-2026 19:25:43 - [ 7] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 15 LEO-I ACS/WFC ACCUM WFCENTER F814W 1 1108 S POS TARG 0.7320, 0.7852; (1108 S) PAR 16 WITH 15 ------------------------------------------------------------------------------------------------------------------------------------ 16 ANY WFC3/UVI ACCUM UVIS F814W 1 1170 S S (1170 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEO-I 1 ACS/WFC ACCUM WFCENTER F814W none none none none 1 N/A ANY 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S LEO-I 3 ACS/WFC ACCUM WFCENTER F814W none none none none 1 N/A ANY 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S LEO-I 5 ACS/WFC ACCUM WFCENTER F814W none none none none 2 N/A ANY 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 N/A S LEO-I 7 ACS/WFC ACCUM WFCENTER F814W none none none none 2 N/A ANY 8 WFC3/UVI ACCUM UVIS F814W none none none none 2 N/A S LEO-I 9 ACS/WFC ACCUM WFCENTER F814W none none none none 3 N/A ANY 10 WFC3/UVI ACCUM UVIS F814W none none none none 3 N/A S LEO-I 11 ACS/WFC ACCUM WFCENTER F814W none none none none 3 N/A ANY 12 WFC3/UVI ACCUM UVIS F814W none none none none 3 N/A S LEO-I 13 ACS/WFC ACCUM WFCENTER F814W none none none none 4 N/A ANY 14 WFC3/UVI ACCUM UVIS F814W none none none none 4 N/A S LEO-I 15 ACS/WFC ACCUM WFCENTER F814W none none none none 4 N/A ANY 16 WFC3/UVI ACCUM UVIS F814W none none none none 4 N/A S 18411( 2) - 29-Jul-2026 19:25:43 - [ 8] Summary Form for Proposal 18411 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS, WFCENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 315D TO 315 D, PAR 10 WITH 9, PAR 12 WITH 11, PAR 14 WITH 13, PAR 16 WITH 15, PAR 2 WITH 1, PAR 4 WITH 3, PAR 6 WITH 5, PAR 8 WITH 7, 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 ANY, LEO-I ------------------------------------------------------------------------------------------------------------------------------------