18042( 3) - 15-Aug-2025 17:10:28 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18042 Version: 3 Check-in Time: 15-Aug-2025 17:10:28 STScI Edit Number: 1 Title Deep UVIS imaging of the Leo II dwarf spheroidal galaxy: deciphering the evolution of a unique Milky Way satellite ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Alessandro Savino University of California - Berkeley Y CoI: Dr. Daniel R. Weisz University of California - Berkeley N CoI: Dr. Ekta Patel Villanova University N CoI: Dr. Andrew Eugene Dolphin Raytheon Company N CoI: Dr. Yumi Choi NOIRLab - (AZ) N ------------------------------------------------------------------------------------ Abstract We propose to obtain deep photometric imaging of the Milky Way (MW) satellite galaxy Leo II. Leo II has unique properties within the MW galactic ecosystem and has great potential to deepen our understanding of the Local Group history. Leo II is the only known MW satellite to have quenched at intermediate ages (6 Gyr ago), bridging the gap between the two main groups of early-quenched and late-quenched satellites. Additionaly, Leo II is one of the most distant MW satellites and is in a low-eccentricity orbit, making it a precious tracer of the properties of the outer MW halo. Yet, this is the only long-known Local Group galaxy that lacks deep photometry and it is readily accessible by HST. The exquisite precision of our proposed observations will enable us to derive an updated, high-resolution, star formation history for this galaxy and investigate the presence of multiple episodes of star formation. Characterizing the frequency, amplitude, and timescales of star formation activity in Leo II will help address open questions connected to the formation and evolution of dwarf spheroidal galaxies, assessing the relative importance of ubiquitous (stellar feedback, interaction with the host) versus stochastic (mergers, gas accretion) drivers of star formation. In addition, we will determine a precise quenching epoch for Leo II which, when combined with state-of-the-art orbital calculations, will allow us to constrain the efficiency of ram pressure stripping at large Galactocentric radii, with implications for the density of the Milky Way circum-galactic medium and the total mass contained in the hot corona of our Galaxy. ------------------------------------------------------------------------------------ Observations Description ------------------------ This proposal will obtain deep UVIS photometry of the Milky Way satellites Leo II. The goal is to construct a deep color-magnitude diagram so that: i) the presence of multiple main sequence turn-offs can be established; ii) an accurate quenching time can be derived for Leo II. This in turn will put constrain on dwarf spheroidal galaxy formation models and on the density of the Milky Way outer circum-galactic medium. The observations are split into 3 identical visits, consisting of 2 orbits each. Each orbit will obtain 1 UVIS F606W and 1 UVIS F814W exposures (with no 18042( 3) - 15-Aug-2025 17:10:28 - [ 2] CR splits). We are also obtaining similar F606W/F814W exposures with ACS/WFC, in parallel. Due to the presence of parallels, the visits have been set up with the SAME_ORIENT requirement. For each orbit of this program, we have applied POS_TARG offsets to obtain a 6-point subpixel dither pattern, following the recommendations of WFC3 ISR 2023-05 (3 2-point dithers, each spaced by ~1 arcsec). ------------------------------------------------------------------------------------ 18042( 3) - 15-Aug-2025 17:10:28 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LEO-II GALAXY, DWARF RA=11H13M28.1300S, J2000 V = 25 SPHEROIDAL DEC=+22D09'10.10" Reference Frame: Other: SIMBAD Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2015.5 7.4859337523641905E-6 -3.300006028439384E-5 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18042( 3) - 15-Aug-2025 17:10:28 - [ 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 LEO-II WFC3/UVI ACCUM UVIS F606W CR-SPLIT=NO 1 667 S (667 POS TARG 0, 0; GS ACQ S S) SCENARIO ONEB1O3; PAR 3-4 WITH 1-2 ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-II WFC3/UVI ACCUM UVIS F814W CR-SPLIT=NO 1 1737 S POS TARG 0, 0 S (1737 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ANY ACS/WFC ACCUM WFC F606W CR-SPLIT=NO 1 728 S (728 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY ACS/WFC ACCUM WFC F814W CR-SPLIT=NO 1 1457 S (1457 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 LEO-II WFC3/UVI ACCUM UVIS F606W CR-SPLIT=NO 1 662 S (662 POS TARG 0.17859, S S) 0.06987; PAR 7-8 WITH 5-6 ------------------------------------------------------------------------------------------------------------------------------------ 6 LEO-II WFC3/UVI ACCUM UVIS F814W CR-SPLIT=NO 1 1731 S POS TARG 0.17859, S (1731 S) 0.06987 ------------------------------------------------------------------------------------------------------------------------------------ 7 ANY ACS/WFC ACCUM WFC F606W CR-SPLIT=NO 1 731 S (731 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY ACS/WFC ACCUM WFC F814W CR-SPLIT=NO 1 1462 S (1462 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-II 1 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S LEO-II 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S ANY 3 ACS/WFC ACCUM WFC F606W none none none none 1 N/A ANY 4 ACS/WFC ACCUM WFC F814W none none none none 1 N/A LEO-II 5 WFC3/UVI ACCUM UVIS F606W none none none none 2 N/A S LEO-II 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 N/A S 18042( 3) - 15-Aug-2025 17:10:28 - [ 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 | ------------------------------------------------------------------------------------------------------------------------------------ ANY 7 ACS/WFC ACCUM WFC F606W none none none none 2 N/A ANY 8 ACS/WFC ACCUM WFC F814W none none none none 2 N/A 18042( 3) - 15-Aug-2025 17:10:28 - [ 6] Visit: 02 Visit Requirements: SAME ORIENT AS 01 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-II WFC3/UVI ACCUM UVIS F606W CR-SPLIT=NO 1 667 S (667 POS TARG 1, 1; GS ACQ S S) SCENARIO ONEB1O3; PAR 3-4 WITH 1-2 ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-II WFC3/UVI ACCUM UVIS F814W CR-SPLIT=NO 1 1737 S POS TARG 1, 1 S (1737 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ANY ACS/WFC ACCUM WFC F606W CR-SPLIT=NO 1 728 S (728 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY ACS/WFC ACCUM WFC F814W CR-SPLIT=NO 1 1457 S (1457 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 LEO-II WFC3/UVI ACCUM UVIS F606W CR-SPLIT=NO 1 662 S (662 POS TARG 1.17859, S S) 1.06987; PAR 7-8 WITH 5-6 ------------------------------------------------------------------------------------------------------------------------------------ 6 LEO-II WFC3/UVI ACCUM UVIS F814W CR-SPLIT=NO 1 1731 S POS TARG 1.17859, S (1731 S) 1.06987 ------------------------------------------------------------------------------------------------------------------------------------ 7 ANY ACS/WFC ACCUM WFC F606W CR-SPLIT=NO 1 731 S (731 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY ACS/WFC ACCUM WFC F814W CR-SPLIT=NO 1 1462 S (1462 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-II 1 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S LEO-II 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S ANY 3 ACS/WFC ACCUM WFC F606W none none none none 1 N/A ANY 4 ACS/WFC ACCUM WFC F814W none none none none 1 N/A LEO-II 5 WFC3/UVI ACCUM UVIS F606W none none none none 2 N/A S LEO-II 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 N/A S 18042( 3) - 15-Aug-2025 17:10:28 - [ 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 | ------------------------------------------------------------------------------------------------------------------------------------ ANY 7 ACS/WFC ACCUM WFC F606W none none none none 2 N/A ANY 8 ACS/WFC ACCUM WFC F814W none none none none 2 N/A 18042( 3) - 15-Aug-2025 17:10:28 - [ 8] Visit: 03 Visit Requirements: SAME ORIENT AS 01 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-II WFC3/UVI ACCUM UVIS F606W CR-SPLIT=NO 1 667 S (667 POS TARG -1, -1; GS ACQ S S) SCENARIO ONEB1O3; PAR 3-4 WITH 1-2 ------------------------------------------------------------------------------------------------------------------------------------ 2 LEO-II WFC3/UVI ACCUM UVIS F814W CR-SPLIT=NO 1 1737 S POS TARG -1, -1 S (1737 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 ANY ACS/WFC ACCUM WFC F606W CR-SPLIT=NO 1 728 S (728 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 ANY ACS/WFC ACCUM WFC F814W CR-SPLIT=NO 1 1457 S (1457 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 LEO-II WFC3/UVI ACCUM UVIS F606W CR-SPLIT=NO 1 662 S (662 POS TARG -1.17859, S S) -1.06987; PAR 7-8 WITH 5-6 ------------------------------------------------------------------------------------------------------------------------------------ 6 LEO-II WFC3/UVI ACCUM UVIS F814W CR-SPLIT=NO 1 1731 S POS TARG -1.17859, S (1731 S) -1.06987 ------------------------------------------------------------------------------------------------------------------------------------ 7 ANY ACS/WFC ACCUM WFC F606W CR-SPLIT=NO 1 731 S (731 S) ------------------------------------------------------------------------------------------------------------------------------------ 8 ANY ACS/WFC ACCUM WFC F814W CR-SPLIT=NO 1 1462 S (1462 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-II 1 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S LEO-II 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S ANY 3 ACS/WFC ACCUM WFC F606W none none none none 1 N/A ANY 4 ACS/WFC ACCUM WFC F814W none none none none 1 N/A LEO-II 5 WFC3/UVI ACCUM UVIS F606W none none none none 2 N/A S LEO-II 6 WFC3/UVI ACCUM UVIS F814W none none none none 2 N/A S 18042( 3) - 15-Aug-2025 17:10:28 - [ 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 | ------------------------------------------------------------------------------------------------------------------------------------ ANY 7 ACS/WFC ACCUM WFC F606W none none none none 2 N/A ANY 8 ACS/WFC ACCUM WFC F814W none none none none 2 N/A 18042( 3) - 15-Aug-2025 17:10:28 - [ 10] Summary Form for Proposal 18042 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS, WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements GS ACQ SCENARIO ONEB1O3, PAR 3-4 WITH 1-2, PAR 7-8 WITH 5-6, POS TARG -1, -1, POS TARG -1.17859, -1.06987, POS TARG 0, 0, POS TARG 0.17859, 0.06987, POS TARG 1, 1, POS TARG 1.17859, 1.06987, SAME ORIENT AS 01 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names ANY, LEO-II ------------------------------------------------------------------------------------------------------------------------------------