0 Assigned ID: 6138 ----- Attempting Submission 1 (Mon Apr 07 18:33:14 GMT 2025) ----- HST Phase I Proposal 6138 successfully submitted. Receipt: # 6138-1 0 ----- Attempting Submission 1 (Mon Aug 04 23:31:46 GMT 2025) ----- Deep UVIS imaging of the Leo II dwarf spheroidal galaxy: deciphering the evolution of a unique Milky Way satellite 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. 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 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). Dr. Savino, Weisz, Dolphin and Choi have extensive experience in working with HST photometry of Local Group galaxies, color-magnitude diagram modelling and star formation history measurements. Dr. Weisz has deep involvement with many observational programs of local galaxies and he currently leads a large legacy HST survey of M31 satellite galaxies (GO-15902). Dr. Dolphin is the main developer of Dolphot, a popular photometric reduction pipeline, is the author of several seminal studies on the measurement of star formation histories, and the developer of MATCH, the software that will be used for our star formation history work. Dr. Patel is an expert in the dynamical modelling of Local Group galaxies. She has used HST-based proper motion measurements to calculate the orbits of Andromeda, the Magellanic Clouds, and nearly twenty of the Milky Way's dwarf satellite galaxies. Her expertise will be pivotal in combining the star formation history of Leo II with its orbital history. Galaxies Intergalactic Medium and the Circumgalactic Medium 6 false false false false false false SUPPORTED GALAXY DWARF SPHEROIDAL This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W Hst Same orient constraint links visits (01 02 03) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18042 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "04-Aug-2025 16:30:58.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2291888 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18042 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18042 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18042 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 116 :internal-dur-at-end 699 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) false 2 0 Hst Same orient constraint links visits (01 02 03) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18042 :visit-id "02" :proposal-version -1 :trans-version "20250624" :date "04-Aug-2025 16:30:59.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2291888 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18042 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18042 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18042 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 116 :internal-dur-at-end 699 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) false 2 0 Hst Same orient constraint links visits (01 02 03) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18042 :visit-id "03" :proposal-version -1 :trans-version "20250624" :date "04-Aug-2025 16:30:49.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2291888 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18042 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18042 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18042 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 116 :internal-dur-at-end 699 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) false 2 0