Final Submission -865891210 Assigned ID: 5297 ----- Attempting Submission 1 (Tue Mar 26 02:03:04 GMT 2024) ----- HST Phase I Proposal 5297 successfully submitted. Receipt: # 5297-1 ----- Attempting Submission 2 (Tue Mar 26 03:13:38 GMT 2024) ----- HST Phase I Proposal 5297 successfully submitted. Receipt: # 5297-2 ----- Attempting Submission 3 (Tue Mar 26 12:08:43 GMT 2024) ----- HST Phase I Proposal 5297 successfully submitted. Receipt: # 5297-3 ----- Attempting Submission 4 (Tue Mar 26 18:03:39 GMT 2024) ----- HST Phase I Proposal 5297 successfully submitted. Receipt: # 5297-4 ----- Attempting Submission 5 (Tue Mar 26 21:17:00 GMT 2024) ----- HST Phase I Proposal 5297 successfully submitted. Receipt: # 5297-5 ----- Attempting Submission 6 (Tue Mar 26 23:51:43 GMT 2024) ----- HST Phase I Proposal 5297 successfully submitted. Receipt: # 5297-6 ----- Attempting Submission 7 (Tue Mar 26 23:53:41 GMT 2024) ----- Resubmitting with visits 52,53,55,56 as retakes of failed exposures (see TTRBHST-93564) 1792262317 ----- Attempting Submission 1 (Fri Aug 16 20:26:58 GMT 2024) ----- ----- Attempting Submission 2 (Thu Oct 17 03:19:21 GMT 2024) ----- HST Phase II Proposal 17787 successfully submitted. Receipt: # 17787-2 ----- Attempting Submission 3 (Thu Oct 17 03:22:07 GMT 2024) ----- ----- Attempting Submission 4 (Sun Jul 05 02:04:52 GMT 2026) ----- DELVE 1: The Least Massive Galaxy? The Milky Way satellite DELVE 1 may be the faintest, smallest, and least-massive galaxy known. However, the alternative possibility that DELVE 1 is a chemically-peculiar, very metal poor star cluster has thus far proven impossible to rule out through spectroscopy; this is because the system has too few bright stars to enable definitive velocity or metallicity dispersion measurements capable of distinguishing between these scenarios. Classifying the system as a dwarf galaxy or star cluster will therefore require an independent avenue that does not rely on ground-based spectroscopy. Here, we request 12 orbits of HST/ACS F606W+F814W imaging to construct a deep color--magnitude diagram of DELVE 1 and test whether the system displays stellar mass segregation. The presence of mass segregation would imply short dynamical times in DELVE 1 consistent with a star cluster, while its absence would provide strong evidence that DELVE 1 is a dark matter dominated dwarf galaxy. Our proposed observations will either confirm DELVE 1 as the least-massive galaxy yet discovered or would suggest that DELVE 1 is the first known example of a uniformly carbon-enhanced star cluster formed directly out of gas enriched by Population III supernovae. Whatever the outcome, these results will have important implications for the nature of dark matter and the formation of the lowest mass galaxies and most metal-poor star clusters. This 12-orbit, 6-visit program images the DELVE 1 stellar system with ACS in the F606W and F814W filters, with WFC3 UVIS coordinated parallels in the same filters. Three pointings (labeled NORTH, SOUTHEAST, SOUTHWEST) have been placed to cover the system contiguously out to at roughly twice its half-light radius for any possible ORIENT. Each of the three pointings is split into 2 visits at the SAME ORIENT: one visit consisting of two orbits of F606W imaging and the other visit consists of one orbit of F814W imaging and one orbit split between F606W and F814W. In other words, 3 pointings x 2 visits/pointing x 2 orbits/visit = 12 prime orbits. For each filter and pointing, we use a ACS-WFC-DITHER-LINE pattern to cover the chip gap. In practice, we use a 4-point LINE for the F606W exposures and a 3-point LINE for the F814W exposures. The single fifth exposure in F606W from the split F606W+F814W orbit is not part of this pattern and is placed at the center of each of the three main pointings. These LINE patterns favor filling the chip gaps and providing CR removal over perfect subpixel sampling. Lastly, we note that there are two bright (G ~ 12) stars covered by our fields. These two stars will certainly saturate but we expect this will not be a major issue (based on similar ACS observations in the archive). However, "glint" artifacts may also occur; the SAME ORIENT restriction will help ensure that any possible glints are localized to the same region of the detector within a given pointing, allowing them to be masked at the analysis stage. William Cerny is a graduate student at Yale University, experienced in the discovery and characterization of ultra-faint Milky Way satellites with deep imaging and spectroscopy. Ting Li is an assistant professor at the University of Toronto. Li is experienced in the characterizations of ultra-faint dwarf galaxies and star clusters in the Milky Way with both large sky surveys and tailored deep observations. Josh Simon is a staff member at Carnegie Observatories. He has extensive experience with all aspects of observational studies of Milky Way satellites, including star formation histories and initial mass function measurements from HST imaging. Eric Bell is a professor at the University of Michigan, with expertise in the analysis of resolved star datasets with HST, for both the stellar halos of nearby massive galaxies and faint and ultra-faint satellites. Andrew Pace is a postdoctoral research associate at Carnegie Mellon University, with expertise on the spectroscopy, dynamics, proper motions, and orbital properties of Milky Way dwarf galaxies and star clusters. Marla Geha is a professor at Yale University with expertise in analyzing various datasets on dwarf galaxies in the Milky Way and Local Group from a range of instruments from ground to space including HST and JWST. Alexander Ji is an assistant professor at the University of Chicago. He is an expert in nucleosynthesis and chemical abundances of metal-poor stars, especially in dwarf galaxies and stellar streams, and using them to classify objects as dwarf galaxies or star clusters. Burcin Mutlu-Pakdil is an assistant professor at Dartmouth College, and has led (and participated in) several previous HST campaigns related to resolved stellar populations of dwarf galaxies that have led to published results. She is an expert in near-field cosmology and has expertise in HST data reduction for ACS and WFC3 which are the primary and parallel instruments used in this proposal. Holger Baumgardt is an associate professor at the University of Queensland. He has experience in the dynamical evolution and simulation of dense stellar systems like star clusters and galactic nuclei. Stellar Populations and the Interstellar Medium Galaxies 6 false false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER SEPOINTING J2000 STELLAR CLUSTER GLOBULAR CLUSTER NORTHPOINTING J2000 STELLAR CLUSTER GLOBULAR CLUSTER SWPOINTING J2000 ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 3.034 LINE false true 4 85.29 ACS-WFC-DITHER-LINE 1 false POS-TARG DITHER 3.034 LINE false true 3 85.29 ACS-WFC-DITHER-LINE 2 false POS-TARG DITHER 3.034 A 2-point subset of Pattern 2 (line); used for re-take observations where one point in 3-point line is OK LINE false false 2 85.29 ACS-WFC-DITHER-LINE 3 false The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v01.xml 2 orbits of F606W imaging on the northern pointing; 4 point DITHER-LINE Cpar with F606W imaging 1771968497000 278.91 (Tic-Visit :proposal-id 17787 :visit-id "01" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:24.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 5 :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 17787 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2588 :al-overhead-at-end 110 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v02.xml The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. 2 orbits of F606W imaging on the southeastern pointing; 4 point DITHER-LINE Cpar with F606W imaging 1771273985000 278.91 (Tic-Visit :proposal-id 17787 :visit-id "02" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:29.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 5 :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 17787 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2588 :al-overhead-at-end 110 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v52.xml The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. 2 orbits of F606W imaging on the southeastern pointing; 4 point DITHER-LINE Cpar with F606W imaging (Tic-Visit :proposal-id 17787 :visit-id "52" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:33.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "52" :obset-id "52" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "52" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 5 :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 17787 :visit-id "52" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2593 :al-overhead-at-end 105 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v03.xml 2 orbits of F606W imaging on the southwestern pointing; 4 point DITHER-LINE Cpar with F606W imaging 1771454676000 278.91 (Tic-Visit :proposal-id 17787 :visit-id "03" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:34.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 5 :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 17787 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2588 :al-overhead-at-end 110 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v53.xml 2 orbits of F606W imaging on the southwestern pointing; 4 point DITHER-LINE Cpar with F606W imaging (Tic-Visit :proposal-id 17787 :visit-id "53" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:36.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "53" :obset-id "53" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "53" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 5 :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 17787 :visit-id "53" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2593 :al-overhead-at-end 105 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v04.xml Northern pointing; one orbit of entirely F814W imaging with UVIS F814 cpar; one orbit of F606W +F814W with UVIS cpar on both filters 1771708777000 278.91 (Tic-Visit :proposal-id 17787 :visit-id "04" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:37.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 104 :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 17787 :visit-id "04" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 13 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 Single 606 exposure to complement the other four from the first visit The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v05.xml Southeastern pointing; one orbit of entirely F814W imaging with UVIS F814 cpar; one orbit of F606W +F814W with UVIS cpar on both filters 1771624078000 278.91 (Tic-Visit :proposal-id 17787 :visit-id "05" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:40.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "05" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 104 :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 17787 :visit-id "05" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 13 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 Single 606 exposure to complement the other four from the first visit The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v55.xml Southeastern pointing; one orbit of entirely F814W imaging replacing partially-failed visit 5 (Tic-Visit :proposal-id 17787 :visit-id "55" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:41.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1145936 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "55" :obset-id "55" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "55" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 104 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v06.xml Southwestern pointing; one orbit of entirely F814W imaging with UVIS F814 cpar; one orbit of F606W +F814W with UVIS cpar on both filters 1771545023000 278.91 (Tic-Visit :proposal-id 17787 :visit-id "06" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:42.0" :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 2291872 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "06" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 104 :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 17787 :visit-id "06" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 13 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 Single 606 exposure to complement the other four from the first visit The scheduling data was generated by CASM 34.1. CASM was run in Phase II mode. /home/willcerny/Desktop/Misc/HST_PhaseII/OP-cache/FINAL_RETAKE_APT_DELVE1_prechangeEXPtimes-v56.xml Southwestern pointing; Two exposures of F814W, replacing the two middle exposures of visit 6 (Tic-Visit :proposal-id 17787 :visit-id "56" :proposal-version -1 :trans-version "20260319" :date "04-Jul-2026 21:57:43.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1145936 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17787 :visit-id "56" :obset-id "56" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17787 :visit-id "56" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2598 :al-overhead-at-end 104 :internal-dur-at-end 1015 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0