18416( 2) - 10-Aug-2026 20:26:31 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18416 Version: 2 Check-in Time: 10-Aug-2026 20:26:31 STScI Edit Number: 0 Title HST Imaging of A Nearby Dwarf Galaxy Surrounded by an Ionized Ring of Pristine Circumgalactic Gas ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Imad Pasha CIERA, Northwestern University Y CoI: Prof. Pieter van Dokkum Yale University N CoI: Chloe Neufeld Yale University N ------------------------------------------------------------------------------------ Abstract It was recently discovered that the nearby dwarf galaxy LEDA 135162 hosts a large, low surface brightness ring surrounding the galaxy, well beyond its optical extent. Followup spectroscopy has revealed that the gas, clumps of which have been ionized by triggered star formation in the ring, is remarkably lacking in almost any [NII] emission, despite being bright in [OIII] and H-alpha, suggesting this ring is composed of low metallicity, diffuse gas from the circumgalactic medium of the system. Unexpectedly, the [OIII]/H-beta line ratios vary considerably from clump to clump in the ring (and even from spaxel to spaxel) as measured from KCWI IFU spectroscopy, suggesting sub-seeing variation in metallicity or ionization conditions. We propose 12 orbits of HST narrowband and broadband imaging to characterize the properties of this unique ring, which provides a rare opportunity to study the spatially resolved circumgalactic medium of a nearby dwarf galaxy in situ, as well as to study the nature of star formation in the low-density, low metallicity regime. ------------------------------------------------------------------------------------ Observations Description ------------------------ We aim to carry out a 12 orbit ACS/WFC program, split between 8 orbits of narrowband imaging (4 orbits in F502N, 4 orbits in F660N), with accompanying 4 orbits of broad band coverage (2 orbits in F475W, 2 orbits in F625W). To maximize signal accumulation per subexposure, particularly in the narrowband filters, we have scheduled 2 images per orbit, maximizing availability at around 1000-1200 seconds per exposure. We complete 1 full 4 point dither pattern (large, at 9.5" scale --- we are aiming to ensure uniform filling of the field, rather than high subpixel aligned sampling) per visit/orient, with each visit comprising 2 orbits. There are no roll angle constraints for any individual visit. Per ETC recommendations, we have input 10e- of post flash per NB exposure, though we seek guidance on this front. ETC: ACS.im.2363734 (see submission comments). Updating from the proposal, we reran the ETC using the same clump input spectrum (0.4" source with a 0.4" extraction aperture) but with an updated exposure time of 1099 seconds (based on the 2 exposure per orbit packing), and 18416( 2) - 10-Aug-2026 20:26:31 - [ 2] determined 10e- of post flash would be sufficient compared to the 27e- for the initial proposal. ------------------------------------------------------------------------------------ 18416( 2) - 10-Aug-2026 20:26:31 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 LEDA-135162 GALAXY, DWARF COMPACT, RA=04H30M46.5857S, J2000 V = 16.0160 INTERACTING GALAXY, DEC=-20D36'25.61" KNOT, RING, STAR FORMING REGION Reference Frame: ICRS Extended: YES 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) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18416( 2) - 10-Aug-2026 20:26:31 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 9.5 Line_Spacing 9.5 Coordinate_Frame POS-TARG Pattern_Orient 20.7 Angle_Between_Sides 69.02 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18416( 2) - 10-Aug-2026 20:26:31 - [ 5] Visit: 01 (F660N Orbits 1-2) 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 LEDA-135162 ACS/WFC ACCUM WFC F660N FLASH=10 1 1099 S POS TARG (4448 S) -2.62089904834826, -23.872154086798375; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEDA-135162 1 ACS/WFC ACCUM WFC F660N 1 none none none 1 N/A LEDA-135162 1 ACS/WFC ACCUM WFC F660N 2 none none none 1 N/A LEDA-135162 1 ACS/WFC ACCUM WFC F660N 3 none none none 2 1125.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F660N 4 none none none 2 1125.0 S 18416( 2) - 10-Aug-2026 20:26:31 - [ 6] Visit: 02 (F660N Orbits 3-4) 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 LEDA-135162 ACS/WFC ACCUM WFC F660N FLASH=10 1 1099 S POS TARG (4448 S) -2.62089904834826, -23.872154086798375; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEDA-135162 1 ACS/WFC ACCUM WFC F660N 1 none none none 1 N/A LEDA-135162 1 ACS/WFC ACCUM WFC F660N 2 none none none 1 N/A LEDA-135162 1 ACS/WFC ACCUM WFC F660N 3 none none none 2 1125.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F660N 4 none none none 2 1125.0 S 18416( 2) - 10-Aug-2026 20:26:31 - [ 7] Visit: 03 (F502N Orbits 1-2) 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 LEDA-135162 ACS/WFC ACCUM WFC F502N FLASH=10 1 1200 S POS TARG see below -2.62089904834826, (4448 S) -23.872154086798375; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEDA-135162 1 ACS/WFC ACCUM WFC F502N 1 none none none 1 1099.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F502N 2 none none none 1 1099.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F502N 3 none none none 2 1125.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F502N 4 none none none 2 1125.0 S 18416( 2) - 10-Aug-2026 20:26:31 - [ 8] Visit: 04 (F502N Orbits 3-4) 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 LEDA-135162 ACS/WFC ACCUM WFC F502N FLASH=10 1 1200 S POS TARG see below -2.62089904834826, (4448 S) -23.872154086798375; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEDA-135162 1 ACS/WFC ACCUM WFC F502N 1 none none none 1 1099.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F502N 2 none none none 1 1099.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F502N 3 none none none 2 1125.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F502N 4 none none none 2 1125.0 S 18416( 2) - 10-Aug-2026 20:26:31 - [ 9] Visit: 05 (F475W Orbits 1-2) 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 LEDA-135162 ACS/WFC ACCUM WFC F475W 1 1200 S POS TARG see below -2.62089904834826, (4472 S) -23.872154086798375; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEDA-135162 1 ACS/WFC ACCUM WFC F475W 1 none none none 1 1105.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F475W 2 none none none 1 1105.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F475W 3 none none none 2 1131.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F475W 4 none none none 2 1131.0 S 18416( 2) - 10-Aug-2026 20:26:31 - [ 10] Visit: 06 (F625W Orbits 1-2) 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 LEDA-135162 ACS/WFC ACCUM WFC F625W 1 1200 S POS TARG see below -2.62089904834826, (4472 S) -23.872154086798375; PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ LEDA-135162 1 ACS/WFC ACCUM WFC F625W 1 none none none 1 1105.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F625W 2 none none none 1 1105.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F625W 3 none none none 2 1131.0 S LEDA-135162 1 ACS/WFC ACCUM WFC F625W 4 none none none 2 1131.0 S 18416( 2) - 10-Aug-2026 20:26:31 - [ 11] Summary Form for Proposal 18416 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1, POS TARG -2.62089904834826, -23.872154086798375 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F475W, F502N, F625W, F660N ------------------------------------------------------------------------------------------------------------------------------------ Target Names LEDA-135162 ------------------------------------------------------------------------------------------------------------------------------------