18390( 2) - 19-Aug-2026 14:53:00 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18390 Version: 2 Check-in Time: 19-Aug-2026 14:53:00 STScI Edit Number: 0 Title The First Deep, High-Resolution UV Systematic Search for the Dwarf Host Galaxies of Nearby Hostless Core-Collapse Supernovae ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Daichi Hiramatsu University of Florida Y CoI: Dr. Sebastian Gomez University of Texas at Austin N CoI: Prof. Edo Berger Harvard University N CoI: Dr. Peter Blanchard Harvard University N CoI: Daichi Tsuna Harvard University N ------------------------------------------------------------------------------------ Abstract Massive stars (above 8 solar masses) end their lives in core-collapse supernovae (CCSNe), but key aspects of this process remain poorly understood with the observed spectral and light-curve diversities, reflecting a range of progenitor properties. Type II SNe progenitors - red supergiants - have been directly observed, and the progenitors of stripped-envelope SNe (Types Ib/c) are thought to be even more massive stars (e.g., Wolf-Rayet stars) and/or equally massive stars in a binary system with sufficient mass loss. Because massive stars have short lifetimes, they are expected to explode near their star-forming birthplaces. However, some CCSNe appear "hostless" with no obvious host galaxy detected, offering unique insight into progenitor environments. Possible explanations include undetected faint dwarf hosts or runaway massive stars ejected from their galaxies. We propose deep, high-resolution UV (F300X) imaging with WFC3/UVIS to search for faint dwarf galaxies with recent star formation at the explosion sites of nine nearby (z < 0.05), spectroscopically confirmed hostless CCSNe (6 Type II, 3 Type Ic-BL). These targets lack clear host associations with high probabilities of chance alignment with unrelated field galaxies. Only HST provides the required UV sensitivity and spatial resolution. We request 2 orbits per target to reach 26.1 AB mag with sufficient signal-to-noise for robust host detection or stringent non-detection limits. These observations will probe the faint end of the galaxy UV luminosity function and inform models of massive star evolution, galaxy formation, and stellar kinematics. ------------------------------------------------------------------------------------ Observations Description ------------------------ To maximize the signal while focusing on UV light to track recent star formation, we place the supernova location at the center of UVIS chip 2 and choose the most transmissive filter whose red edge is less than 300nm - F300X. We calculate the on-source exposure times required by WFC3/UVIS F300X to obtain signal-to-noise ratios greater than 10. With four dithers (for optimal cosmic-ray rejection and point-spread function sampling) for a 2'' source with a 1'' extraction radius, we find 80 minutes of on-source time (or 2 orbits including the overhead) to reach m_F300X = 26.1 (AB) for all the hostless core-collapse supernova sample. ------------------------------------------------------------------------------------ 18390( 2) - 19-Aug-2026 14:53:00 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 SN2022AEY GALAXY, UNDESIGNATED RA=16H04M8.1900S, J2000 Z = g=24.7 DEC=+29D27'43.78" 0.015 Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 2 SN2023IWY GALAXY, UNDESIGNATED RA=18H00M18.6218S, J2000 Z = 0.03 g=24.7 DEC=+26D24'32.04" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 3 SN2020DGD GALAXY, UNDESIGNATED RA=15H45M35.5328S, J2000 Z = 0.03 g=24.7 DEC=+29D18'38.51" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 4 SN2020SJE GALAXY, UNDESIGNATED RA=01H17M21.9600S, J2000 Z = 0.03 g=23.3 DEC=+44D11'41.53" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 5 SN2021STU GALAXY, UNDESIGNATED RA=00H08M58.2189S, J2000 Z = g=24.7 DEC=-14D32'55.14" 0.034 Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 6 SN2023VKZ GALAXY, UNDESIGNATED RA=08H28M36.1736S, J2000 Z = g=24.7 DEC=+57D12'31.86" 0.035 Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 7 SN2021KKX GALAXY, UNDESIGNATED RA=15H48M3.8132S, J2000 Z = g=24.7 DEC=+82D34'34.02" 0.0383 Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 18390( 2) - 19-Aug-2026 14:53:00 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 8 SN2018IDH GALAXY, UNDESIGNATED RA=07H07M48.3110S, J2000 Z = 0.04 g=24.7 DEC=+36D28'25.95" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 9 SN2022WLM GALAXY, UNDESIGNATED RA=05H56M46.6335S, J2000 Z = 0.05 g=23.3 DEC=+48D06'20.84" Reference Frame: ICRS 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 ------------------------------------------------------------------------------------------------------------------------------------ 18390( 2) - 19-Aug-2026 14:53:00 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18390( 2) - 19-Aug-2026 14:53:00 - [ 5] Visit: 01 (SN2022AEY) 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 SN2022AEY WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2022AEY 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2022AEY 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2022AEY 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2022AEY 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 6] Visit: 02 (SN2023iwy) 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 SN2023IWY WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2023IWY 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2023IWY 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2023IWY 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2023IWY 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 7] Visit: 03 (SN2020dgd) 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 SN2020DGD WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2020DGD 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2020DGD 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2020DGD 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2020DGD 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 8] Visit: 04 (SN2020sje) 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 SN2020SJE WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2020SJE 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2020SJE 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2020SJE 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2020SJE 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 9] Visit: 05 (SN2021stu) 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 SN2021STU WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2021STU 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2021STU 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2021STU 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2021STU 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 10] Visit: 06 (SN2023vkz) 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 SN2023VKZ WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2023VKZ 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2023VKZ 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2023VKZ 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2023VKZ 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 11] Visit: 07 (SN2021kkx) 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 SN2021KKX WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2021KKX 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2021KKX 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2021KKX 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2021KKX 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 12] Visit: 08 (SN2018ldh) 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 SN2018IDH WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2018IDH 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2018IDH 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2018IDH 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2018IDH 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 13] Visit: 09 (SN2022wlm) 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 SN2022WLM WFC3/UVI ACCUM UVIS2 F300X FLASH=15 1 1200 S PATTERN 1 1 S (4800 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 | ------------------------------------------------------------------------------------------------------------------------------------ SN2022WLM 1 WFC3/UVI ACCUM UVIS2 F300X 1 none none none 1 N/A S SN2022WLM 1 WFC3/UVI ACCUM UVIS2 F300X 2 none none none 1 N/A S SN2022WLM 1 WFC3/UVI ACCUM UVIS2 F300X 3 none none none 2 N/A S SN2022WLM 1 WFC3/UVI ACCUM UVIS2 F300X 4 none none none 2 N/A S 18390( 2) - 19-Aug-2026 14:53:00 - [ 14] Summary Form for Proposal 18390 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=15 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PATTERN 1 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F300X ------------------------------------------------------------------------------------------------------------------------------------ Target Names SN2018IDH, SN2020DGD, SN2020SJE, SN2021KKX, SN2021STU, SN2022AEY, SN2022WLM, SN2023IWY, SN2023VKZ ------------------------------------------------------------------------------------------------------------------------------------