18066( 2) - 11-Aug-2025 17:14:11 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18066 Version: 2 Check-in Time: 11-Aug-2025 17:14:11 STScI Edit Number: 0 Title Discovery of Very Low-Mass Galaxy Candidates within the Local Group ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Kristen B W McQuinn Space Telescope Science Institute Y CoI: Dr. Yao-Yuan Mao University of Utah N CoI: Prof. Matthew Buckley Rutgers the State University of New Jersey N CoI: Prof. David Shih Rutgers the State University of New Jersey N CoI: Dr. Andrew Eugene Dolphin Raytheon Company N CoI: Dr. Roger Cohen Eureka Scientific Inc. N CoI: Dr. Erik Tollerud Space Telescope Science Institute N CoI: Tehreem Hai Rutgers the State University of New Jersey N CoI: Cody Leishman University of Utah N CoI: Alexis Brown University of Utah N ------------------------------------------------------------------------------------ Abstract The discoveries of ultra-faint dwarfs satellites of the Milky Way and M31 were a paradigm shift for our understanding of heirarchical structure formation and helped frame our thinking of how reionization impacts the growth of low-mass structures. That paradigm is now changing again as more ultra-faint dwarfs are detected at farther distances and in different environments. For example, we now have evidence that *isolated* ultra-faint dwarfs are not necessarily quenched by reionization and there are tentative detections of ultra-faint dwarfs that are gas-rich. Despite these advances, interpretations are limited as these types of systems are very faint and hard to find, and therefore sample sizes are still quite small. Here, we report the discovery of 4 ultra-faint dwarf galaxy candidates found via ground-based optical surveys. Each system is visible by-eye in the optical images and the color-magntiude diagrams of the resolved stars suggest they are very low-mass galaxies within the Local Group. These intriguing systems appear to be outside the virial radius of the Milky Way and M31 (i.e., are likely isolated) and are an opportuity to futher probe the regime of ultra-faint dwarfs as a function of different environments. We are requesting 1-orbit of HST WFC3 imaging for each candidate to confirm and characterize these systems. The data will enable robust measurements of distances, structural parameters, integrated magnitudes, and star formation histories. ------------------------------------------------------------------------------------ Observations Description ------------------------ The observations include WFC3/UVIS imaging in the F606W and F814W filters. The data will be used to confirm that the targets are galaxies, measure the distances to the systems from resolved stars (TRGB and/or Horizontal Branch methods), characterize the stellar properites (star formation histories and metallicities), and measure the structural parameters of the galaxies. 18066( 2) - 11-Aug-2025 17:14:11 - [ 2] Each of the 4 targets will be observed for 1 orbit. We use a 2.5x2.5 pixel dither implemented using pos-targ (equivalent to the WFC3 UVIS DITHER LINE pattern) to improve PSF sampling and handle hot pixels. The first pointing is executed with both filters in WFC3. After the dither, the observations are then repeated. The exposure time per orbit is split roughly equally between the F606W and F814W filters. Each visit is 1-orbit. We have place ORIENT constraints to ensure we maximum coverage of the stellar disk and prevent the galaxy centers from falling in the WFC3 chip gap, while avoiding nearby bright stars. ------------------------------------------------------------------------------------ 18066( 2) - 11-Aug-2025 17:14:11 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 PHOENIX-K GALAXY, DWARF RA=01H46M55.8279S, J2000 V = 17 SPHEROIDAL DEC=-41D26'7.01" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 2 GEMINI-M GALAXY, DWARF RA=07H53M30.8858S, J2000 V = 17 SPHEROIDAL DEC=+15D59'41.86" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 3 CETUS-L GALAXY, DWARF RA=00H13M40.0711S, J2000 V = 17 SPHEROIDAL DEC=-14D34'26.22" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 4 VIRGO-B GALAXY, DWARF RA=11H58M16.1502S, J2000 V = 17 SPHEROIDAL DEC=+06D55'59.14" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18066( 2) - 11-Aug-2025 17:14:11 - [ 4] Visit: 01 (PHOENIX-K) Visit Requirements: ORIENT 240D TO 278 D 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 PHOENIX-K WFC3/UVI ACCUM UVIS F606W 1 538 S (538 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 PHOENIX-K WFC3/UVI ACCUM UVIS F814W FLASH=8 1 538 S (538 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 PHOENIX-K WFC3/UVI ACCUM UVIS F606W 1 539 S (539 POS TARG 0.099, 0.106 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 PHOENIX-K WFC3/UVI ACCUM UVIS F814W FLASH=8 1 539 S (539 SAME POS AS 3 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ PHOENIX-K 1 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S PHOENIX-K 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S PHOENIX-K 3 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S PHOENIX-K 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S 18066( 2) - 11-Aug-2025 17:14:11 - [ 5] Visit: 02 (GEMINI-M) Visit Requirements: ORIENT 250D TO 289 D 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 GEMINI-M WFC3/UVI ACCUM UVIS F606W 1 532 S (532 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 GEMINI-M WFC3/UVI ACCUM UVIS F814W FLASH=8 1 532 S (532 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 GEMINI-M WFC3/UVI ACCUM UVIS F606W 1 532 S (532 POS TARG 0.099, 0.106 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 GEMINI-M WFC3/UVI ACCUM UVIS F814W FLASH=8 1 533 S (533 SAME POS AS 3 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ GEMINI-M 1 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S GEMINI-M 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S GEMINI-M 3 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S GEMINI-M 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S 18066( 2) - 11-Aug-2025 17:14:11 - [ 6] Visit: 03 (CETUS-L) Visit Requirements: ORIENT 235D TO 257 D 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 CETUS-L WFC3/UVI ACCUM UVIS F606W 1 531 S (531 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 CETUS-L WFC3/UVI ACCUM UVIS F814W FLASH=8 1 532 S (532 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 CETUS-L WFC3/UVI ACCUM UVIS F606W 1 532 S (532 POS TARG 0.099, 0.106 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 CETUS-L WFC3/UVI ACCUM UVIS F814W FLASH=8 1 532 S (532 SAME POS AS 3 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ CETUS-L 1 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S CETUS-L 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S CETUS-L 3 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S CETUS-L 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S 18066( 2) - 11-Aug-2025 17:14:11 - [ 7] Visit: 04 (VIRGO-B) Visit Requirements: ORIENT 272D TO 300 D 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 VIRGO-B WFC3/UVI ACCUM UVIS F606W 1 532 S (532 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 VIRGO-B WFC3/UVI ACCUM UVIS F814W FLASH=8 1 532 S (532 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 VIRGO-B WFC3/UVI ACCUM UVIS F606W 1 533 S (533 POS TARG 0.099, 0.106 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 VIRGO-B WFC3/UVI ACCUM UVIS F814W FLASH=8 1 533 S (533 SAME POS AS 3 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ VIRGO-B 1 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S VIRGO-B 2 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S VIRGO-B 3 WFC3/UVI ACCUM UVIS F606W none none none none 1 N/A S VIRGO-B 4 WFC3/UVI ACCUM UVIS F814W none none none none 1 N/A S 18066( 2) - 11-Aug-2025 17:14:11 - [ 8] Summary Form for Proposal 18066 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=8 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 235D TO 257 D, ORIENT 240D TO 278 D, ORIENT 250D TO 289 D, ORIENT 272D TO 300 D, POS TARG 0.099, 0.106, SAME POS AS 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names CETUS-L, GEMINI-M, PHOENIX-K, VIRGO-B ------------------------------------------------------------------------------------------------------------------------------------