0 Assigned ID: 6125 ----- Attempting Submission 1 (Mon Apr 07 12:33:01 GMT 2025) ----- HST Phase I Proposal 6125 successfully submitted. Receipt: # 6125-1 ----- Attempting Submission 2 (Tue Apr 08 12:06:53 GMT 2025) ----- HST Phase I Proposal 6125 successfully submitted. Receipt: # 6125-2 ----- Attempting Submission 3 (Tue Apr 08 19:53:34 GMT 2025) ----- HST Phase I Proposal 6125 successfully submitted. Receipt: # 6125-3 ----- Attempting Submission 4 (Wed Apr 09 16:54:38 GMT 2025) ----- 0 ----- Attempting Submission 1 (Thu Aug 07 13:12:25 GMT 2025) ----- Discovery of Very Low-Mass Galaxy Candidates within the Local Group 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. 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. 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. Our team is comprised of scientists responsible for the discovery and characterization of the ultra-faint dwarf galaxy Pegasus W. PI McQuinn and Co-I Cohen are experts on resolved stellar populations, measuring distances to galaxies, reconstructing star formation histories, and measuring the structural parameters of galaxies. Co-I Hai is a graduate student working with PI Dr. McQuinn, with experience working with HST imaging of low-mass galaxies and resolved stellar populations. They all have significant expertise with HST images and will be primarily responsible for the data handling. Co-I Tollerud is a SAGA team member and an expert in HST data analysis and will aid in measuring the structural parameters of the galaxies. Dr. Mao brings expertise in the formation of low-mass galaxies in the context of CDM and hierarchical structure formation and has experience studying low-mass satellites around Milky Way analog galaxies in the SAGA survey. Drs. Mao, Shih, and Buckley are all experts on dark matter and built the basis of the search algorithm used to find the galaxy candidates. Dr. Mao's students, Co-Is Brown and Leishman, aided in the development of the algorithm and the discovery of the targets. Dr. Dolphin, author of the photometry software DOLPHOT and the CMD-fitting code MATCH that we will use on the data, brings significant technical expertise and knowledge of stellar populations. Stellar Populations and the Interstellar Medium 6 false false false false false false SUPPORTED GALAXY DWARF SPHEROIDAL J2000 J2000 GALAXY DWARF SPHEROIDAL J2000 J2000 GALAXY DWARF SPHEROIDAL J2000 J2000 GALAXY DWARF SPHEROIDAL J2000 J2000 WFC3/UVIS Imaging No No F606W WFC3/UVIS 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 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W The scheduling data was generated by CASM 33.1. 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