0 Assigned ID: 4540 ----- Attempting Submission 1 (Wed May 24 19:47:17 GMT 2023) ----- HST has been the premier observatory for deriving star formation histories (SFHs) based on resolved stars for the last two decades due to its unique combination of spatial resolution, photometric depth, and wavelength coverage. The resolved color-magnitude diagrams obtained by HST for the ultra-faint dwarf galaxies (M_V > -7; UFDs) of the Milky Way (MW) have indicated these galaxies were quenched around the epoch of reionization, making them fossil remnants of this important cosmological era. However, new SFH measurements of UFDs thought to have fallen into the MW's halo along with the Magellanic Clouds indicate that these UFDs may have been able to form stars for longer than those that were directly accreted onto the MW. This type of variation in SFH could be indicative of local differences in the ultraviolet radiation field during the epoch of reionization and therefore may provide an avenue for constraining models of inhomogeneous reionization. To enable these analyses, we propose to measure the SFHs of the 32 objects observed as part of HST Treasury Proposal 14734. These objects are primarily recently-discovered UFDs with no existing SFH measurements. We will perform a uniform analysis, enabling robust comparisons between SFHs within the sample. We will additionally use the few bright dwarfs in the sample with existing SFH measurements to constrain systematics (i.e., stellar model choice) and enable comparisons to other datasets. The Treasury data reach the oldest main sequence turnoff and were taken in the F606W and F814W filters, known as workhorses for resolved stellar populations -- the data are therefore supremely well-suited to our proposed goal. Dr. Garling is an expert in Bayesian statistics and resolved stellar populations and is leading the development of the computational methods for color-magnitude diagram (CMD) modelling. He also has theoretical experience working with cosmological simulations and semi-analytic models of galaxy evolution and will provide interpretation of measured star formation histories. Dr. McQuinn will be a primary point of contact for these development efforts as she has considerable experience modelling resolved stellar populations observed with HST using the MATCH program, allowing her to advise on technical aspects of the analysis. Dr. Gennaro is an expert on resolved stellar populations whose knowledge of topics like the stellar initial mass and multiplicity functions will aid in the development of methods to include these effects in our CMD models. Dr. Girardi is an expert on stellar models and will provide guidance on the varieties of models available in the literature and their differences. Ms. Richstein has experience performing photometry on HST/ACS data and is leading the data processing for the HST Treasury Program 14734 we are proposing to analyze. She will provide photometric catalogs for our analysis. Ms. Richstein is being advised by Dr. Kallivayalil, who is an expert on the dynamics of Local Group galaxies and will advise on environmental implications of our results. Stellar Populations and the Interstellar Medium Galaxies 0 Regular false false false false false false SUPPORTED