0 Assigned ID: 710 ----- Attempting Submission 1 (Fri Apr 10 19:34:21 GMT 2015) ----- HST Phase I Proposal 710 (UDFs.apt) successfully submitted. Receipt: # 710-1 ----- Attempting Submission 2 (Fri Apr 10 19:41:56 GMT 2015) ----- HST Phase I Proposal 710 (UDFs.apt) successfully submitted. Receipt: # 710-2 ----- Attempting Submission 3 (Fri Apr 10 19:43:54 GMT 2015) ----- Ultra-faint dwarf galaxies (UFDs) include the faintest, lowest-metallicity, and most dark-matter dominated galaxies known, providing unique and particularly powerful laboratories for our understanding of galaxy formation at the earliest times and in the lowest mass dark matter halos that host long-lived stars. Among the most striking discoveries about UFDs is that they appear to contain exclusively ancient stars (> 10 Gyr), an insight made possible by extremely deep HST imaging. Remarkably, this is exactly the age distribution expected in a scenario where the Missing Satellites Problem is primarily solved via early reionization-related feedback. Motivated by these exciting results from HST, this proposal is aimed at understanding the cosmological formation of UFDs using ultra-high resolution cosmological simulations. We aim to determine the relationship between the global reionization history and the observed star formation histories of low-mass dwarfs. We will also determine the relationship betwen halo mass and stellar mass at the dwarf regime, and use these results to inform our interpretation of dwarf galaxy counts in the Local Group in the context of LCDM cosmology. Our simulations, based on the FIRE star formation scheme, will carefully track the impact of local stellar feedback and global reionization feedback on the star formation histories of dwarf galaxies. Using zoom-in simulations over a range of dwarf-sized halos, we will explore the physical origin of the transition between uniformly (and anciently) quenched UFDs to the slightly larger "classical" dwarfs that appear to be star-forming at z=0 in the absence of environmental quenching. UNRESOLVED STAR FORMATION 0 Regular false false false false true SUPPORTED