0 Assigned ID: 43 ----- Attempting Submission 1 (Fri Apr 03 15:49:40 GMT 2015) ----- HST Phase I Proposal 43 (ffprop23.apt) successfully submitted. Receipt: # 43-1 ----- Attempting Submission 2 (Fri Apr 10 20:26:03 GMT 2015) ----- HST Phase I Proposal 43 (ffprop23.apt) successfully submitted. Receipt: # 43-2 ----- Attempting Submission 3 (Fri Apr 10 22:32:03 GMT 2015) ----- The formation and abundance of the lowest luminosity galaxies represents one of the most significant issues facing galaxy formation today. The faintest galaxies probe feedback at its most effective and may offer clues to the particle nature of dark matter itself. Currently, we are secure in our accounting of the faint ~10^6.5 Msun dwarfs only in the Local Group -- a regime that has gotten much attention via its association with the "Missing Satellites" and "Too Big to Fail" problems. However, the Local Group is just one system with limited statistical power -- it may not be typical of the Universe at large. We propose to mine the deepest HST pointings, including the new Frontier Fields, so as to measure the stellar mass function to unparalleled depths in three distinct environments: cluster cores, cluster outskirts, and the field. Through comparison to corresponding high-resolution cosmological simulations, we will test LCDM-based galaxy formation at unprecedented scales and over the widest possible environmental range. Only HST provides the photometric depth and precision needed to study galaxies at this critical low-mass regime (<10^8 Msun). Yet, while extremely deep HST imaging has existed in the field, the Frontier Fields supply the missing ingredient, by allowing us to probe the high-density regime in comparable detail. In addition to addressing the failures of galaxy formation models, our proposed program will produce a variety of observational data products (e.g. stellar mass, photo-z, and environment catalogs) as well as high-resolution simulations that will be of significant value to the community in conducting complementary analyses in the HST Deep Fields. COSMOLOGY 0 Regular false false false false false SUPPORTED