Neutral Hydrogen in Local Group Dwarf Galaxies

The gas content of the faintest and lowest mass dwarf galaxies provide means to study the evolution of these unique objects. The evolutionary histories of low mass dwarf galaxies are interesting in their own right, but may also provide insight into fundamental cosmological problems. These include the nature of dark matter, the disagreement between the number of observed Local Group dwarf galaxies and that predicted by lambda cold dark matter models, and the discrepancy between the observed census of baryonic matter in the Milky Way's environment and theoretical predictions. This thesis explores these questions by studying the neutral hydrogen (HI) component of dwarf galaxies. First, limits on the HI mass of the ultra-faint dwarfs are presented, and the HI content of all Local Group dwarf galaxies is examined from an environmental standpoint. We find that those Local Group dwarfs within 270 kpc of a massive host galaxy are deficient in HI as compared to those at larger galactocentric distances. Ram-pressure arguments are invoked, which suggest halo densities greater than 0.0002-0.0003 per cubic centimeter out to distances of at least 70 kpc, values which are consistent with theoretical models and suggest the halo may harbor a large fraction of the host galaxy's baryons. We also find that accounting for the incompleteness of the dwarf galaxy count, known dwarf galaxies whose gas has been removed could have provided at most 210 million Solar Masses of HI gas to the Milky Way. Second, we examine the possibility of discovering unknown gas-rich ultra-faint galaxies in the Local Group using HI. The GALFA-HI Survey catalog is searched for compact, isolated HI clouds which are most similar to the expected HI characteristics of low mass dwarf galaxies. Fifty-one Local Group dwarf galaxy candidates are identified through column density, brightness temperature, and kinematic selection criteria, and their properties are explored. Third, we present hydrodynamic simulations of dwarf galaxies experiencing a constant velocity and density wind which emulates relative motion of the dwarf and the host's hot halo. These simulations resolve instabilities which can contribute to gas loss, such as the effects of the Kelvin Helmholtz instability. The results of these simulations support the hypothesis that rapid gas loss occurs when the ram pressure stripping criterion is met, with complete stripping occurs with a timescale of about half a Gyr. This stripping would occur in less than an orbital period for ultra-faint like dwarfs. Models which do not meet the ram-pressure stripping criterion show slower but constant mass loss which does not depend on the residual dwarf's gas mass. Extrapolating the stripping timescales, we show low-mass dwarf galaxies can be stripped on timescales between 1.1 and 3.3 Gyrs. These simulations are a first step towards accurate ram-pressure and dynamical mass loss rates for low mass Local Group galaxies orbiting within a hot halo. They suggest that the lack of low mass galaxies within 250 kpc of the Milky Way can be explained via ram-pressure and dynamical interactions between the satellite galaxy's gas and the hot halo.