HI emission and absorption-line studies of nearby, gas-rich galaxies

Abstract

HI absorption-line studies provide a unique insight into the growth and evolution of galaxies. Next-generation radio telescopes will allow us to conduct the first large, blind absorption-line surveys, studying the evolution of neutral gas at redshifts that are currently largely unexplored. In this thesis, we have conducted a detailed study of HI emission and absorption in nearby galaxies to investigate the expected detection rate of intervening absorption in future surveys and how various factors influence this detection rate. We conducted a survey for intervening absorption using new ATCA observations. 23 sightlines, with impact parameters of 6-46kpc were searched for absorption, resulting in one new detection (a detection rate of 4.3%). Our detection rate is significantly lower than in previous studies, but as our selection criteria much more closely match that of future blind surveys, this suggests our results are far more representative of the true detection rate. We also find that the structure of the background radio sources significantly affects the detection rate, with many sources having complex structures which dramatically reduces the absorption-line sensitivity. In a separate study, we applied the technique of HI spectral stacking to search, for the first time, for a signal of intervening absorption in HIPASS. In particular, we were interested in searching for absorption at impact parameters to large (<100-200kpc) to detect in individual observations. While the presence of HI emission limits the interpretation of our results, we do see possible evidence of absorption at impact parameters <60kpc, and our analysis clearly demonstrates the potential of this new technique to search for weak absorption-line signals in the outskirts of galaxies. Combined, the results in this thesis advance our understanding of the conditions in which intervening absorption arises, and provide important information in the preparation for future large absorption-line surveys.