Maria Patterson

Properties of Star Formation and the Interstellar Medium in Galaxy Outskirts

The ability of spiral galaxies to sustain current star formation is not well-understood, since galaxies should have already used up all available gas. However, recent observations find that spiral galaxies frequently have extended halos of extraplanar gas that may help resolve part of this problem. The origin of this gas, however, is unknown. It could be ejected by star formation activity in the disk or be accreted externally. This thesis aims to investigate the origins of extraplanar gas by studying the role and prevalence of accreting gas in spiral galaxies with three complementary projects. We study the metallicities of star forming regions in the outer disc and halo of the nearby galaxies M81 and M31 to help determine how processed or pristine the gas in the outskirts is. We also model deep neutral hydrogen observations of NGC 5055 from the Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey to look for evidence of current gas accretion and identify its source. In a third project, we use deep Hα and UV imaging to compare the presence of extraplanar gas with star formation and search for current star formation in the low density gas outskirts of a subset of the galaxies in the HALOGAS sample. This research helps to further our understanding of the origin of gaseous halos in spiral galaxies and of how star formation is fueled.