"Hot Gaseous Halos Around Galaxies"
The modern theory of galaxy formation makes a generic prediction that all massive galaxies, elliptical or spiral, ought to be surrounded by hot gaseous halos. Elliptical galaxies are indeed generally surrounded by hot gas, but for many reasons spiral galaxies offer a better laboratory for testing the theory and its implications for missing baryons. Unfortunately, hot halos around spiral galaxies have eluded detection for decades. In this thesis, we present the first two detections of hot halos around spiral galaxies (NGC1961 and UGC12591), as well as a number of results characterizing the hot gas and its implications. Both the hot halos we detect have similar properties, and it is likely that neither contains more than a third of the missing baryons from either galaxy. We also compile constraints on the mass of hot halos around the Milky Way and other massive spirals, based on upper limits in emission and absorption. For the Galaxy the most robust constraint is based on the dispersion measure of pulsars in the Magellanic Clouds, which argues against a Galactic hot halo containing more than a quarter of the missing baryons from the Galaxy. In addition, we perform a stacking analysis of ROSAT All-Sky Survey images of isolated galaxies, in which we detect X-ray emission from L* spirals and ellipticals at high confidence, and find moderate evidence that the emission is extended on scales of tens of kpc. We infer hot halo masses consistent with our other studies, and are able to strongly exclude models with more massive hot halos that would have contained the missing baryons from these galaxies. Finally, we introduce significant improvements for the spatial analysis of extended X-ray emission, which we use to trace the emission around the isolated elliptical NGC 720 to below 1/10 of the background. We are able to settle a discrepancy between two different studies of the hot halo around this galaxy, suggesting this galaxy does not have most of its missing baryons in its hot halo. We conclude that the missing baryons must either be in a cooler phase, or outside the virial radius.