Determining the Origins and Impact of Hot Gas in the Milky Way.

Abstract

The Milky Way's circumgalactic medium (CGM) contains million degree gas that is volume-filling on >10 kpc scales based on X-ray emission from the ROSAT All-Sky Survey, detections of OVII absorption lines in AGN spectra, ubiquitous detections of OVII-OVIII emission lines in ~1000 blank-sky spectra, and the discovery of the ~10 kpc outflow from the Galactic center known as the Fermi bubbles. Analyses on the line strengths in small samples (<20) constrain the characteristic plasma density, but the dominant hot gas structure is debated in the literature. This is a crucial uncertainty since different galaxy formation mechanisms predict a wide range of hot gas morphologies and masses, and the Fermi bubbles are recently discovered objects that are interacting with the ambient CGM. In this dissertation, I constrain the global hot gas density structure by comparing predictions from parametric density models with all-sky samples of OVII and OVIII line strength measurements. I find that a spherical power law density profile extending to the Milky Way's virial radius with a slope of -3/2 reproduces how the absorption and emission line strengths vary across the sky. These results imply the hot gas accounts for less than 50% of the Galactic missing baryons, a hot gas metallicity of greater than one third solar, and that most of the hot gas formed from an accretion shock during the Milky Way's formation. I also compare the absorption line shapes and centroids to kinematic models to show that the hot gas rotates with a flat rotation curve of 183 +/- 41 km/s. For the Fermi bubbles, a volume-filled bubble and shell model that is hotter than the surrounding medium and over-pressurized with log(T) = 6.60-6.70 is consistent with the observed line intensities and ratios. I infer a bubble expansion rate, age, and energy injection rate from these constraints, and these results indicate the bubbles likely formed from a Sgr A* accretion event. This analysis uses the most comprehensive observation samples and modeling techniques to reveal how and when million degree gas formed in the Milky Way.