From Observations to Origins and Fates: Disentangling Gas Flows Around Galaxies

Hafen, Zachary Harris

doi:https://doi.org/10.21985/n2-05vm-1q98

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From Observations to Origins and Fates: Disentangling Gas Flows Around Galaxies

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As much as half of the gas mass in our Galaxy's dark matter halo may reside not in the galaxy itself, but in the surrounding volume, the circumgalactic medium (CGM). The vast gas content of the CGM, loosely defined as the volume immediately outside the galaxy but inside the dark matter halo, is crucially involved in galaxy formation: accretion from the CGM onto the central galaxy provides the material necessary to fuel observed star formation rates, while galactic winds ejected into the CGM are are primarily responsible for regulating star formation in galaxies up to Milky-Way mass galaxies. The relevance of the CGM was cemented over the past decade through Hubble Space Telescope observations at z<1 and ground-based spectroscopy at higher redshifts. Despite their individual importance, determining the exact roles of circumgalactic gas flows and differentiating them in observations is an outstanding problem in galaxy formation. This thesis addresses this problem through the use of hydrodynamic galaxy formation simulations. In Chapter 2 I present mock observations of cool CGM absorption systems selected without regard to host galaxy and use them to inform early efforts to observationally distinguish halo-scale inflows and outflows. I show that metallicity distribution of these absorption systems originates from the CGM across a wide halo mass range, and argue that for most observed systems metallicity cannot be used to identify the origin of contributing gas. In Chapters 3 and 4 I systematically quantify the channels through which the simulated circumgalactic medium gains and loses mass, thereby building the necessary foundation for future work to quantify the real-universe equivalent. The CGM is a strongly halo-mass-dependant interplay of winds and accretion, ranging from diffuse, low-mass halos in equilibrium with the UV background at halo masses around 1e10 solar masses, to highly dynamic, bursty, and windy halos at halo masses around 1e11 solar masses, to hot and diffuse halos with intermittent cool gas at halo masses around 1e12 solar masses. I show that a full model of the CGM of Milky Way-mass halos must include gaseous halos of satellite galaxies that can both fuel and accrete from the CGM.

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