2015 Theses Doctoral

Using Hydrodynamic Simulations to Understand the Structure and Composition of the Circumgalactic Medium of Milky Way-sized Galaxies

Salem, Munier

We explore the structure and evolution of baryons within Milky Way-sized halos (M ~ 10¹² Msun) via hydrodynamic simulations. First, we employ a two-fluid model to study the dynamics of a relativistic, diffusive cosmic ray proton (CR) fluid interacting with the thermal interstellar medium (ISM). This model was implemented into the eulerian hydrodynamics code enzo, used throughout this dissertation. After testing this model on analytically tractable scenarios in one dimension, it is unleashed upon an idealized disk simulation in a rapidly-star forming setting, where we find evidence for robust, mass-loaded winds driven by the diffusive CR fluid. These winds reduce the galaxy’s star formation rate (SFR) and circulate on order as much mass into winds as into forming stars. We then extend this model to a cosmological setting where the diffuse CR fluid proves capable of redistributing star formation within the forming disk, reducing the overly-peaked rotation curves in non-CR runs and producing thin, extended disks with visible spiral structure. From these same runs, we then explore the effect of CRs on the circumgalactic medium (CGM) within the halo, comparing our results to observed metal column lines from L ~ L* galaxies and gamma-ray emission observed by Fermi LAT. From this body of work, we find the cosmic ray population of forming galaxies likely alters the system’s baryonic structure and dynamics in fundamental ways, and has a measurable impact on both the galaxy’s disk and CGM. Complimenting this work, we also explore the distribution of baryons in the CGM via simulations of the Large Magellanic Cloud (LMC) orbiting through our own MW halo. We perform a broad suite of “wind tunnel” simulations to constrain models for the MW CGM’s density profile. From this work, we find that the extent of HI gas along the LMC’s leading edge is a direct, localized probe of MW CGM gas density at r ~ 50 kpc from the Galactic Center. Assuming a β-profile for the diffuse gas density, we find the MW CGM may host ~ 10 − 25% of the Galaxy’s expected baryons.