The morphology and kinematics of neutral hydrogen in the vicinity of z=0
  galaxies with Milky Way masses -- a study with the Illustris simulation
  We analyze the properties of the circumgalactic gas (CGM) around 120 galaxies
with stellar and dark matter halo masses similar to that of the Milky Way. We
focus on the morphology and kinematics of the neutral hydrogen and how this
depends on f_g, the ratio of gas-to-stellar mass within the optical radius. In
gas-rich galaxies with f_g > 0.1, gas temperatures rise slowly from center of
the halo out to the virial radius and average neutral gas column densities
remain above 10^19 atoms cm^-2 out to radii of 50-70 kpc. In gas-poor galaxies
with f_g < 0.1, gas temperatures rise quickly outside the edge of the disk to
10^6 K, and then remain fixed out to radii of 100 kpc. The column density of
neutral gas quickly drops below 10^19 atoms cm^-2 at radii of 10 kpc. Neutral
gas distributions are also more asymmetric in gas-poor galaxies. Most of the
differences between gas-poor and gas-rich galaxies in the Illustris simulation
can be attributed to the effects of "radio-mode" AGN feedback. In the Illustris
simulation, the circumgalactic gas is found to rotate coherently about the
center of the galaxy with a maximum rotational velocity of around 200 km/s. In
gas-rich galaxies, the average coherence length of the rotating gas is 40 kpc,
compared to 10 kpc in gas-poor galaxies. In the very most gas-rich systems, the
CGM can rotate coherently over scales of 70-100 kpc. We discuss our results in
the context of recent observations of the CGM in low mass galaxies via UV
absorption-line spectroscopy and deep 21cm observations of edge-on spiral
galaxies.
