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 analyse the properties of the circumgalactic gas (circumgalactic medium, 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 centre of the halo out to the virial radius and average neutral gas column densities remain above 10¹⁹ 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 disc to ~10⁶ K, and then remain fixed out to radii of 100 kpc. The column density of neutral gas quickly drops below 10¹⁹ 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 centre of the galaxy with a maximum rotational velocity of around 200 km s^-1. 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 21 cm observations of edge-on spiral galaxies.