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 component  of  the  CGM  and  how  this  depends  on  the  ratio  of  gas-to-stellar mass  within  the  optical  radius.  In  gas-rich  galaxies,  gas  temperatures  rise monotonically from center of the halo out to the virial radius. Average neutral gas column densities remain higher than 10**19 atoms cm−2 all the way from the center of the galaxy out to radii of 50-70 kpc. In gas-poor galaxies with fg < 0.1, gas temperatures remain fixed at ~10^6 K from the edge of the disk out to  radii  of  100  kpc.  The  column  density  of  neutral  gas  drops  below  1019 atoms  cm−2  at  radii  of  10  kpc.  The  neutral  gas  distributions  are  also more asymmetric in gas-poor galaxies. Most of these trends can be explained by  the  fact  that  in  the  Illustris  simulation,  gas-poor  galaxies  with  Milky  Way masses have massive (108Mo) black holes that accrete at few percent of Eddington, and that energy is being dumped into the halo at large (100 kpc) radii in the form of bubbles of hot gas in these systems. We also find that the circumgalactic  gas  rotates  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  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.