The Physical Nature of Circum-Galactic Gas

Recent observations by the Hubble Space Telescope (HST), combined with results from ground-based observatories, have begun to systematically probe the gaseous halos of typical (L^star) galaxies at low redshift. At the same time, complementary campaigns at high redshift are enabling us to trace the time evolution of halo gas. The striking correlation between O VI absorption in galactic halos and the specific star formation rate of central galaxies revealed by the Cosmic Origins Spectrograph (COS), in particular, highlights the close connection between circum-galactic gas and galaxy evolution. It is now also appreciated that the mass in heavy elements in galactic halos is comparable to (or greater than) the mass in heavily elements inside galaxies. The properties of halo gas are thus intimately connected to the processes driving galaxy evolution. However, the physical origin of observed galaxy-halo gas correlations and of halo gas in general is presently not understood. We propose to model the halo gas of galaxies, focusing on systems probed by HST observations, with cosmological simulations of unprecedented resolution and with a much more complete and physically predictive implementation of feedback from stars and massive black holes than previously done. We will carry out detailed radiative transfer calculations, which are critical to accurately map the simulations to observables but which have been neglected or oversimplified in previous studies. Finally, we will systematically compare of our radiative transfer results with all the main existing CGM measurements. This comprehensive study will enable us to test our current best models of galaxy formation and evolution. It will also allow us to quantify the implications of HST results for key physical processes, including galactic gas accretion, the formation of cool clouds in hot halos, galactic winds driven by stars and black holes, heavy element dispersal and gas recycling, and their role in galaxy evolution. In particular, we will investigate the processes that may drive the observed dichotomy between blue star-forming discs and red quiescent ellipticals, and thus shed light on the physics of this key problem in galaxy evolution. In addition to HST, our work will have ramifications for all NASA missions used to study galaxy evolution, including the James Webb Space Telescope, WFIRST, Spitzer, Chandra, NuSTAR, and Herschel.