The Multiphase Circum- and Intergalactic Media at the Nexus Between Galaxy Formation and Cosmology
Nir Mandelker (Yale)
Galaxies are not closed boxes. Rather, it has become clear in recent years that it is the flow of gas into and out of galaxies that shapes their evolution. Cycles of gas accretion, star-formation, galactic outflows and reaccretion, intimately link galaxies to the circumgalactic medium (CGM, gas outside galaxies but within dark matter halos) and the intergalactic medium (IGM, gas outside dark matter halos). While the diffuse gas in these regions has traditionally been very difficult to study, recent advances in both observations and numerical simulations are now providing a wealth of information on the gas around galaxies, revealing complex, multiphase and multiscale structure. I will describe my ongoing efforts to study the phase-structure of the C/IGM by combining novel cosmological magnetohydrodynamic moving-mesh simulations that I modified to overcome difficulties in resolving the diffuse gas in these regions, with analytic modeling and idealized numerical experiments that address detailed questions about physical processes affecting multiphase gas in the C/IGM. I will present two main results from these studies. I will first show how non-linear thermal instabilities cause hot gas to "shatter" into small-scale clouds of cool and dense gas. I will then present a model for the interaction of cold accretion streams with the ambient hot CGM by considering the Kelvin-Helmholtz Instability in a dense, supersonic, self-gravitating, and radiatively cooling cylinder. These have helped us to understand how the C/IGM are shaped by a complex interplay of hydrodynamical, thermal, and gravitational instabilities which we are beginning to put together in a coherent framework.