THE ORIGIN AND DISTRIBUTION OF COLD GAS IN THE HALO OF A MILKY-WAY-MASS GALAXY

Abstract

We analyze an adaptive mesh refinement hydrodynamic cosmological simulation\nof a Milky Way-sized galaxy to study the cold gas in the halo. HI observations\nof the Milky Way and other nearby spirals have revealed the presence of such\ngas in the form of clouds and other extended structures, which indicates\non-going accretion. We use a high-resolution simulation (136-272 pc throughout)\nto study the distribution of cold gas in the halo, compare it with\nobservations, and examine its origin. The amount (10^8 Msun in HI), covering\nfraction, and spatial distribution of the cold halo gas around the simulated\ngalaxy at z=0 are consistent with existing observations. At z=0 the HI mass\naccretion rate onto the disk is 0.2 Msun/yr. We track the histories of the 20\nsatellites that are detected in HI in the redshift interval 0.5>z>0 and find\nthat most of them are losing gas, with a median mass loss rate per satellite of\n3.1 x 10^{-3} Msun/yr. This stripped gas is a significant component of the HI\ngas seen in the simulation. In addition, we see filamentary material coming\ninto the halo from the IGM at all redshifts. Most of this gas does not make it\ndirectly to the disk, but part of the gas in these structures is able to cool\nand form clouds. The metallicity of the gas allows us to distinguish between\nfilamentary flows and satellite gas. We find that the former accounts for at\nleast 25-75% of the cold gas in the halo seen at any redshift analyzed here.\nPlacing constraints on cloud formation mechanisms allows us to better\nunderstand how galaxies accrete gas and fuel star formation at z=0.\n