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We analyze an adaptive mesh refinement hydrodynamic cosmological simulationof a Milky Way-sized galaxy to study the cold gas in the halo. HI observationsof the Milky Way and other nearby spirals have revealed the presence of suchgas in the form of clouds and other extended structures, which indicateson-going accretion. We use a high-resolution simulation (136-272 pc throughout)to study the distribution of cold gas in the halo, compare it withobservations, and examine its origin. The amount (10^8 Msun in HI), coveringfraction, and spatial distribution of the cold halo gas around the simulatedgalaxy at z=0 are consistent with existing observations. At z=0 the HI massaccretion rate onto the disk is 0.2 Msun/yr. We track the histories of the 20satellites that are detected in HI in the redshift interval 0.5>z>0 and findthat most of them are losing gas, with a median mass loss rate per satellite of3.1 x 10^{-3} Msun/yr. This stripped gas is a significant component of the HIgas seen in the simulation. In addition, we see filamentary material cominginto the halo from the IGM at all redshifts. Most of this gas does not make itdirectly to the disk, but part of the gas in these structures is able to cooland form clouds. The metallicity of the gas allows us to distinguish betweenfilamentary flows and satellite gas. We find that the former accounts for atleast 25-75% of the cold gas in the halo seen at any redshift analyzed here.Placing constraints on cloud formation mechanisms allows us to betterunderstand how galaxies accrete gas and fuel star formation at z=0.

The Origin and Distribution of Cold Gas in the Halo of a Milky Way-Mass Galaxy