Gas Accretion onto the Milky Way
  The Milky Way is surrounded by large amounts of gaseous matter that are
slowly being accreted over cosmic timescales to support star formation in the
disk. The corresponding gas-accretion rate represents a key parameter for the
past, present, and future evolution of the Milky Way. In this article, I
discuss our current understanding of gas accretion processes in the Galaxy by
reviewing past and recent observational and theoretical studies. The first part
of this review deals with the spatial distribution of the different gas phases
in the Milky Way halo, the origin of the gas, and its total mass. The second
part discusses the gas dynamics and the physical processes that regulate the
gas flow from the outer Galactic halo to the disk. From the most recent studies
follows that the present-day gas accretion rate of the Milky Way is a few solar
masses per year, which is sufficient to maintain the Galaxy's star-formation
rate at its current level.
