High - Velocity Clouds in the Nearby Spiral Galaxy M 83

We present deep H I 21-cm and optical observations of the face-on spiral galaxy M 83 obtained as part of a project to search for high-velocity clouds (HVCs) in nearby galaxies. Anomalous-velocity neutral gas is detected toward M 83, with 5.6×10 M⊙ of H I contained in a disk rotating 40–50 km s more slowly in projection than the bulk of the gas. We interpret this as a vertically extended thick disk of neutral material, containing 5.5% of the total H I within the central 8 kpc. Using an automated source detection algorithm to search for small-scale H I emission features, we find eight distinct, anomalous-velocity H I clouds with masses ranging from 7×10 to 1.5×10 M⊙ and velocities differing by up to 200 km s compared to the H I disk. Large on-disk structures are coincident with the optical spiral arms, while unresolved off-disk clouds contain no diffuse optical emission down to a limit of 27 r mag per square arcsec. The diversity of the thick H I disk and larger clouds suggests the influence of multiple formation mechanisms, with a galactic fountain responsible for the slowlyrotating disk and on-disk discrete clouds, and tidal effects responsible for off-disk cloud production. The mass and kinetic energy of the H I clouds are consistent with the mass exchange rate predicted by the galactic fountain model. If the HVC population in M 83 is similar to that in our own Galaxy, then the Galactic HVCs must be distributed within a radius of less than 25 kpc. Subject headings: galaxies: individual (M 83) — galaxies: ISM — galaxies: kinematics and dynamics — radio lines: galaxies