High-velocity Clouds in the Nearby Spiral

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 × 107 M of H i contained in a disk rotating 40–50 km s−1 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 × 105 to 1.5 × 107M and velocities differing by up to 200 km s−1 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 slowly rotating 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.