The neutral gaseous halos of Milky Way-type galaxies

Various scenarios have been proposed to explain the origin of the Galactic high-velocity clouds, including tidal stripping from companions, a galactic fountain, and remnants of Local Group galaxy formation. Each of these scenarios predicts differing cloud characteristics such as distance and mass, implying widely varying properties for the Galaxy's gaseous halo. To eliminate the difficulties of studying the Galactic halo from within, we have embarked on a program to study the nature of neutral halo gas in external galaxies using deep VLA 21cm spectral line observations and deep optical imaging. Here we present the results for two nearby, face-on spiral galaxies, M 83 and M 51.; Significant amounts of anomalous-velocity gas are detected toward both galaxies, with about 108 $M\odot$ of H I lying in a slowly rotating disk. An automated source detection process was devised and implemented to search for small-scale H I emission features. Toward M 83, 14 distinct, anomalous-velocity H I clouds are detected, with masses ranging from 7 × 105–1.5 × 107 $M\odot$ and velocities differing by up to 200 km s−1 from that of the galaxy's H I disk. The clouds are projected both on and off the disk, with large on-disk structures coincident with the optical spiral arms and unresolved off-disk clouds containing no diffuse optical emission down to a limit of 27 r′ mag per square arcsec. In M 51, 23 anomalous-velocity H I clouds are detected, including several previously-known, tidally extended H I tails. The remaining clouds are found on and off the optical disk, with identical H I/optical correspondence as seen in M 83.; The varying nature of the detected H I clouds requires the presence of multiple formation mechanisms, with a galactic fountain responsible for the extended anomalous disk and inner-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 expected mass exchange rate under the galactic fountain model. If the clouds in M 83 and M 51 are drawn from a similar population as the Galactic HVCs, then the distances to the latter must be less than about 25 kpc.