AAS 203rd Meeting, January 2004
Session 116 Elliptical and Spiral Galaxies
Poster, Thursday, January 8, 2004, 9:20am-4:00pm, Grand Hall

## [116.03] High Velocity Clouds in M 83 and M 51

E.D. Miller, J.N. Bregman (U. Michigan)

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 solar masses of HI lying in a slowly rotating disk. An automated source detection process was devised and implemented to search for small-scale HI emission features. Toward M 83, 14 distinct, anomalous-velocity HI clouds are detected, with masses ranging from 7 \times 105--1.5 \times 107 solar masses and velocities differing by up to 200 km/s from that of the galaxy's HI disk. In M 51, 23 anomalous-velocity HI clouds are detected, including several previously-known, tidally extended HI tails. The clouds are projected on and off the disk of both galaxies.

The varying nature of the detected HI clouds requires the presence of multiple formation mechanisms, with a galactic fountain responsible for the extended anomalous disk and inner-disk clouds, and tidal effects responsible for off-disk clouds. The mass and kinetic energy of the 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.