The Neutral Gaseous Halos of Milky Way-type Galaxies

Eric D. Miller
Ph.D. Dissertation
Joel N. Bregman, Advisor

Press release for Jan 2004 AAS meeting    HTML
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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 halo gas in external galaxies, and here we present the results of deep HI and optical observations of 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 7x105-1.5x107 solar masses and velocities differing by up to 200 km/s from that of the galaxy's HI 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, 20 anomalous-velocity HI clouds are detected, including several previously-known, tidally extended HI tails. The remaining clouds are found on and off the optical disk, with identical HI/optical correspondence as seen in M 83.

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 discrete clouds, and tidal effects responsible for off-disk cloud production. The mass and kinetic energy of the HI 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.

Last updated: Fri Feb 18 11:12:20 EST 2005