AAS 201st Meeting, January, 2003
Session 127. Interstellar Medium: Hot and Cold
Oral, Thursday, January 9, 2003, 10:00-11:30am, 606-607

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[127.05D] HI around Milky Way-type Galaxies

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 halo gas in external galaxies, and here we present the results of deep HI and optical observations of two nearby, face-on spiral galaxies, M51 and M83. Significant amounts of anomalous-velocity gas are detected toward both galaxies. In M51, tidally extended HI tails are confirmed, and additional discrete HI components are detected. A very extended optical tail is also observed, and it is shown that these extended HI and stellar features are uncorrelated. Toward M83, several distinct, anomalous-velocity HI clouds are detected, with masses ranging from 106-107 solar masses and velocities differing by up to 150 km/s from that of the galaxy's HI disk. These clouds have no visible optical counterparts to indicate that they are faint companion galaxies, down to a limit of 27 R magnitudes per square arcsec. In addition, both galaxies show a significant amount of extended, intermediate-velocity HI emission. As we will discuss, these results point to a scenario wherein both tidal interactions and a galactic fountain are necessary to explain different aspects of the observations.

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.