IAU Symposium Abstract S186-020P

The High-Velocity Clouds : Galactic or Intergalactic?

Hugo van Woerden1, Bart P. Wakker2,
Ulrich J. Schwarz1, Reynier F. Peletier1,
and Peter M.W. Kalberla3
1 Kapteyn Institute, Groningen, The Netherlands < hugo@astro.rug.nl >
2 Astr. Dept., Univ. Wisconsin, Madison WI, USA < wakker@astro.wisc.edu >
3 Astronomische Institute, Univ. Bonn, Germany < kalb@astro.uni-bonn.de >

In a recent analysis, Blitz, Spergel et al. (1997, BAAS 28, 1349 and 1350) suggest that most of the high-velocity clouds (HVCs) are relatively unprocessed, extragalactic remnants of the gas which formed the Local Group of galaxies. We present evidence that at least one major HVC, Complex A, lies in the Milky Way Halo, at z = 3 - 7 kpc from the Galactic plane; for Complex M, Danly et al. (1993, ApJ 416, L 29) had already found z < 5 kpc. We further present data showing that the metallicity in the largest HVC, Complex C, is at least of order 0.1 solar. These and other absorption-line results indicate that several major HVC complexes are neither primordial nor extragalactic, but rather may originate in a Galactic Fountain; alternatively they may be the result of interaction of infalling intergalactic gas, or tidal debris from the Magellanic Clouds, with the Milky Way Halo. For the smaller HVCs, some of which have much higher velocities, a location in the Local Group remains a possibility.