H-alpha from High Velocity Clouds
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Optical emission lines provide an important new window on the HVCs. Recent studies of the H-alpha line reveal that ionized gas is pervasively associated with the neutral hydrogen in HVCs. The Wisconsin H-Alpha Mapper (WHAM) instrument has so far detected H-alpha from high-velocity clouds in the M, A, and C complexes. We find a close spatial correspondence between the neutral and ionized portions of the HVCs with some evidence that the ionized gas envelopes the neutral part of the clouds. The velocities of the H-alpha and 21-cm detections are well correlated, but the intensities are not. If the clouds are photoionized, the H-alpha intensity is a direct measure of the Lyman continuum flux in the Galactic halo. Forthcoming observations of the H-alpha line in combination with other emission lines will give new insights into the high-velocity cloud phenomena and will also probe the physical conditions of their environment.
12 pages, 10 figures. To be published in the Proceedings of the Stromlo Workshop on High Velocity Clouds, eds. B. K. Gibson and M. E. Putnam (ASP Conf. Series)
12 pages, 10 figures. To be published in the Proceedings of the Stromlo Workshop on High Velocity Clouds, eds. B. K. Gibson and M. E. Putnam (ASP Conf. Series)