Hα from Hvcs

Optical emission lines provide an important new window on the HVCs. Recent studies of the Hα line reveal that ionized hydrogen is pervasively associated with the neutral hydrogen in HVCs. The Wis-consin Hα Mapper (WHAM) instrument has so far detected Hα 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 Hα and 21-cm detections are well correlated, but the intensities are not. If the clouds are photoionized, the Hα intensity is a direct measure of the Lyman continuum flux in the Galactic halo. Forthcoming observations of the Hα 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.