H-I ABSORPTION LIMITS AND EMISSION MAPPING FOR HIGH-VELOCITY CLOUDS

We have carried out systematic emission and absorption surveys for high-velocity H I in the direction of 63 bright continuum sources. We detected high-velocity emission along 18 of these 63 directions with a threshold about a factor of 2 better than previous, much more extensive surveys. No high-velocity absorption was found in any of the directions. This negative result argues against any appreciable amounts of H I being "hidden" by a highly depressed spin temperature TS in low-density regions. The absence of absorption even in clouds with column densities up to NH I ∼ 1020 cm-2 requires either an extragalactic cosmic-ray/X-ray flux or some mechanical energy penetrating into the halo from the galactic disk. The sky distribution of the high-velocity emission and the column density distribution agree with larger surveys. There is no evidence for a large population of high-velocity clouds with small peak column density. Detailed emission maps in the vicinity of each continuum source show that the variations in column density within a high-velocity cloud are similar to the variations between high-velocity clouds. A previously discovered high-velocity cloud at high galactic latitude, in the direction of part of the Virgo Cluster of galaxies, was mapped. This small cloud is moving away from the galactic plane at v ≳ 30 km s-1 and is probably not self-gravitating.