A SENSITIVE SEARCH FOR GALACTIC HIGH-VELOCITY H-I CLOUDS

We have conducted a sensitive search for Galactic high-velocity 21 cm H I clouds along and near the sight lines to quasars. The 5 a completeness limit is N-H = 7 x 10(17) cm(-2) which is a factor of 3 more sensitive than other recent surveys. Approximately 37% of the observed directions show high-velocity gas at \V-LSR\ > 100 km s(-1), excluding that which can be associated with the Galactic warp. This is a significant increase in the sky-covering factor above the 18% derived from previous studies. Virtually all of the newly detected emission is associated with previously known high-velocity cloud (HVC) complexes; there is no evidence for a large population of low column density clouds not associated with the complexes. HVC complexes appear to be surrounded by extensive low column density, neutral envelopes. The large observed covering factor for Galactic HVCs may have important implications for the hypothesis that large gaseous halos are responsible for many of the narrow metal-line absorption systems seen in the spectra of quasars.