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 complete ness limit is N-H = 7 x 10(17) cm(-2) which is a factor of 3 more sens itive than other recent surveys. Approximately 37% of the observed dir ections show high-velocity gas at \V-LSR\ > 100 km s(-1), excluding th at which can be associated with the Galactic warp. This is a significa nt increase in the sky-covering factor above the 18% derived from prev ious studies. Virtually all of the newly detected emission is associat ed with previously known high-velocity cloud (HVC) complexes; there is no evidence for a large population of low column density clouds not a ssociated with the complexes. HVC complexes appear to be surrounded by extensive low column density, neutral envelopes. The large observed c overing factor for Galactic HVCs may have important implications for t he hypothesis that large gaseous halos are responsible for many of the narrow metal-line absorption systems seen in the spectra of quasars.