DISTRIBUTION AND ORIGIN OF HIGH-VELOCITY CLOUDS .2. STATISTICAL-ANALYSIS OF THE WHOLE-SKY SURVEY

A sensitive, almost complete, whole-sky survey of high-velocity clouds (HVCs) has been made available by Bajaja et al. (1985) and Hulsbosch & Wakker (1988, Paper I). This paper (Paper II in a series on HVCs) is dedicated to the analysis of the statistical properties of these surveys.The main conclusions drawn from the statistics are the following: (1) It is now clear that the fact that HVCs used to be observed only at negative velocities was due to the incomplete and less-sensitive coverage of the southern sky, in spite of earlier statements to the contrary (Oort & Hulsbosch 1978; Hulsbosch 1979).(2) The distributions of flux with galactic longitude and latitude indicate that gas at high velocity concentrates toward the plane and is evenly distributed in longitude.(3) Extrapolating the relation between the sky area covered by high-velocity gas and the detection limit suggests that at a level of congruent-to 2 10(17) cm-2 between 30% and 60% of the sky may be covered with such gas. Such column densities are detectable by means of ultra-violet resonance lines of for instance C II or Si II. However, no high-velocity absorption is detected in a sample of 19 stars more distant than 1.5 kpc (Danly 1989), indicating that in general HVCs are at large distances.