High-velocity clouds: review of observational properties

Earlier extended surveys with moderate resolution and sensitivity had shown that high-velocity gas can appear as small clouds (approximately 20) either isolated, or in elongated complexes with lenths of up to 300. The profiles were rather broad (halfwidths of 25 km/s). Recent high-resolution observations of a number of individual clouds reveal much small-scale structure. The most conspicuous property is the existence of small (6'), bright (up to 65 K), and narrow (5 km/s) cores, embedded in smooth envelopes (20, 4 K, 25 km/s). Also sharp gradients; ringlike structures and large velocity gradients (up to 10 km/s/arc-degr.) are reported. New highly sensitive observations show the existence of many small (10), faint (up to 0.3 K), isolated clouds with very high negative velocities. (Auth.)