THE HI GAS KINETIC TEMPERATURE OF HIGH GALACTIC LATITUDE CIRRUS CLOUDS

We present the results of a combined absorption and emission study of the H I 21 cm line toward several cirrus clouds at high galactic latit ude. The absorption measurements were obtained with the Very Large Arr ay (VLA) synthesis telescope, the emission data with the Effelsberg te lescope. The three cirrus complexes studied were selected from the Deu l & Burton (1990) study. Deul & Burton had shown that these cirrus clo uds have associated H I emission with radial velocities near 0 km s-1 as well as with more extreme velocities of v < -20 km s-1. Absorption against background continuum sources was detected in 40% of the lines of sight investigated. The H I line was always found to be optically t hin, tau < 0.6. The 0 km s-1 component was found in each direction whe re absorption was detected; in addition, three directions showed absor ption at intermediate negative velocities. The derived apparent spin t emperatures vary between 20 K and > 350 K. The emission profiles revea l much more widely distributed gas than is visible in absorption. Only the cool clumps in the cirrus show up in absorption. The observations suggest a collision of a high velocity cloud with the galactic disk g as, by which different parts of the cirrus are at present seen in diff erent phases of shock completion. This model may explain the large obs erved differences in the H I emission and absorption profiles over sma ll distances on the sky, as well as the variation of the far-infrared brightness ratio.