A HIGH-RESOLUTION OPTICAL AND RADIO STUDY OF MILKY-WAY HALO GAS

Optical interstellar absorption lines of Ti II and Ca II and the 21 cm emission line of H I have been observed at high-resolution (6 and 1 k m s-1, respectively) and high detection sensitivity along 25 lines of sight in the Galactic halo. The sample includes 16 distant halo stars matched with one or more nearly aligned foreground stars as well as lo cal stars along five extragalactic sight lines. The data show substant ial interstellar material, at both low and intermediate velocities, be tween 250 and 1000 pc beyond the Galactic plane. As much as one-third of the total gas observed in Ca II absorption may be beyond 1 kpc, and the gaseous Ti II may lie in an even thicker layer. The directly dete rmined gaseous titanium abundance above the Galactic plane exceeds tha t in the disk, on the average, by a factor of 4 to 6 and, for individu al cloud components, is further enhanced at higher LSR velocity. Thirt y-three discrete high-latitude clouds, with well-determined distance l imits, are detected in Ca ii absorption and 17 discrete clouds, includ ing three high-velocity clouds, are identified in H I emission. The ki nematics of the high-latitude gas observed in Ti II and Ca II absorpti on is characterized by significant peculiar velocities with respect to a model corotating halo.