A high-resolution optical and radio study of Milky Way halo gas

Abstract

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 km 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 local stars along five extragalactic sight lines. The data show substantial interstellar material, at both low and intermediate velocities, between 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 determined gaseous titanium abundance above the Galactic plane exceeds that in the disk, on the average, by a factor of 4 to 6 and, for individual cloud components, is further enhanced at higher LSR velocity. Thirty-three discrete high-latitude clouds, with well-determined distance limits, are detected in Ca II absorption and 17 discrete clouds, including three high-velocity clouds, are identified in H I emission. The kinematics of the high-latitude gas observed in Ti II and Ca II absorption is characterized by significant peculiar velocities with respect to a model corotating halo.