Ultraviolet observations of halo clouds

Abstract

Since the earliest optical absorption line studies of Munch and Zirin (1961) identified clouds of gas located at large distances from the galactic plane, considerable effort has gone into trying to understand the origin and nature of Milky Way halo gas. Subsequent high resolution optical absorption studies (Albert 1981; Blades et al 1989) have expanded on the early results, demonstrating clearly that (1) halo clouds are more likely to have velocities outside the range allowed by galactic rotation and (2) halo clouds show smaller depletion of refractory elements compared to their disk counterparts (i.e. the SpitzerRoutley effect). Further insight into the nature of halo gas has been provided by absorption studies in the near ultraviolet region of the spectrum (Savage and deBoer 1979, 1981; Pettini and West 1983; Savage and Massa 1987; Danly 1987, 1989). Despite the lower (25 km/s) resolution, the wider range of ionization states available for study in the ultraviolet and the greater sensitivity provided by higher f-value lines of more abundant species have permitted several additional pieces of observational information to be uncovered. These include: (i) the nearly ubiquitous presence of C IV and Si IV in the halo, (2) the higher scale height of the high ionization species, (3) the very low colmnn density, low ionization gas that pervades the halo, (4) the strong bias toward infall of gas in the northern galactic hemisphere and (5) the strong asymmetry in the characteristics of halo gas toward the NGP vs that toward the SGP. Comprehensive absorption line and 21-cm surveys have provided insight into the gross characteristics of halo gas and have lead to several plausible theoretical explanations for its origin. However, no definitive conclusions have been reached as to the nature of its support or ionization, or to the origin of diffuse halo gas or of the high velocity clouds (HVCs). Differentiation between competing theories for halo gas will only be achieved by in-depth studies of physical characteristics on a component-by-component basis. This is the hope and expectation for GHRS (the Goddard High Resolution Spectrometer on HST): to observe and analyze ultraviolet species at ground based resolution. In order to make the most efficient use of data available in advance of the launch of HST, we have been combining high-resolution, ground based optical data with lower resolution IUE data on several important species observed toward high latitude halo stars. The selected lines of sight are those known from previous work to have complex, multicomponent spectra where at least one cloud lies outside the bulk of the HI layer (e.g. z>250 pc). Here we present results toward one of our stars, HD 203664.