Observations of the gaseous galactic halo toward 3C 273 with the Goddard High Resolution Spectrograph

We present an analysis of observations of 24 absorption lines produced near zero redshift by Milky Way disk and halo gas along the sight line to 3C 273. The observations were obtained with the intermediate-resolution mode of the Goddard High Resolution Spectrograph (GHRS) using the large entrance aperture. The spectral spread function has FWHM ∼ 20 km s^-1 and broad wings. The species detected include C II, C II*, C IV, O I, N V, Mg II, Si II, Si IV, S II, Mn II, Fe II, and Ni II. The O I line is contaminated by Earth atmospheric emission, and one of the Si IV doublet lines is contaminated by what is likely an intergalactic H I Lyman-α line. The set of absorption lines provides information about neutral and highly ionized gas in the Galactic disk and halo. The data permit estimates of reliable column densities and depletions for Si II, S II, Mn II, and Ni II. A value of N(H I) is obtained from profile fitting to the Faint Object Survey Lyman-α measurement of Bahcall et al. (1991a). We analyze the GHRS data using standard curve-of-growth techniques and the apparent optical depth method and find that S based on S II is overabundant by 0.16 dex, while Si, Mn, and Ni have modest depletions (-0.67 to -0.81 dex) which are representative of the low-density interstellar medium. The suggested S overabundance is probably due to the presence of substantial amounts of S II in the warm ionized gas toward 3C 273. The presence of the ionized gas probably also influences the results for Si, Mn, and Ni. An estimate of the column density of C II* yields information about the cooling rate per nucleon in the gas toward 3C 273. The cooling rate is 7 times lower than the average found in the Galactic disk. Reliable column densities are obtained for Si IV, C IV, and N V. The amount of highly ionized gas toward 3C 273 is compared with that found for other sight lines to Galactic stars and to stars in the LMC and SMC. The amount of highly ionized gas perpendicular to the Galactic plane is about 2-3 times that seen toward the LMC and SMC. The highly ionized gas (including recent observations of O VI by Davidsen et al. 1992 seems best explained as being associated with the cooling flow of a Galactic fountain with additional contributions to the absorption from gas associated with the energetic events that produced Galactic radio loops I and IV. The data provide kinematical information about gas motions along an extended path through the halo. The high-ionization lines have average velocities approximately 10 km s^-1 more negative than the weaker low-ionization lines, suggestive of inflow of hot gas along this high-latitude sight line. The very strongly saturated lines for Fe II, Mg II, Si II, and C II exhibit full extents at half-absorption depths ranging from ∼ 120 km s^-1 up to ∼ 150 km s^-1. These lines are evidently tracing a high-velocity dispersion phase of the gas toward 3C 273 that is apparent in H I 21 cm emission-line measurements. High-velocity gas absorption-line components with 100 km s^-1 < |υ| < 1000 km s^-1 are not evident in any of the profiles for the strong low-ionization species or for the highly ionized species. The GHRS absorption-line equivalent widths for all detected species are compared with the measurements for other extragalactic sight lines, including those to the LMC, the SMC, and H1821+643. In the case of the strong low-ionization lines, the absorption-line equivalent widths for 3C 273 are intermediate to those for the SMC sight line and the LMC and H1821+643 sight lines. The strong low-ionization lines mostly measure the kinematical conditions along the sight line to each object. The high-ionization lines are less saturated and mostly provide a measure of the column density, which is greater toward 3C 273 than for the other paths. The 3C 273 Milky Way disk and halo absorption-line data are compared with measurements for QSO damped Lyman-α absorption-line systems and found to be similar. The low-ionization lines compared are strongly saturated. Therefore, the correspondence implies a similarity of the kinematical behavior of the turbulent phases of the different absorbing systems and does not necessarily imply similar elemental abundances.