The Hubble Space Telescope Quasar Absorption Line Key Project. XV. Milky Way Absorption Lines^*

This paper presents the results of an analysis of the Milky Way absorption lines found in the Hubble Space Telescope (HST) Quasar Absorption Line Key Project database for 83 QSOs observed with the Faint Object Spectrograph G190H and G270H gratings, of which 16 QSOs are also observed with the G130H grating. The HST Key Project observations are supplemented with high-quality 21 cm H I emission-line observations mostly obtained with the NRAO 43 m radio telescope. The Milky Way halo gas exhibits "mixed ionization" absorption with high-ionization absorption from Si IV and C IV substantially weaker than the extremely strong intermediate- and low-ionization absorption from Si III, Si II, C II, Mg II, and Fe II. For a sample of 16 QSOs observed in the far-UV, the median velocity equivalent widths of very strong lines of Si IV, Si III, and Si II are 60, 180, and 180 km s^-1, respectively. Velocity equivalent widths this large for Si III and Si II imply the existence of high velocity dispersion moderate- (Si III) and low-ionization (Si II) gas along many paths through the Galactic halo. Measures of the Galactic damped Lyα line toward 14 QSOs permit the determination of N(H I)_Lyα through the gaseous disk and halo of the Galaxy. The values of N(H I)_Lyα range from 0.64 × 10²⁰ to 3.37 × 10²⁰ cm^-2 with N(H I)_Lyα| sin b| averaging (1.29 ± 0.49) × 10²⁰ cm^-2. A comparison of N(H I)_Lyα with N(H I)_{21 cm} reveals that N(H I)_Lyα/N(H I)_{21 cm} for the 10 sight lines where the value of N(H I)_Lyα is not significantly affected by geocoronal emission ranges from 0.62 and 0.91. This difference is probably produced by a combination of systematic and random errors and contribution from the small angular scale structure in the H I distribution. Such structure can produce different column densities when sampling gas with an infinitesimal beam in the UV (the angular size of the QSO) compared to the much larger 21^' beam of the NRAO 43 m radio telescope.

     The overall strength of the Mg II λλ2796 and 2803 absorption appears to be correlated with the presence of high-velocity gas along the line to sight. Velocity-resolved Mg II absorption associated with high-velocity gas in the Magellanic Stream is detected toward eight QSOs, including PKS 0003+15, PG 0043+039, PKS 0637-75, 3C 454.3, PKS 2251+11, PG 2302+029, PKS 2340-36, and PKS 2344+09. Velocity-resolved Mg II absorption toward 15 QSOs is not accompanied by the existence of associated H I emission. Interesting objects in this category include PKS 0232-04 (l = 1745, b = -562), which has a high-velocity cloud (HVC) at v ∼ +270 km s^-1 detected in Mg II, and PG 1116+215 (l = 2233,b = 682) with a HVC at +200 km s^-1 detected in Mg II, C II, Si IV, and possibly C IV. The HVC toward PKS 0232-04 is interesting because all known H I HVCs in this general region of the sky have negative velocity rather than positive velocity. For 15 QSOs known to lie in the direction of H I HVCs, the Mg II lines have extremely strong principal absorption components, suggesting the detection of blended low- and high-velocity absorption. These lines of sight imply the detection of Mg II absorption by the high-velocity gas in HVC complexes C and A, in the outer Galaxy warp, and in the Magellanic Stream, as well as toward three smaller clouds. There are 11 QSO sight lines with very strong Mg II absorption for which there is no evidence for high-velocity H I emission. However, six of these sight lines lie near known H I HVCs. There are 38 QSOs with weak Mg II principal absorption and no known H I HVCs. These objects provide information about the H I absorption characteristics of disk and halo gas well away from H I HVCs. The sky covering factor of high-velocity Mg II is large, with 41 and 71 QSO lines of sight showing either resolved high-velocity Mg II absorption or principal absorption that is so strong that blended low- and high-velocity Mg II absorption is suggested.