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 G1 90H and G270H gratings, of which 16 QSOs are also observed with the G130H g rating. The HST Key Project observations are supplemented with high-quality 21 cm H I emission-line observations mostly obtained with the NRAO 43 m ra dio telescope. The Milky Way halo gas exhibits "mixed ionization" absorptio n 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 S i iv, Si m, 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 h igh velocity dispersion moderate(Si III) and low-ionization (Si II) gas alo ng many paths through the Galactic halo. Measures of the Galactic damped Ly alpha line toward 14 QSOs permit the determination of N(H I), through the gaseous disk and halo of the Galaxy. The values of N(H I)(Ly alpha) range f rom 0.64 x 10(20) to 3.37 X 10(20) cm(-2) with N(H I)(Ly alpha)\ sin b \ av eraging (1.29 +/- 0.49) x 10(20) cm(-2). A comparison of N(H I)(Ly alpha) w ith N(H I)(21 cm) reveals that N(H I)(Ly alpha)/N(H I)(21) (cm) for the 10 sight lines where the value of N(H I)(Ly alpha) is not significantly affect ed by geocoronal emission ranges from 0.62 and 0.91. This difference is pro bably produced by a combination of systematic and random errors and contrib ution 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. lambda lambda 2796 and 2803 absorption a ppears to be correlated with the presence of high-velocity gas along the li ne to sight. Velocity-resolved Mg II absorption associated with high-veloci ty gas in the Magellanic Stream is detected toward eight QSOs, including PK S 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 11 absorption towa rd 15 QSOs is not accompanied by the existence of associated H I emission. Interesting objects in this category include PKS 0232-04 (l = 174 degrees5, b = -56 degrees2), which has a high-velocity cloud (HVC) at v similar to 270 km s(-1) detected in Mg 11, and PG 1116+215 (l = 223 degrees3, b = 68 d egrees2) with a HVC at +200 km s(-1) detected in Mg 11, C II, Si Iv, and po ssibly C rv. 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, sugg esting the detection of blended low- and high-velocity absorption. These li nes of sight imply the detection of Mg 11 absorption by the high-velocity g as in HVC complexes C and A, in the outer Galaxy warp, and in the Magellani c Stream, as well as toward three smaller clouds. There are 11 QSO sight li nes with very strong Mg II absorption for which there is no evidence for hi gh-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 kno wn H I HVCs. These objects provide information about the H II absorption ch aracteristics of disk and halo gas well away from H II HVCs. The sky coveri ng factor of high-velocity Mg II is large, with 41 and 71 QSO lines of sigh t showing either resolved high-velocity Mg II absorption or principal absor ption that is so strong that blended low- and high-velocity Mg II absorptio n is suggested.