The Hubble Space Telescope Quasar Absorption Line Key Project. III. First Observational Results on Milky Way Gas

We report on absorption by Milky Way disk and halo gas seen in Quasar Absorption Line Key Project measurements of 15 quasars obtained with the Faint Object spectrograph (FOS) of the Hubble Space Telescope (HST). The measurements extend from 1150 to 3270 A for four objects and from 1600 to 3270 A for 11 objects and have a velocity resolution (FWHM) of 220-240 km s^-1^ and signal-to-noise ratio of typically 25 per resolution element. The data processing techniques are presented in Key Project Papers I and II. Milky Way absorption lines comprise 44% of all absorption lines found in the first group of 13 Key Project spectra. The Milky Way lines observed in the highest quality data for 3C 273 and H 1821+643 include lines from H I, C II, C II^*^, C IV, O I, N I, Mg I, Mg II, Al II, Al III, Si II, Si III, Si IV, S II, Mn II, Fe II, and Zn II. Strong singly ionized metal lines of Fe II λλ2600.17 and 2586.65 and Mg II λλ2796.35 and 2803.53 are detected in all the spectra. High-quality H I 21 cm emission spectra are used to make small adjustments to the FOS wavelength scale to bring the UV data onto an LSR velocity system. The strong metal lines of Fe II and Mg II permit a sensitive search for metal-line analogs to the high-velocity clouds seen in H I 21 cm emission. Toward three quasars we detect resolved, very high negative velocity (v < - 250 km s^-1^) metal-line absorption. Toward four quasars we detect blended high negative velocity absorption (-250 km s^-1^ < v < - 100 km s^-1^). Therefore, seven of 15 sight lines observed in this limited sample exhibit high-velocity or very high velocity metal-line absorption. Lower limits to the Mg-to-H abundance ratio of 0.059,0.12, and 0.32 times the solar abundance are obtained for the three detections of very high velocity Mg II absorption toward PKS 2251 + 11, PG 0043 + 039, and 3C 454.3, respectively. However, cloud clumping may influence this result, since the H I reference column density from 21 cm emission measurements is obtained with a 21' beam. The sight-line to PG 1259 + 593 reveals absorptions in the lines of Mg II, Fe II, C II, and Si II which are in part due to high-velocity cloud Complex C III. Two sight lines (H1821 + 643 and 3C 351) which extend through the warped outer Galaxy exhibit strong absorption by singly ionized metals (C II, Si II, Fe II, Mg II) and highly ionized gas (C IV) at velocities corresponding to absorption in the outer Galaxy. Combining our measurements of C IV with data from the literature, we derive a Galactic C IV exponential scale height of 4.9 kpc and a midplane density of 7.1 x 10^-9^ atoms cm^-3^. However, the C IV distribution is so patchy that the measurements are found to be equally well fitted by a spherical halo model with a Galactocentric exponential scale length of 6.9 kpc. A comparison of absorption in high-redshift damped Lyα systems with the absorption produced by the Milky Way shows that an appreciable fraction of the damped Lyα absorption-line systems have mixed ionization absorption-line characteristics roughly similar to that found for six sight lines through the Milky Way disk and halo. Since the low-ionization lines are highly saturated, this similarity does not imply similar abundances in the neutral and weakly ionized gas but instead probably suggests a similarity in the kinematical behavior of the different absorbing media.