Probing the Galactic Disk and Halo. III. The Galactic and Intergalactic Sight Line to H1821+643

We present intermediate-resolution (FWHM ∼10-20 km s-1) GHRS observations of interstellar and intergalactic absorption lines in the spectrum of the bright quasar H1821+643. The interstellar species detected include S II, Si II, Mg II, C IV, and N V. The intergalactic lines include Lyα (six lines), Lyβ (2 lines), Lyγ, and C III. The Galactic direction of H1821+643 (l = 94°.0, b = 27°.4) allows for the study of gas in the warped outer Galaxy since the effects of Galactic rotation cause absorption by distant gas to be Doppler-shifted by velocities approaching -190 km s-1. We detect by Mg II and Si II extending from approximately +40 km s-1 to -150 km s-1, indicating the presence of gas containing these metals in the outer Galaxy. Over the velocity range from -100 to -150 km s-1, we find Si II/H I, Mg II/H I, and S II/H I = 0.92±0.20, ≥0.25, and ≤ 1.5 times the abundances of these elements in the Sun. However, the presence of C IV in this gas implies corrections must be made for the effects of ionization. A simple model allowing for the ionization by the EUV background suggests the measurements are consistent with absorption in a gas with an intrinsic metallicity of roughly 1/10 solar. Strong C IV and N V absorption is found in broad absorption centered near -8 km s-1. This absorption is well represented with a component having a Doppler spread parameter, b ∼32 km s-1, N(C IV) = (10.4±1.4) × 1013 cm-2, and N(N V) = (14.3±2.6) x 1013 cm-2. The inferred column density ratio of C IV to N V of approximately 0.7 and the large line width suggests this gas is probably hot (T> 2 × 105 K) and may be associated with Galactic radio loop III. The C IV doublet absorption also exhibits strong components centered near -70 and -120 km s-1, which are not seen in N V. The -70 km s-1 C IV absorption probably is associated with highly ionized gas situated 1.5 kpc above the Perseus spiral arm. The -120 km s-1 absorption is likely associated with gas above the outer spiral arm of the Milky Way. Assuming corotation of disk and halo gas, a velocity of -120 km s-1 occurs at a Galactocentric distance of 24 kpc and a distance away of the plane of 7 kpc. The ionization of this distant gas may be produced by photoionization from the EUV extragalactic background radiation. We observe five intergalactic Lyα lines with Wλ(rest)> 60 mÅ over a redshift path of 0.057 implying a low-redshift Lyα cloud density in agreement with the local number density found toward 3C 273 by Morris et al. (1991). Combining the results for the two sight lines we obtain a Lyα cloud density dN/dz = 73±19 for 0 < z <0.3.