Far Ultraviolet Spectroscopic Explorer observations of the galactic and intergalactic medium toward H1821+643

We have obtained moderate-resolution spectra of the bright QSO H1821+643 (z = 0.297) in the wavelength interval 990-1185 Å with the Far Ultraviolet Spectroscopic Explorer (FUSE). Strong, complex O VI λ1032 and Fe II λ1145 absorption arising in gas above several spiral arms and the outer warp of our Galaxy is detected. We have identified absorption by a high negative-velocity (-215 km s^-1) component of O VI, which corresponds to the limiting velocity of v_LSR ∼ -190 ± 20 km s^-1 for Milky Way gas along this line of sight, assuming corotation of disk and halo gas and a flat rotation curve for the outer Galaxy. We report the detection of four absorbers in the intergalactic medium, through detections of Lyβ at z = 0.02438 and 0.1213, Lyδ, Lyε, Lyζ in a system at z = 0.2249, C III λ977 at z = 0.1705, and O VI λ1032 at z = 0.12137. The FUSE data show that the Lyman absorbers at z = 0.225 are composed of two components separated by ∼70 km s^-1. Finally, we have observed associated absorption from O III λ832, O IV λ787, and possibly S V λ786 at the redshift of the QSO (z ∼ 0.297). Ne VIII λλ770, 780 is not detected in this associated system. When combined with the previous detection of associated O VI λ1032, we conclude that this system is a multiphase absorber, and we discuss the origin of the absorption in this context.