HST/COS observations of galactic high-velocity clouds: four active galactic nucleus sight lines through complex C

We report ultraviolet spectra of Galactic high-velocity clouds (HVCs) in Complex C, taken by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), together with new 21 cm spectra from the Green Bank Telescope. The wide spectral coverage and higher signal-to-noise ratio, compared to previous HST spectra, provide better velocity definition of the HVC absorption, additional ionization species (including high ions), and improved abundances in this halo gas. Complex C has a metallicity of 10%-30% solar and a wide range of ions, suggesting dynamical and thermal interactions with hot gas in the Galactic halo. Spectra in the COS medium-resolution G130M (1133-1468 angstrom) and G160M (1383-1796 angstrom) gratings detect ultraviolet absorption lines from eight elements in low-ionization states (O I, N I, C II, S II, Si II, Al II, Fe II, P II) and three elements in intermediate-and high-ionization states (Si III, Si IV, C IV, N V). Our four active galactic nucleus sight lines toward Mrk 817, Mrk 290, Mrk 876, and PG 1259+593 have high-velocity Hi and Ovi column densities, log N-H (I) = 19.39-20.05 and logN(OVI) = 13.58-14.10, with substantial amounts of kinematically associated photoionized gas. The high-ion abundance ratios are consistent with cooling interfaces between photoionized and collisionally ionized gas: N(C IV)/N(O vi) approximate to 0.3-0.5, N(Si IV)/N(O VI) approximate to 0.05-0.11, N(N V)/N(O VI) approximate to 0.07-0.13, and N(Si IV)/N(Si III) approximate to 0.2.