Hubble-COS Observations of Galactic High-Velocity Clouds: Four AGN Sight Lines through Complex C

TL;DR

This study uses Hubble COS ultraviolet spectra and Green Bank 21-cm data to analyze the composition, ionization, and dynamics of high-velocity clouds in Complex C, revealing their metallicity, ionization states, and interactions with the Galactic halo.

Contribution

It provides detailed ionization and abundance measurements of Complex C HVCs using new high-quality UV spectra, improving understanding of their physical conditions and interactions.

Findings

Complex C has 0.1-0.3 solar metallicity.

Detected multiple ionization states indicating interactions with hot halo gas.

High-ion ratios suggest cooling interfaces between different gas phases.

Abstract

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 S/N, compared to previous HST spectra, provide better velocity definition of the HVC absorption, additional ionization species, and improved abundances in this halo gas. Complex C has a metallicity of 0.1-0.3 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 A) and G160M (1383-1796 A) gratings detect ultraviolet absorption lines from 8 elements in low ionization stages (O I, N I, C II, S II, Si II, Al II, Fe II, P II) and 3 elements in intermediate and high-ionization states (Si III, Si IV, C IV, N V). Our four AGN sight lines toward Mrk 817, Mrk 290, Mrk 876, and PG1259+593 have high-velocity H I and O VI column densities, log N_HI = 19.39-20.05 and log N_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 gas and collisionally ionized gas: N(C IV)/N(O VI) = 0.3-0.5, N(Si IV)/N(O VI) = 0.05-0.11, N(N V)/N(O VI) = 0.07-0.13, and N(Si IV)/N(Si III) = 0.2.