The Milky Way halo as a QSO absorption-line system^⋆

New results from an HST/STIS absorption-line catalogue of Galactic high-velocity clouds

¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam-Golm, Germany
e-mail: pherenz@astro.physik.uni-potsdam.de
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
⁴ Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received: 10 October 2012
Accepted: 17 December 2012

Abstract

We use archival UV absorption-line data from HST/STIS to statistically analyse the absorption characteristics of the high-velocity clouds (HVCs) in the Galactic halo towards more than 40 extragalactic background sources. We determine absorption covering fractions of low- and intermediate ions (O i, C ii, Si ii, Mg ii, Fe ii, Si iii, C iv, and Si iv) in the range f_c = 0.20−0.70. For detailed analysis we concentrate on Si ii absorption components in HVCs, for which we investigate the distribution of column densities, b-values, and radial velocities. Combining information for Si ii and Mg ii, and using a geometrical HVC model we investigate the contribution of HVCs to the absorption cross section of strong Mg ii absorbers in the local Universe. We estimate that the Galactic HVCs would contribute on average  ~52 percent to the total strong Mg ii cross section of the Milky Way, if our Galaxy were to be observed from an exterior vantage point. We further estimate that the mean projected covering fraction of strong Mg ii absorption in the Milky Way halo and disc from an exterior vantage point is ⟨f_c,sMgII⟩ = 0.31 for a halo radius of R = 61 kpc. These numbers, together with the observed number density of strong Mg ii absorbers at low redshift, indicate that the contribution of infalling gas clouds (i.e., HVC analogues) in the halos of Milky Way-type galaxies to the cross section of strong Mg ii absorbers is  < 34 percent. These findings are in line with the idea that outflowing gas (e.g., produced by galactic winds) in the halos of more actively star-forming galaxies dominate the absorption-cross section of strong Mg ii absorbers in the local Universe.

We use archival UV absorption-line data from HST/STIS to statistically analyse the absorption characteristics of the high-velocity clouds (HVCs) in the Galactic halo towards more than 40 extragalactic background sources. We determine absorption covering fractions of low- and intermediate ions (O i, C ii, Si ii, Mg ii, Fe ii, Si iii, C iv, and Si iv) in the range f_c = 0.20−0.70. For detailed analysis we concentrate on Si ii absorption components in HVCs, for which we investigate the distribution of column densities, b-values, and radial velocities. Combining information for Si ii and Mg ii, and using a geometrical HVC model we investigate the contribution of HVCs to the absorption cross section of strong Mg ii absorbers in the local Universe. We estimate that the Galactic HVCs would contribute on average  ~52 percent to the total strong Mg ii cross section of the Milky Way, if our Galaxy were to be observed from an exterior vantage point. We further estimate that the mean projected covering fraction of strong Mg ii absorption in the Milky Way halo and disc from an exterior vantage point is ⟨f_c,sMgII⟩ = 0.31 for a halo radius of R = 61 kpc. These numbers, together with the observed number density of strong Mg ii absorbers at low redshift, indicate that the contribution of infalling gas clouds (i.e., HVC analogues) in the halos of Milky Way-type galaxies to the cross section of strong Mg ii absorbers is  < 34 percent. These findings are in line with the idea that outflowing gas (e.g., produced by galactic winds) in the halos of more actively star-forming galaxies dominate the absorption-cross section of strong Mg ii absorbers in the local Universe.