An absorption-selected survey of neutral gas in the Milky Way halo

New results based on a large sample of Ca ii, Na i, and H i spectra towards QSOs^⋆

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: nbekhti@astro.uni-bonn.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
⁴ Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Hawthorn, 3122 Victoria, Australia

Received: 18 December 2011
Accepted: 22 March 2012

Abstract

Aims. We aim at analysing systematically the distribution and physical properties of neutral and mildly ionised gas in the Milky Way halo, based on a large absorption-selected data set.

Methods. Multi-wavelength studies were performed combining optical absorption line data of Ca ii and Na i with follow-up H i 21-cm emission line observations along 408 sight lines towards low- and high-redshift QSOs. We made use of archival optical spectra obtained with UVES/VLT. H i data were extracted from the Effelsberg-Bonn H i survey and the Galactic All-Sky survey. For selected sight lines we obtained deeper follow-up observations using the Effelsberg 100-m telescope.

Results. Ca ii (Na i) halo absorbers at intermediate and high radial velocities are present in 40–55% (20–35%) of the sightlines, depending on the column density threshold chosen. Many halo absorbers show multi-component absorption lines, indicating the presence of sub-structure. In 65% of the cases, absorption is associated with H i 21-cm emission. The Ca ii (Na i) column density distribution function follows a power-law with a slope of β ≈  −2.2 (−1.4).

Conclusions. Our absorption-selected survey confirms our previous results that the Milky Way halo is filled with a large number of neutral gas structures whose high column density tail represents the population of common H i high- and intermediate-velocity clouds seen in 21-cm observations. We find that Na i/Ca ii column density ratios in the halo absorbers are typically smaller than those in the Milky Way disc, in the gas in the Magellanic Clouds, and in damped Lyman α systems. The small ratios (prominent in particular in high-velocity components) indicate a lower level of Ca depletion onto dust grains in Milky Way halo absorbers compared to gas in discs and inner regions of galaxies.