Ca II and Na I absorption signatures from extraplanar gas in the halo of the Milky Way^*

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: nbekhti@astro.uni-bonn.de
² Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany e-mail: prichter@astro.physik.uni-potsdam.de
³ Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia e-mail: tobias.westmeier@csiro.au
⁴ Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia e-mail: mmurphy@swin.edu.au

Received: 14 November 2007
Accepted: 11 June 2008

Abstract

Aims. We analyse absorption characteristics and physical conditions of extraplanar intermediate- and high-velocity gas to study the distribution of the neutral and weakly ionised Milky Way halo gas and its relevance for the evolution of the Milky Way and other spiral galaxies.

Methods. We combine optical absorption line measurements of / and 21 cm emission line observations of  along 103 extragalactic lines of sight towards quasars (QSOs) and active galactic nuclei (AGN). The archival optical spectra were obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope, while the 21 cm  observations were carried out using the 100-m radio telescope at Effelsberg.

Results. The analysis of the UVES spectra shows that single and multi-component / absorbers at intermediate and high velocities are present in about 35 percent of the sight lines, indicating the presence of neutral extraplanar gas structures. In some cases the / absorption is connected with  21 cm intermediate- or high-velocity gas with  column densities in the range of 10¹⁸ to  (i.e., the classical IVCs and HVCs), while other / absorbers show no associated  emission. The observed  line widths vary from  km s^-1 to 32.0 km s^-1 indicating a range of upper gas temperature limits of 250 K up to about 22 500 K.

Conclusions. Our study suggests that the Milky Way halo is filled with a large number of neutral gaseous structures whose high column density tail represents the population of common  high-velocity clouds seen in 21 cm surveys. The  column density distribution follows a power-law with a slope of , thus comparable to the distribution found for intervening metal-line systems toward QSOs. Many of the statistical and physical properties of the  absorbers resemble those of strong ( Å)  absorbing systems observed in the circumgalactic environment of other galaxies, suggesting that both absorber populations may be closely related.