EDP Sciences
Free access
Volume 487, Number 2, August IV 2008
Page(s) 583 - 594
Section Galactic structure, stellar clusters, and populations
DOI http://dx.doi.org/10.1051/0004-6361:20079067
Published online 24 June 2008

A&A 487, 583-594 (2008)
DOI: 10.1051/0004-6361:20079067

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

N. Ben Bekhti1, P. Richter2, T. Westmeier3, and M. T. Murphy4

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

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 $\ion{Ca}{ii}$/ $\ion{Na}{i}$ and 21 cm emission line observations of $\ion{H}{i}$ 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 $\ion{H}{i}$ 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 $\ion{Ca}{ii}$/ $\ion{Na}{i}$ 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 $\ion{Ca}{ii}$/ $\ion{Na}{i}$ absorption is connected with $\ion{H}{i}$ 21 cm intermediate- or high-velocity gas with $\ion{H}{i}$ column densities in the range of 1018 to  $10^{20}\,\mathrm{cm}^{-2}$ (i.e., the classical IVCs and HVCs), while other $\ion{Ca}{ii}$/ $\ion{Na}{i}$ absorbers show no associated $\ion{H}{i}$ emission. The observed $\ion{H}{i}$ line widths vary from $\Delta v_{\it FWHM}=3.2$ 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 $\ion{H}{i}$ high-velocity clouds seen in 21 cm surveys. The $\ion{Ca}{ii}$ column density distribution follows a power-law $f(N)=CN^{\beta}$ with a slope of $\beta \approx -1.6$, thus comparable to the distribution found for intervening metal-line systems toward QSOs. Many of the statistical and physical properties of the $\ion{Ca}{ii}$ absorbers resemble those of strong ( $W_\mathrm{\lambda 2796}>0.3\,\AA{}$) $\ion{Mg}{ii}$ absorbing systems observed in the circumgalactic environment of other galaxies, suggesting that both absorber populations may be closely related.

Key words: Galaxy: halo -- ISM: structure -- quasars: absorption lines -- galaxies: halo

© ESO 2008

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