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Home List of Titles 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 Hi spectra towards QSOs
Please use this identifier to cite or link to this item: http://hdl.handle.net/1959.3/227612
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- 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 Hi spectra towards QSOs
- Ben Bekhti, N.; Winkel, B.; Richter, P.; Kerp, J.; Klein, U.; Murphy, M. T.
- 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.
- Publication type
- Journal article
- Research centre
- Swinburne University of Technology. Faculty of Information and Communication Technologies. Centre for Astrophysics and Supercomputing
- Astronomy and Astrophysics, Vol. 542 (Jun 2012), article no. A110
- Publication year
- FOR Code(s)
- 0201 Astronomical and Space Sciences
- Absorption lines; Galaxies; Haloes; ISM structure; Milky Way halo
- EDP Sciences
- Publisher URL
- Copyright © 2012 ESO. The published version is reproduced in accordance with the copyright policy of the publisher.
- Full text
- Peer reviewed