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- Spin temperatures and covering factors for HI 21-cm absorption in damped Lyman α systems
- Curran, Stephen J.; Murphy, Michael T.; Pihlstrom, Ylva M.; Webb, J. K.; Purcell, Cormac R.
- We investigate the practice of assigning high spin temperatures to damped Lyman α absorption systems (DLAs) not detected in H i 21-cm absorption. In particular, Kanekar & Chengalur have attributed the mix of 21-cm detections and non-detections in low-redshift (zabs≤ 2.04) DLAs to a mix of spin temperatures, while the non-detections at high redshift were attributed to high spin temperatures. Below zabs= 0.9 , where some of the DLA host galaxy morphologies are known, we find that 21-cm absorption is normally detected towards large radio sources when the absorber is known to be associated with a large intermediate (spiral) galaxy. Furthermore, at these redshifts, only one of the six 21-cm non-detections has an optical identification and these DLAs tend to lie along the sight-lines to the largest background radio continuum sources. For these and many of the high-redshift DLAs occulting large radio continua, we therefore expect covering factors of less than the assumed/estimated value of unity. This would have the effect of introducing a range of spin temperatures considerably narrower than the current range of ΔTs≳ 9000 K , while still supporting the hypothesis that the high-redshift DLA sample comprises a larger proportion of compact galaxies than the low-redshift sample.
- Publication type
- Journal article
- Monthly Notices of the Royal Astronomical Society, Vol. 356, no. 4 (Feb 2005), pp. 1509-1518
- Publication year
- Wiley-Blackwell Publishing
- Publisher URL
- Copyright © 2004 the authors. Journal compilation copyright © 2004 Royal Astronomical Society. The accepted manuscript of the paper is reproduced here in accordance with the copyright policy of the publisher. The definitive publication is available at www.interscience.wiley.com.