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Title:
Spin temperatures and covering factors for HI 21-cm absorption in damped Lyman α systems
Authors:
Curran, S. J.; Murphy, M. T.; Pihlström, Y. M.; Webb, J. K.; Purcell, C. R.
Affiliation:
AA(School of Physics, University of New South Wales, Sydney NSW 2052, Australia), AB(Institute of Astronomy, Madingley Road, Cambridge CB3 0HA), AC(National Radio Astronomy Observatory, PO Box 0, Socorro, NM87801, USA), AD(School of Physics, University of New South Wales, Sydney NSW 2052, Australia), AE(School of Physics, University of New South Wales, Sydney NSW 2052, Australia)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 356, Issue 4, pp. 1509-1518. (MNRAS Homepage)
Publication Date:
02/2005
Origin:
MNRAS
MNRAS Keywords:
galaxies: ISM, quasars: absorption lines, cosmology: observations, early Universe
DOI:
10.1111/j.1365-2966.2004.08594.x
Bibliographic Code:
2005MNRAS.356.1509C

Abstract

We investigate the practice of assigning high spin temperatures to damped Lyman α absorption systems (DLAs) not detected in HI 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.

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