Open AccessSpin temperatures and covering factors for H i 21‐cm absorption in damped Lyman α systems
Author(s) -
Curran S. J.,
Murphy M. T.,
Pihlström Y. M.,
Webb J. K.,
Purcell C. R.
Publication year - 2005
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2004.08594.x
Subject(s) - redshift , physics , astrophysics , galaxy , spiral galaxy , absorption (acoustics) , spin (aerodynamics) , optics , thermodynamics
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 ( z abs ≤ 2.04) DLAs to a mix of spin temperatures, while the non‐detections at high redshift were attributed to high spin temperatures. Below z abs = 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 Δ T s ≳ 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.