2006MNRAS.371..356C

In this paper, we investigate the possible reasons why Hi 21-cm absorption in damped Lyman α systems (DLAs) has only been detected at low redshift. To date, no 21-cm absorption has yet been detected at z_abs> 2.3, and at redshifts less than this, there is a mix of detections and non-detections in the DLAs searched. This has been attributed to the morphologies of the galaxies hosting the DLAs, where at low redshift the DLAs comprise both large and compact galaxies, which are believed to have low and high spin temperatures, respectively. Likewise, at high redshift the DLA population is believed to consist exclusively of compact galaxies of high spin temperature. However, in a previous paper, we found that by not assuming or assigning an often uncertain value for the coverage of the radio continuum source by the 21-cm absorbing gas, that there is generally no difference in the spin temperature/covering factor ratio between the 21-cm detections and non-detections or between the low- and high-redshift samples. Furthermore, only one of the 18 non-detections has a known host morphology, thus making any link between morphology and 21-cm delectability highly speculative.

We suggest that the lack of 21-cm absorption detections at high redshift arises from the fact that these DLAs are at similar angular diameter distances to the background quasars (i.e. the distance ratios are always close to unity). Above z_abs∼ 1.6, the covering factor becomes largely independent of the DLA-QSO distance, making the high-redshift absorbers much less effective at covering the background continuum emission. At low redshift, small distance ratios are strongly favoured by the 21-cm detections, whereas large ratios are favoured by the non-detections. This mix of distance ratios gives the observed mix of detections and non-detections at z_abs≲ 1.6. In addition to the predominance of large distance ratios and non-detections at high redshift, this strongly suggests that the observed distribution of 21-cm absorption in DLAs is dominated by geometric effects.