A deep search for 21-cm absorption in high redshift damped Lyman-α systems

We present deep GMRT 21-cm absorption spectra of 10 damped Lyman-alpha systems (DLAs), of which 8 are at redshifts zgreater than or similar to1.3. HI absorption was detected in only one DLA, the z=0.5318 absorber toward PKS 1629+12. This absorber has been identified with a luminous spiral galaxy; the spin temperature limit (T(s)less than or equal to310 K) derived from our observations continues the trend of DLAs associated with bright spirals having low spin temperatures. In seven of the remaining 9 systems, the observations place strong lower limits on the spin temperature of the HI gas. We combine this sample with data taken from the literature to study the properties of all known DLAs with 21-cm absorption studies. The sample of DLAs which have been searched for 21-cm absorption now consists of 31 systems, with T-s estimates available in 24 cases; of these, 16 are at z<2 and 8 at z>2, with 11 (all at z<1) having optical identifications. For the latter 11 DLAs, we find that all of the low T-s DLAs have been identified with large, luminous galaxies, while all the DLAs with high spin temperature (T-s greater than or similar to 1000 K) have been identified either with LSBs or dwarfs. Further, we find no correlation between impact parameter and spin temperature; it is thus unlikely that the high measured T-s values for DLAs arise from lines of sight passing through the outskirts of large disk galaxies. Instead, the spin temperature of DLAs appears to correlate with the host galaxy type. The trend (noted earlier by Chengalur & Kanekar 2000) that low z DLAs exhibit both high and low T-s values while high redshift (z greater than or similar to 3) DLAs only show high spin temperatures is present in this expanded data set. Based on this difference in spin temperatures, the Gehan test rules out the hypothesis that DLAs at z>2 and DLAs at z<2 are drawn from the same parent population at similar to 99% confidence level. Finally, we use the new GMRT spectra along with 2 spectra from the literature to estimate upper limits on the fraction of cold HI, f(CNM), in DLAs at z greater than or similar to 3. For local spirals, f(CNM) similar to 0.5; in contrast, we find that f(CNM)<0.3 in all 7 high z absorbers, and f(CNM)<0.1 in 5 of the 7 cases.