Implications of 21-cm observations for damped Lyα systems

We present Giant Metrewave Radio Telescope HI 21-cm absorption observations, of candidate and confirmed damped Lyα systems. The derived spin temperatures Tspin are high, in all cases, ∼1000 K or higher. We have also collated from the published literature a list of damped absorbers for which 21-cm observations exist, and discuss the implications of the observations for the nature of these systems. A cross-comparison of the HI 21-cm profiles (which trace the cold gas) with the low ionization metal profiles (which trace all the neutral HI, both cold and warm) shows that in all cases for which both spectra are available, the 21-cm absorption coincides in velocity with the deepest metal line feature. This is consistent with models in which the deep metal line features arise from discrete clouds but not with models where the deepest features are the result of velocity crowding. We also find that the typical derived spin temperatures of damped Lyα systems are considerably higher than those in the Milky Way or nearby spiral galaxies. The only exceptions are systems that are known to be associated with the discs of spirals; these do, in fact, show low spin temperature. In a multi-phase medium the derived spin temperature is a weighted average of the temperatures of the individual phases. High apparent Tspin values are hence to be expected from small, low metallicity objects since these objects should (as per existing theoretical models of the formation of a multi-phase interstellar medium (ISM) in the Milky Way and high-redshift proto-galaxies) have a lower fraction of the cold phase in their ISM as compared with large galaxies. The high Tspin is hence consistent with the observed low metallicities of damped Lya absorbers as well as with recent findings that damped absorption is associated with a variety of galaxy types (as opposed to being confined to the discs of large spirals). Finally, although the number of systems for which observations are available is small, we suggest that the following trend may be identified: at low redshift, damped Lyα absorption arises from a range of systems, including spiral galaxy discs, while, at high redshift, absorption occurs predominantly in smaller systems.