Relationships between the HI 21-cm line strength, MgII equivalent width and metallicity in damped Lyman α absorption systems

We present the results of a survey for 21-cm absorption in four never previously searched damped Lyman alpha absorption systems ( DLAs) with the Westerbork Synthesis Radio Telescope. The one detection is presented and discussed in Curran et al. and here we add our results to other recent studies in order to address the important issues regarding the detection of cold gas, through 21-cm absorption, in DLAs. Although, due to the DLAs identified with spiral galaxies, there is a mix of spin temperature/covering factor ratios at low redshift, two recent high redshift end points confirm that this ratio does not generally rise much above T-spin/f similar to 10(3) K over the whole redshift range searched ( up to z(abs) = 3.39). That is, if the covering factors of many of these galaxies were a factor of >= 2 smaller than for the spirals (which span 120 <= T-spin/ f <= 520 K), then no significant difference in the spin temperatures between these two classes would be required. Furthermore, although it is difficult to separate the relative contributions of the spin temperature and covering factor, the new results confirm that 21-cm detections tend to occur at low angular diameter distances, where the coverage of a given absorption cross-section is maximized. This indicates a dominant contribution by the covering factor. Indeed, the two new high redshift detections occur towards two extremely compact radio sources (<= 0.04 arcsec), although the one other new detection, which may have an impact parameter in excess of 75 kpc, occurs towards one of the largest radio sources. Finally, we also find an apparent 21-cm line strength-Mg II equivalent width correlation, which appears to be due to a coupling of the velocity structure between the components that each species traces. That is, the gas seen in 21-cm absorption could be the same as that seen in optical absorption. Combined with the known equivalent width-metallicity relation, this may be manifest as a spin temperature-metallicity anticorrelation, which is non-evolutionary in origin.