7 Summary and discussion

We have presented the first results from the CORALS survey for DLAs in a radio-selected sample of QSOs. The sample consists of 66 $z_{\rm em} \geq 2.2$ QSOs, 58 of which have been observed by us using the ESO 3.6 m, the AAT and the VLT facilities, while the remaining eight were culled from the literature. All the new spectra are presented in Fig. 1. A total of 22 DLAs has been identified, 19 of which are at intervening redshifts.

We find that the comoving mass density of neutral gas implied by these DLAs is $\log \Omega_{\rm DLA} h = -2.59^{+0.17}_{-0.24}$ at a mean $\langle z_{\rm abs} \rangle= 2.37$, in good agreement with previous surveys. Similarly, the number density of DLAs per unit redshift in radio-selected QSOs, n(z) =0.31^+0.09_-0.08, is only $\sim$1$\sigma$ higher than that in optical, magnitude-limited samples. Within our own sample we also find that n(z) is higher, but again by only $\sim$1$\sigma$, in QSOs with B > 20, compared with sightlines towards brighter quasars. These results indicate that at redshifts z = 2-3.5 DLA surveys using optically selected QSOs probably underestimate the number of DLAs, and the gas mass they trace, by no more than a factor of about 2, in broad agreement with the predictions by Pei & Fall (1995). In particular, we have not uncovered a population of high column density $N\rm (H~I) > 10^{21}$cm^-2absorbers which had been missed in previous searches limited to QSOs brighter than $B \simeq 20$.

These conclusions are somewhat tentative because of the small size of the CORALS sample. Our value of $\Omega _{\rm DLA}$ is dominated by two very high column density systems, both of which occur in moderately bright ( B = 19.5, 20) QSOs, and the column density distribution function of DLAs is clearly not well sampled with the relatively small number of QSOs in our survey.

The next important step in this work is to determine the metallicities and dust content of CORALS DLAs. The possibility of a dust bias in DLA selection has previously been appealed to in order to explain the lack of high column density, metal-rich absorbers (e.g. Prantzos & Boissier 1999). This explanation now seems less likely, regardless of whether or not CORALS DLAs prove to be more metal-rich, simply because high column density DLAs do not appear to be significantly more common in fainter QSOs, at least within the statistical limitations of our survey. Nonetheless, determining the metallicities of the new DLAs discovered here remains an important goal, because it will allow us to assess whether the low element abundances found so far are indeed typical of the full DLA population.

What could then be the reason for the observed dearth of high column density, metal-rich DLAs? We consider it unlikely that gravitational lensing may be the answer. In principle one may conjecture that close alignment of QSOs with foreground galaxies may produce a tendency for such sightlines to be deflected away from the inner regions of galaxies, where interstellar clouds with high N(H I) and high Z may be preferentially intercepted. However, to date no statistical evidence for lensing of QSOs by DLAs has been found (Le Brun et al. 2000) and, in any case, lensing would be most effective at significantly lower redshifts than those considered here (Smette et al. 1997).

A more plausible explanation is that there is simply a cross-sectional bias against detecting DLAs in sightlines that pass through the centres of galaxies. Observationally, one could argue that high N(H I), high Z DLAs have already been found in the Lyman break galaxies (e.g. Pettini et al. 2000) which indeed have typical linear sizes one order of magnitude smaller than the impact parameters of most DLAs (Giavalisco et al. 1996; Calzetti & Giavalisco 2000; see also Fig. 1 of Pettini 2001). Theoretical studies of DLAs also support this interpretation. For example, the models of Mathlin et al. (2001), who simulated DLA surveys by sampling model galaxies at random impact parameters, predict that the locus of high column density and metal rich absorbers should be populated, but that DLAs with these properties are intrinsically rare due to the small cross-sectional area presented by the inner galactic regions where they are found.

As already emphasised, the CORALS data set is too small to sample properly the column density distribution function and a considerably larger survey is required in order to provide the statistical coverage that will determine the true incidence of N(H I) > 10²¹ cm^-2 absorbers. A statistically larger survey will not only improve our determinations of $\Omega _{\rm DLA}$ and n(z), but will also make it possible to investigate the evolution (or lack thereof) of these quantities with redshift. It will be particularly interesting to examine the possibility of higher dust bias at larger redshifts as suggested by the data in Fig. 3; such data will offer an insight into the evolution of dust at early epochs. One promising prospect for extending the work presented here is the FIRST QSO survey (Gregg et al. 1996; White et al. 2000). Although spectral follow-up has so far been limited to bright R < 19 targets, future follow-up of FIRST sources at fainter optical magnitudes would provide an excellent complement to the CORALS survey.

Acknowledgements

It is a pleasure to acknowledge the consistent support of this project by the ESO and AAT Time Assignment Panels and the professional and efficient help of the telescope staff at the AAT, ESO 3.6-m and VLT. In particular, we are grateful to the ESO Paranal science operations staff for their expert execution of our service observations. We thank Mauro Giavalisco and Lisa Storrie-Lombardi for obtaining spectra of two of our targets and Joop Schaye for useful comments on an earlier draft of this paper.