3 The CORALS survey sample

We have based our new survey for DLAs on the radio-selected Parkes quarter-Jansky sample (Shaver et al. 1996; Jackson et al. in prep.; Hook et al. in prep.). This complete sample of flat-spectrum radio sources was taken from the Parkes Catalogue (Wright & Otrupcek 1990), which consists of radio and optical data for almost 8300 radio sources for the sky south of declination +27deg. Over most of its area, it is complete to $S_{2.7}~\rm GHz = 0.25$ Jy. The Parkes quarter-Jansky sample consists of all (878 in total) flat-spectrum ( $\alpha > -0.4$, measured at 2.7 and 5.0 GHz) sources with declinations between +2.5$^{\circ}$ and $-80^{\circ}$, excluding low galactic latitudes ( $\mid b\mid\, < 10^{\circ}$) and regions around the Magellanic Clouds. The sample is complete to $S_{\rm 2.7}$ GHz = 0.25 Jy over 4.39 sr and to 0.60 Jy over a further 1.16 sr.

Using this Parkes compilation of radio sources as the parent sample, optical identification and B-band magnitudes for all targets with $\delta > -45^{\circ}$ were achieved using the COSMOS Southern Sky Catalogue (Drinkwater et al. 1995) in the first instance or imaging at the ESO 3.6-m telescope at La Silla for the faintest QSOs. Therefore, optical counterparts were identified for every radio source in the Parkes compilation. From this complete sample of 878 sources with no optical magnitude limit or other selection bias, low resolution spectra (FWHM = 12-14 Å) were obtained for the 442 stellar identifications (QSOs and BL Lacs) with the EFOSC on the ESO 3.6-m to determine redshifts. We note that since compiling the CORALS sample, the catalogue of PKS sources has been revised slightly to include some extra targets (Jackson et al. in prep.; Hook et al. in prep.). However, it is important to stress that this will not affect the results presented here, since the important factor in our sample definition is that it is optically complete.

In our ground-based search for DLAs in this sample we restricted ourselves to QSOs with $z_{\rm em} \geq 2.2$, so as to be able to record a sufficient portion of the spectrum blueward of the Ly$\alpha$ emission down to the onset of the atmospheric cut-off. This left us with a final sample of 66 QSOs in which we could search for DLAs in the range $1.8 < z_{\rm abs} < z_{\rm em}$ with sufficient sensitivity to measure reliable values of the column density N(H I).