1 Introduction

The presence of intervening gas in the sightline to a high redshift QSO will cause absorption lines to be superimposed on the quasar's continuum. This relatively simple physical process allows us to study in detail the properties of gas clouds in front of QSOs, whether in the intergalactic medium or in the galaxies giving rise to high column density damped Lyman alpha Systems (DLAs). DLAs are one of the key components of the universe at high redshift because, although relatively rare, they account for most of the neutral gas available for star formation.

Measuring the redshift evolution of $\Omega _{\rm DLA}$ (the total amount of neutral gas traced by DLAs expressed as a fraction of the closure density) has traditionally been seen as a tool for probing the history of assembling galaxies and measuring the rate at which they convert gas into stars (e.g. Lanzetta et al. 1995; Pei & Fall 1995; Storrie-Lombardi et al. 1996; Storrie-Lombardi & Wolfe 2000; Rao & Turnshek 2000). Similarly, chemical abundances in DLAs have been used to trace the metallicity evolution of galaxies over most of the Hubble time (e.g. Pettini et al. 1997; Pettini et al. 1999; Prochaska & Wolfe 1999, 2000). However, the interpretation of both DLA abundances and $\Omega _{\rm DLA}$ as indicators of galaxy evolution is pivotal upon the assumption that DLAs are representative of the bulk of "normal'' galaxies at each redshift sampled. Therefore, it is rather surprising that, at least for current samples of DLAs, no strong evolution of either $\Omega _{\rm DLA}$ or metallicity ( $Z_{\rm DLA}$) has been revealed by the above studies from $z \sim 3.5$ down to $z \sim 0.5$. One possible explanation is that DLAs are representative of only a particular evolutionary stage in a galaxy's lifetime. Alternatively, current DLA samples may be incomplete due to a selection bias. In this paper, we specifically investigate the possibility that dust extinction may have produced incomplete samples of DLAs in previous surveys based on optically-selected QSOs.

DLAs appear to remain relatively metal-poor ( $Z_{\rm DLA} \approx \frac{1}{10} - \frac{1}{30} Z_{\odot}$) from $z \sim 3.5$ to $z \sim 0.5$(Pettini et al. 1999; Prochaska & Wolfe 2000). However, it has been noted by several authors that the absorbers with the highest values of hydrogen column density are all of low metallicity and dust content (e.g. Pettini et al. 1997; Boissé et al. 1998; Pettini et al. 1999; Prantzos & Boissier 2000; Savaglio 2001). Whilst relatively metal-rich damped systems do exist, they tend to have values of N(H I) towards the lower end of the range considered for DLAs, $N\rm (H~I)\geq 2 \times 10^{20}$ cm^-2. Boissé et al. (1998) and Prantzos & Boissier (2000) in particular have pointed out an apparent anti-correlation between N(H I) and metallicity. Since the census of metal abundances (using the column-density weighted average) is dominated by the highest N(H I) absorbers, such an anti-correlation could explain the lack of metallicity evolution in the present samples of DLAs. This anti-correlation between N(H I) and metallicity may reflect a bias against high-column-density, metal-rich DLAs with sufficient dust to obscure background QSOs from the eyes of magnitude-limited optical surveys.

Further evidence that optical magnitude-limited QSO samples may be incomplete due to dust bias comes from consideration of the spectral energy distribution of quasars with and without DLAs. Fall et al. (1989) and Pei et al. (1991) found evidence that QSOs with DLAs have systematically steeper continuum slopes than quasars with no damped absorber. A similar conclusion was reached by Carilli et al. (1998) who found that a high fraction of their "red'' QSO sample had associated 21 cm absorbers compared with QSOs selected solely for the presence of Mg II absorbers. By modelling the extinction effect of dust, Pei & Fall (1995) estimated that at z = 3 between 27% and 44% of QSOs and 23%-38% of DLAs may be missing from B-band selected quasar samples.

The objective of the Complete Optical and Radio Absorption Line System (CORALS) survey presented here is simple and straightforward: compile a homogeneous sample of radio-selected QSOs and obtain medium resolution spectra of every target, regardless of optical magnitude. In this way it should be possible to determine quantitatively the severity of the dust bias implied by the observations reviewed above. The present paper is the first in the CORALS series. Here we define the sample (Sect. 3), describe the observations and present the QSO spectra (Sect. 4), identify the DLAs (Sect. 5), and calculate their number density and $\Omega _{\rm DLA}$(Sect. 6).

We adopt an $\Omega_M = 1.0, \Omega_{\Lambda} = 0$ cosmology and use H₀ = 65 km s^-1 Mpc^-1 throughout the paper.