The CORALS survey I: New estimates of the number density and gas content of damped Lyman alpha systems free from dust bias

We present the first results from the Complete Optical and Radio Absorption Line System (CORALS) survey. We have compiled a homogeneous sample of radio-selected QSOs from the Parkes Catalogue and searched for damped Lyman alpha systems (DLAs) towards every target, irrespective of its optical magnitude. This approach circumvents selection effects - particularly from intervening dust - which have long been suspected to affect DLA surveys in optically-selected, magnitude-limited QSO samples. The CORALS data set consists of 66 z_em >= 2.2 QSOs in which 22 DLAs with absorption redshifts 1.8 <= z_abs <= z_em have been identified over a total redshift interval Delta z = 55.46. Three of the DLAs are classified as ``associated'' systems with z_abs ~ z_em; of the 19 intervening DLAs, 17 are new discoveries. In this first paper of the CORALS series we describe the sample, present intermediate resolution spectroscopy and determine the population statistics of DLAs. We deduce a value of the neutral gas mass density traced by DLAs (expressed as a fraction of the closure density) log Omega_DLA h = -2.59^+0.17_-0.24, and a number density of DLAs per unit redshift n(z) = 0.31^+0.09_-0.08, both at a mean redshift < z > = 2.37. Both values are only marginally higher than those measured in optically selected samples of QSOs. Taking into account the errors, we conclude that dust-induced bias in previous surveys may have led to an underestimate of these quantities by at most a factor of two. While n(z) is greater in fainter (B > 20) QSOs, the effect is only at the ~ 1sigma level and we have not uncovered a previously unrecognised population of high column density (N(H I) > 10²¹ cm^-2) DLAs in front of faint QSOs. These conclusions are tentative because of the limited size of our data set; in particular the distribution of column densities is poorly sampled at the high end where a much larger survey of radio-selected QSOs is required the improve the statistics. The work presented here is based in part on data obtained with the ESO facilities on La Silla (EFOSC/3.6-m) and Paranal (FORS1/UT1). Figure 1 is only available in electronic form at http://www.edpsciences.org