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

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
² SIRTF Science Center, Caltech, California, USA e-mail: lyan@ipac.caltech.edu
³ Astrophysics: Department of Physics, Nuclear and Astrophysics Laboratory, Keble Road, Oxford, OX1 3RH, UK e-mail: ihook@gemini.edu
⁴ Institute of Astronomy, Madingley Rd., Cambridge, CB3 0HA, UK e-mail: pettini@ast.cam.ac.uk
⁵ Astrophysics: Department of Physics, Nuclear and Astrophysics Laboratory, Keble Road, Oxford, OX1 3RH, UK e-mail: jvw@astro.ox.ac.uk
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany e-mail: pshaver@eso.org

Corresponding author: S. Ellison, sellison@eso.org

Received: 19 June 2001
Accepted: 12 September 2001

Abstract

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 QSOs in which 22 DLAs with absorption redshifts have been identified over a total redshift interval . Three of the DLAs are classified as "associated" systems with ; 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) , and a number density of DLAs per unit redshift , both at a mean redshift . 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 is greater in fainter () QSOs, the effect is only at the ~1σ level and we have not uncovered a previously unrecognised population of high column density ( 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.