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

Abstract

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 zem ≥ 2.2 QSOs in which 22 DLAs with absorption redshifts 1.8 ≤ z abs ≤ zem have been identified over a total redshift interval ∆z = 55.46. Three of the DLAs are classified as "associated" systems with z abs ∼ zem; 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 ΩDLAh = −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 ∼1σ level and we have not uncovered a previously unrecognised population of high column density (N(H I) > 10 21 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.

Evidence for a magnitude-dependent bias in the Hamburg/ESO survey for damped Lyman–$\alpha$ systems

“…In the redshift range covered by the survey, 19 DLA systems have been discovered. Over the whole survey, we find that the number density n(z) and cosmological density of gas Ωgas have comparable values to the ones obtained by CORALS (Ellison et al 2001).…”

“…In the redshift range covered by the survey, 19 DLA systems have been discovered. Over the whole survey, we find that the number density n(z) and cosmological density of gas Ωgas have comparable values to the ones obtained by CORALS (Ellison et al 2001).However, the number densities of DLA systems n(z) in two sub-samples of equal absorption distance path defined by the magnitude of the background QSOs differ by a factor of ≈ 5. We estimate that the probability that n(z) is equal in the two sub-samples is < 0.003.…”

“…Direct evidence that dust may strongly affect the statistics of DLA systems is crucially 2 Smette et al Figure 1: Distribution of QSO magnitudes in the three major surveys for DLA systems at z abs ≈ 2: in Black, small hashing spacing: , the recently completed Hamburg/ESO survey, in Red, filled histograms: the CORALS survey (Ellison et al 2001), and in Green, large hashing spacing: the LBQS survey, whose results are part of the study of Wolfe et al (1995). Note the strong difference in BJ magnitude coverage between the three surveys.…”

“…Fig. 1); (ii) a comparison between optically-and radioselected DLA samples (Ellison et al 2001) does not provide strong constraints because of the small size of the radio-selected sample. Instead, the evidence for dust in DLA systems is mostly indirect (Pettini et al 1997;Boissé et al 1998;Ledoux et al 2003).…”

The neutral hydrogen cosmological mass density atz= 5

“…However, several studies have shown that most DLAs are not associated with significant amounts of dust (e.g. Murphy & Liske 2004;Vladilo, Prochaska & Wolfe 2008), and DLAs towards radio-selected QSOs, which are insensitive to the presence of dust, have a similar N H I distribution to those in optically selected QSOs (Ellison et al 2001;Jorgenson et al 2006). Pontzen & Pettini (2009) find that the cosmic H I mass density may be underestimated by 3-23 per cent at z ∼ 3 due to selection biases from dust.…”