The UCSD Radio‐selected Quasar Survey for Damped Lyα Systems

As large optical quasar surveys for damped Lya become a reality and the studyof star forming gas in the early Universe achieves statistical robustness, itis now vital to identify and quantify the sources of systematic error. Becausethe nature of optically-selected quasar surveys makes them vulnerable to dustobscuration, we have undertaken a radio-selected quasar survey for damped Lyasystems to address this bias. We present the definition and results of thissurvey. We then combine our sample with the CORALS dataset to investigate theHI column density distribution function f(N) of damped Lya systems towardradio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is inexcellent agreement with that of optically-selected samples at low N(HI), animportant yet expected result given that obscuration should have negligibleeffect at these gas columns. However, because of the relatively small size ofthe radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is notwell constrained at large N(HI) and the first moment of the HI distributionfunction, Omega_g, is, strictly speaking, a lower limit. The power-law is steepenough, however, that extrapolating it to higher column densities implies onlya modest, logarithmic increase in Omega_g. The radio-selected value of Omega_g= 1.15 x 10^-3, agrees well with the results of optically-selected surveys.While our results indicate that dust obscuration is likely not a major issuefor surveys of damped Lya systems, we estimate that a radio-selected sample ofapproximately 100 damped Lya systems will be required to obtain the precisionnecessary to absolutely confirm an absence of dust bias.Comment: 12 pages, 9 Figures. Accepted to ApJ April 11, 200