The UCSD Radio-Selected Quasar Survey for Damped Lyman alpha System

Abstract

As large optical quasar surveys for damped Lya become a reality and the study of star forming gas in the early Universe achieves statistical robustness, it is now vital to identify and quantify the sources of systematic error. Because the nature of optically-selected quasar surveys makes them vulnerable to dust obscuration, we have undertaken a radio-selected quasar survey for damped Lya systems to address this bias. We present the definition and results of this survey. We then combine our sample with the CORALS dataset to investigate the HI column density distribution function f(N) of damped Lya systems toward radio-selected quasars. We find that f(N) is well fit by a power-law f(N) = k_1 N^alpha_1, with log k_1 = 22.90 and alpha_1 = -2.18. This power-law is in excellent agreement with that of optically-selected samples at low N(HI), an important yet expected result given that obscuration should have negligible effect at these gas columns. However, because of the relatively small size of the radio-selected sample, 26 damped Lya systems in 119 quasars, f(N) is not well constrained at large N(HI) and the first moment of the HI distribution function, Omega_g, is, strictly speaking, a lower limit. The power-law is steep enough, however, that extrapolating it to higher column densities implies only a 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 issue for surveys of damped Lya systems, we estimate that a radio-selected sample of approximately 100 damped Lya systems will be required to obtain the precision necessary to absolutely confirm an absence of dust bias.