THE UCSD RADIO-SELECTED QUASAR SURVEY FOR DAMPED Ly SYSTEMS

As large optical quasar surveys for DLAs 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 DLAs to address this bias. We present the definition and results of this survey. We then combine our sample with the CORALS data set to investigate the H i column density distribution function fH i(N;X) of DLAs toward radio-selected quasars.We find that fH i(N;X) is well fitted by a power law fH i(N;X) k1N 1, with log k1 22:90 and 1 2:18þ0:200:26. This power law is in excellent agreement with that of optically selected sam-ples at low NH i, 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 DLAs in 119 quasars, fH i(N;X) is not well constrained at large NH i, and the first moment of the H i distribution function, 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 g. The radio-selected value of g 1:15þ0:370:38; 103 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 DLAs, we estimate that a radio-selected sample of 100 DLAs will be required to obtain the precision necessary to absolutely confirm an absence of dust bias. Subject headinggs: galaxies: evolution — intergalactic medium — quasars: absorption lines Online material: color figures 1