Discovery of Damped Lyα Systems at Redshifts Less than 1.65 and Results on Their Incidence and Cosmological Mass Density

Abstract

We present results from an efficient, nontraditional survey to discover damped Lyα (DLA) absorption systems with neutral hydrogen column densities N ≥ 2 × 1020 atoms cm-2 and redshifts z 0.5 A are DLA systems. (2) The incidence of DLA systems per unit redshift, nDLA, decreases as a function of decreasing redshift. The low-redshift data are consistent with the larger incidence of DLA systems seen at high redshift and the inferred low incidence for DLA at z = 0 derived from 21 cm observations of gas-rich spirals. However, the errors in our determination are large enough that it is not clear if the decrease per comoving volume begins to be significant at z ≈ 2 or possibly does not set in until z ≈ 0.5. (3) On the other hand, the cosmological mass density of neutral gas in low-redshift DLA absorbers, ΩDLA, is observed to be comparable to that observed at high redshift. In particular, there is no observed trend that would indicate that ΩDLA at low redshift is approaching the value at z = 0, which is a factor of ≈4-6.5 lower than ΩDLA. (4) The low-redshift DLA absorbers exhibit a larger fraction of very high column density systems in comparison to determinations both at high redshift and at z = 0. In addition, at no redshift is the column density distribution of DLA absorbers observed to fall off in proportion to ~N with increasing column density, a trend that is theoretically predicted for disklike systems. We discuss this and other mounting evidence that DLA absorption arises not solely in luminous disks but in a mixture of galaxy types. Although we have doubled the sample of confirmed low-redshift DLA systems, we are still confronted with the statistics of small numbers. As a result, the errors in the low-redshift determinations of nDLA and ΩDLA are substantial. Therefore, aside from the above evolutionary trends, we also discuss associated limitations caused by small-number statistics and the robustness of our results. In addition, we note concerns due to gravitational lensing bias, reliance on the Mg II statistics, dust obscuration, and the sensitivity of local H I 21 cm emission surveys.