Discovery of DampedLyα Systems at Redshifts Less than 1.65 and Results on Their Incidence and Cosmological Mass Density^*

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 < 1.65. In the past, identification of DLA systems at z < 1.65 has been difficult because of their rare incidence and the need for UV spectroscopy to detect Lyα absorption at these low redshifts. Our survey relies on the fact that all known DLA systems have corresponding Mg II absorption. In turn, Mg II absorption systems have been well studied, and their incidence at redshifts 0.1 < z < 2.2 as a function of the Mg II rest equivalent width, W, is known. Therefore, by observing the Lyα line corresponding to identified low-redshift Mg II systems and determining the fraction of these that are damped, we have been able to infer the statistical properties of the low-redshift DLA population. In an earlier (1995) paper we presented initial results from an archival study with data from the Hubble Space Telescope (HST) and IUE. Now, with new data from our HST GO program, we have more than doubled the sample of Mg II systems with available ultraviolet spectroscopic data. In total we have uncovered 12 DLA lines in 87 Mg II systems with W ≥ 0.3 Å. Two more DLA systems were discovered serendipitously in our HST spectra. At the present time the total number of confirmed DLA systems at redshifts z < 1.65 is 23.

     The significant results of the survey are as follows. (1) The DLA absorbers are drawn almost exclusively from the population of Mg II absorbers that have W ≥ 0.6 Å. Moreover, half of all absorption systems with both Mg II W and Fe II W > 0.5 Å are DLA systems. (2) The incidence of DLA systems per unit redshift, n_DLA, 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, Ω_DLA, is observed to be comparable to that observed at high redshift. In particular, there is no observed trend that would indicate that Ω_DLA at low redshift is approaching the value at z = 0, which is a factor of ≈4-6.5 lower than Ω_DLA. (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 n_DLA and Ω_DLA 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.