Discovery of damped Ly alpha 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 Ly alpha (DLA) absorption systems with neutral hydrogen column densities N(HI) greater than or equal to 2 x 10(20) 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 Ly alpha 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(0)(lambda 2796), is known. Therefore, by observing the Ly alpha 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)(lambda 2796) greater than or equal to 0.3 Angstrom. 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)(lambda 2796) greater than or equal to 0.6 Angstrom. Moreover, half of all absorption systems with both Mg II W(0)(lambda 2796) and Fe II W(0)(lambda 2600) > 0.5 Angstrom 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 approximate to 2 or possibly does not set in until z approximate to 0.5. (3) On the other hand, the cosmological mass density of neutral gas in low-redshift DLA absorbers, Omega (DLA), is observed to be comparable to that observed at high redshift. In particular, there is no observed trend that would indicate that Omega (DLA) at low redshift is approaching the value at z = 0, which is a factor of approximate to4-6.5 lower than Omega (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 approximate toN(HI)(-3) 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 Omega (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.