Sandhya M. Rao

Damped Lyα Systems at z < 1.65: The Expanded Sloan Digital Sky Survey Hubble Space Telescope Sample

We present results of our Hubble Space Telescope Cycle 11 survey for low-redshift (z 0.1 ?. (2) The mean H I column density of Mg II systems with 0.3 ? ? W < 0.6 ? is = (9.7 ? 2.2) ? 1018 cm-2. For the larger REW systems in our sample, = (3.5 ? 0.7) ? 1020 cm-2. (3) The DLA incidence per unit redshift for 0 < z < 5 is nDLA(z) = n0(1 + z)?, where n0 = 0.044 ? 0.005 and ? = 1.27 ? 0.11. This parameterization is consistent with no evolution for z 2 (?? = 0.7, ?M = 0.3) but exhibits significant evolution for z 2. (4) The cosmological neutral gas mass density due to DLAs is constant in the redshift interval 0.5 < z < 5.0 to within the uncertainties, ?DLA ? 1 ? 10-3. This is larger than ?gas(z = 0) by a factor of ?2. (5) The slope of the H I column density distribution does not change significantly with redshift. However, the low-redshift distribution is marginally flatter due to the higher fraction of high column density systems in our sample. (6) Finally, using the precision of Mg II survey statistics, we show that under the assumption of constant DLA fraction and H I column density suggested by our current sample, there may be evidence of a decreasing ?DLA from z = 0.5 to 0.We present results of our HST Cycle 11 Survey for low-redshift (z 0.1 A. (2) The mean N(HI) of MgII systems with 0.3 A \le W2796 ~ 2. (4) Omega_{DLA} is constant for 0.5<z<5.0 to within the uncertainties. This is larger than Omega_{gas}(z=0) by a factor of ~2. (5) The slope of the N(HI) distribution does not change significantly with redshift. However, the low redshift distribution is marginally flatter due to the higher fraction of high N(HI) systems in our sample. (6) Finally, using the precision of MgII survey statistics, we find that there may be evidence of a decreasing Omega_{DLA} from z=0.5 to z=0. We reiterate the conclusion of Hopkins, Rao, & Turnshek that very high columns of neutral gas might be missed by DLA surveys because of their very small cross sections, and therefore, that Omega_{DLA} might not include the bulk of the neutral gas mass in the Universe. (Abridged)

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 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.

Mg II absorption systems in Sloan Digital Sky Survey QSO spectra

We present the results of a MgII absorption-line survey using QSO spectra from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than 0.3\AA and redshifts

Average extinction curves and relative abundances for quasi-stellar object absorption-line systems at 1 ≤zabs < 2

0.366 \le z \le 2.269

Low-Redshift Damped Lyα Galaxies toward the Quasars B2 0827+243, PKS 0952+179, PKS 1127–145, and PKS 1629+120

were identified and measured. We find that the

Neutral hydrogen in galaxies at the present epoch

\lambda2796

Lensing, reddening and extinction effects of Mg ii absorbers from z= 0.4 to 2

rest equivalent width (

Discovery of z = 0.0912 and z = 0.2212 Damped Lyα Absorption-Line Systems Toward the Quasar OI 363: Limits on the Nature of Damped Lyα Galaxies*

W_0^{\lambda2796}

A ground-based imaging study of galaxies causing damped Lyman α (DLA), sub-DLA and Lyman limit system absorption in quasar spectra

) distribution is described very well by an exponential function

A Groundbased Imaging Study of Galaxies Causing DLA, subDLA, and LLS Absorption in Quasar Spectra ⋆

\partial N/\partial W_0^{\lambda2796} = \frac{N^*}{W^*} e^{-\frac{W_0}{W^*}}