The Evolution of Neutral Gas in the Universe as Traced by Damped Lyman Alpha Systems

We discuss our recent results on the statistical properties of damped Lyman alpha systems (DLAs) at low redshift (z<1.65) (Rao & Turnshek 2000). Contrary to expectations, we found that the cosmological neutral gas mass density as traced by DLAs, $\Omega_{DLA}$, does not evolve from redshifts $z \approx 4$ to $z \approx 0.5$ and that extrapolation to z=0 results in a value that is a factor of ~6.5 times higher than what is derived from galaxies at the current epoch using HI 21 cm emission measurements. We review the current status of HI measurements at low redshift and at the current epoch, and discuss possible causes of this discrepancy.Comment: 10 pages, 4 figures, uses newpasp.sty. To appear in ASP Conference Series, "Extragalactic Gas at Low Redshift", J. Mulchaey and J. Stocke, ed

Damped Lyman Alpha Surveys and Statistics - A Review

The history and progress of DLA research over the past several decades is reviewed here. Larger datasets and deeper surveys, particularly over the last couple of years, have improved our knowledge of the neutral gas content and distribution in the universe at all observable redshifts, including the present epoch. New results on the statistics of DLAs at z<1.65 from our HST-UV surveys are presented and discussed in the context of recent results at z=0 and at high redshift. We find that Omega_{DLA}(z>0) remains roughly constant to within the uncertainties; the z=0 value of the neutral gas mass density, Omega_{g}, is a factor of ~2 less than Omega_{DLA}. The DLA incidence, n(z), undergoes rapid evolution between redshifts 5 and 2, but is consistent with the no-evolution curve in the current concordance cosmology for z ~< 2. We also show that if the local Schmidt law relating surface density of gas and star formation rate (SFR) is valid at the DLA redshifts, then the DLA SFR density is too low for them to provide a significant contribution to the cosmic star formation history (SFH) at z >~ 1. This implies that the DLAs are unlikely to be the same population as the star forming galaxies (i.e., the Lyman break and sub-millimeter galaxies) that dominate the SFH of the high redshift universe. We suggest that this discrepancy and the DLA ``missing metals'' problem could be the result of missing very high column density gas due to its very small absorption cross section. (abridged)Comment: To appear in "Probing Galaxies through Quasar Absorption Lines", Proceedings IAU Colloquium 199, 2005; Williams, Shu, & Menard, ed

Ca II Absorbers in the Sloan Digital Sky Survey: Statistics

We present the results of a survey for CaII 3934,3969 absorption-line systems culled from ~ 95,000 Sloan Digital Sky Survey (SDSS) Data Release 7 and Data Release 9 quasar spectra. With 435 doublets identified in the catalog, this list is the largest CaII catalog compiled to date, spanning redshifts z < 1.34, which corresponds to the most recent ~ 8.9 Gyrs of the history of the Universe. We derive statistics on the CaII rest equivalent width distribution (REW) and incidence (number density per unit redshift). We find that the lambda3934 REW distribution cannot be described by a single exponential function. A double exponential function is required to produce a satisfactory description. The function can be written as a sum of weak and strong components: dn/dW = (N_wk*/W_wk*) exp(-W/W_wk*) + (N_str*/W_str*) exp(-W/W_str*). A maximum likelihood fit to the unbinned data indicates: N_wk*=0.140 +/- 0.029, W_wk*=0.165 +/- 0.020 A, N_str*=0.024 +/- 0.020, and W_str*=0.427 +/- 0.101 A. This suggests that the CaII absorbers are composed of at least two distinct populations. The incidence (product of integrated absorber cross section and their co-moving number density) of the overall CaII absorber population does not show evidence for evolution in the standard cosmology. The normalization of the no-evolution curve, i.e., the value of the CaII incidence extrapolated to redshift z=0, for lambda 3934 >= 0.3 A, is n_0=0.017 +/- 0.001. In comparison to MgII surveys, we found that only 3% of MgII systems in the SDSS have CaII, confirming that it is rare to identify CaII in quasar absorption-line surveys. We also report on some preliminary investigations of the nature of the two populations of CaII absorbers, and show that they can likely be distinguished using their MgII properties.Comment: Submitted to MNRA

CaII Absorbers in the Sloan Digital Sky Survey: Element Abundances and Dust

We present measurements of element abundance ratios and dust in CaII~absorbers identified in SDSS DR7+DR9. In an earlier paper we formed a statistical sample of 435 CaII absorbers and postulated that their statistical properties might be representative of at least two populations of absorbers. Here we show that if the absorbers are roughly divided into two subsamples with CaII rest equivalent widths larger and smaller than $W_0^{\lambda 3934} = 0.7$ \AA, they are then representative of two physically different populations. Comparisons of abundance ratios between the two CaII absorber populations indicate that the weaker $W_0^{\lambda 3934}$ absorbers have properties consistent with halo-type gas, while the stronger absorbers have properties intermediate between halo- and disk-type gas. We also show that, on average, the dust extinction properties of the overall sample is consistent with a LMC or SMC dust law, and the stronger absorbers are nearly 6 times more reddened than their weaker counterparts. The absorbed-to-unabsorbed composite flux ratio at $\lambda_{rest} = 2200$ \AA\ is $\mathcal{R} \approx 0.73$ and $E(B-V) \approx 0.046$ for the stronger CaII absorbers ($W_0^{\lambda 3934} \ge 0.7$ \AA), and $\mathcal{R} \approx 0.95$ and $E(B-V) \approx 0.011$ for the weaker CaII absorbers ($W_0^{\lambda 3934} < 0.7$ \AA).Comment: Submitted to MNRAS. 19 pages, 12 figure

A Comparative Study of Damped Lyman Alpha Galaxies

We compare the properties of a sample of local (z=0) gas-rich galaxies studied in 21 cm emission to a sample of 13 low-redshift (z < 1) damped Lyman alpha (DLA) galaxies identified as the counterparts of low-redshift DLA systems found in QSO absorption-line surveys. This absorption-selected sample has average redshift = 0.5. We find that many of the properties of the two samples are comparable. However, consideration of the statistical results on all known low-redshift DLA systems indicates that there is: (1) a somewhat higher incidence and cosmological mass density for the low-redshift DLA systems in comparison to expectations at z=0 and (2) an unexpectedly high rate of occurrence of very large column density [N(HI) > 1E21 atoms/cm^2] low-redshift DLA systems; both of these results are discussed by Rao & Turnshek in these proceedings. These differences, coupled with imaging studies, suggest that there may be an excess of low-redshift DLA galaxies in the form of dwarf and/or low surface brightness galaxies. Some examples of low-redshift DLA galaxies are shown by Nestor et al. in these proceedings.Comment: 9 pages, 6 figures, uses newpasp.sty. To appear in ASP Conference Series, "Extragalactic Gas at Low Redshift", J. Mulchaey and J. Stocke, ed

MMT Survey for Intervening MgII Absorption

We present the results from a spectroscopic survey for intervening MgII absorption in the spectra of 381 background QSOs conducted at the Multiple Mirror Telescope. This survey complements our earlier SDSS EDR MgII survey, extending our results to lower redshift ($z \simeq 0.15$) and weaker MgII $\lambda2796$ rest equivalent width ($W_0^{\lambda2796} \simeq 0.1$\AA). We confirm two major results from that survey: the transition in the $W_0^{\lambda2796}$ distribution at $W_0^{\lambda2796} \approx 0.3$\AA, and the $W_0^{\lambda2796}$-dependent evolution of the incidence of systems. The nature of $\partial^2N/\partial z \partial W_0^{\lambda2796}$ is consistent with the idea that multiple physically-distinct components/processes contribute to the incidence of MgII absorption systems in a $W_0$-dependent manner and evolve at different rates. A significant decrease in the total proper absorption cross section is detected in our MMT data for systems as weak as 1.0 \AA $\le W_0^{\lambda2796} < 1.5$\AA at $z\lesssim 0.4$. We discuss this $W_0$-dependent evolution in the context of the evolution of galaxy structures, processes including superwinds and interactions, and damped-Ly$\alpha$ absorbers. We also consider the possibility that the observed redshift and $W_0^{\lambda2796}$ dependence of the incidence of absorption in spectroscopic surveys for low-ion/neutral gas results from the effects of dust-induced extinction.Comment: 16 pages, 7 figures, accepted for publication in The Astrophysical Journa

The Statistical Properties of Neutral Gas at z<1.65 from UV Measurements of Damped Lyman Alpha Systems

We derive the statistical properties of neutral gas at redshifts 0.11<z<1.65 from UV measurements of quasar Lyman alpha absorption lines corresponding to 369 MgII systems with $W^{\lambda2796}_{0} \ge 0.3$ \AA. In addition to the 41 damped Lyman alpha (DLA) systems presented in Rao et al. (2006), the current DLA sample includes 29 newly discovered DLAs. Of these, 26 were found in our HST ACS prism survey for DLAs (Turnshek et al. 2015) and three were found in a GALEX archival search. In addition, an HST COS Cycle 19 survey yielded no DLAs that could be used for this study. Formally, this DLA sample includes 70 systems with $N_{\rm HI}\ge 2\times 10^{20}$ atoms cm$^{-2}$. We find that the incidence of DLAs, or the product of their gas cross section and their comoving number density, can be described by $n_{\rm DLA}(z) = (0.027 \pm 0.007) (1+z)^{(1.682 \pm 0.200)}$ over the redshift range 0<z<5. The cosmic mass density of neutral gas can be described by $\Omega_{\rm DLA}(z) = (4.77 \pm 1.60)\times10^{-4} (1 + z)^{(0.64\pm 0.27)}$. The low-redshift column density distribution function is well-fitted by a power law of the form $f(N) \sim N^\beta$ with $\beta = -1.46 \pm 0.20$. It is consistent with the high-redshift as well as z=0 estimates at the high column density end but, lies between them at the low column density end. We discuss possible $N_{\rm HI}$ and metallicity bias in MgII-selected DLA samples and show that such biases do not exist in the current data at z<1.65. Thus, at least at z<1.65, DLAs found through MgII selection statistically represent the true population of DLAs. However, we caution that studies of DLA metallicities should take into the account the relative incidence of DLAs with respect to $W^{\lambda2796}_{0}$ (or gas velocity spread) in order to correctly measure the mean neutral-gas cosmic metallicity of the universe.Comment: 16 page

Luminous Red Galaxies: Selection and classification by combining optical and infrared photometry

We describe a new method of combining optical and infrared photometry to select Luminous Red Galaxies (LRGs) at redshifts $z > 0.6$. We explore this technique using a combination of optical photometry from CFHTLS and HST, infrared photometry from the WISE satellite, and spectroscopic or photometric redshifts from the DEEP2 Galaxy Redshift Survey or COSMOS. We present a variety of methods for testing the success of our selection, and present methods for optimization given a set of rest-frame color and redshift requirements. We have tested this selection in two different regions of the sky, the COSMOS and Extended Groth Strip (EGS) fields, to reduce the effect of cosmic/sample variance. We have used these methods to assemble large samples of LRGs for two different ancillary programs as a part of the SDSS-III/ BOSS spectroscopic survey. This technique is now being used to select $\sim$600,000 LRG targets for SDSS-IV/eBOSS, which began observations in Fall 2014, and will be adapted for the proposed DESI survey. We have found these methods can select high-redshift LRGs efficiently with minimal stellar contamination; this is extremely difficult to achieve with selections that rely on optical photometry alone.Comment: 10 pages, 13 figures, Published in the Astrophysical Journa

Chandra ACIS-S Observations of Three Quasars with Low-Redshift Damped Ly-alpha Absorption: Constraints on the Cosmic Neutral-Gas-Phase Metallicity at Redshift z \approx 0.4

Chandra X-Ray Observatory (CXO) ACIS-S spectra of three quasars which lie behind three foreground damped Lyman alpha (DLA) absorbers are analyzed in order to attempt to determine the amount of photoelectric absorption due to metals present in their x-ray spectra. These absorbers are the three largest neutral hydrogen column density absorption-line systems known at low redshift (0.313 \le z_abs \le 0.524). They have HI column densities which lie between 3E21 and 5E21 atoms/cm^2. At these redshifts the amount of photoelectric absorption at x-ray energies is primarily an indicator of the oxygen abundance. Since the column densities of these systems are so high, one would expect accurate metallicity measurements of them to yield a robust estimate of the column-density-weighted cosmic neutral-gas-phase metallicity at z \approx 0.4. We consider cases where the DLA gas has solar element abundance ratios and ones with the alpha-group element abundance ratios enhanced. For the adopted assumptions, the column-density-weighted cosmic neutral-gas-phase metallicity of the non-enhanced elements (e.g. Zn) at z \approx 0.4 likely lies in the range \approx 0.04-0.38 Z_solar.Comment: 15 pages, 6 figures. Accepted for publication in the Astrophysical Journa

Properties of Low-Redshift Damped Lyman Alpha Galaxies

Images of five QSO fields containing six damped Lyman alpha (DLA) systems at redshifts 0.09<z<0.53 are presented. Identifications for the DLA galaxies giving rise to the DLA systems are made. The observed and modeled characteristics of the DLA galaxies are discussed. The DLA galaxies have impact parameters ranging from < 4 kpc to \approx 34 kpc and luminosities in the range \approx 0.03L* to \approx 1.3L*. Their morphologies include amorphous low surface brightness systems, a probable dwarf spiral, and luminous spirals.Comment: 8 pages, 6 figures, uses newpasp.sty. To appear in ASP Conference Series "Extragalactic Gas at Low Redshift", J. Mulchaey and J. Stocke, ed