GALAXIES IN ABSORPTION: A STUDY OF CHEMICAL AND KINEMATIC PROPERTIES OF SUB-DAMPED LYMAN-ALPHA QUASAR ABSORBERS

Study of the chemical composition of the interstellar medium (ISM) in galaxies over cosmic time is essential for a coherent understanding of galaxy formation and evolu- tion. Absorption lines in the spectra of quasars can be used as powerful, luminosity- independent probes of the properties of gas in and around galaxies and have been used extensively to study galaxies, the circumgalactic medium (CGM) and the intergalac- tic medium (IGM). The Damped Lyman-α systems (DLAs), with neutral hydrogen column densities of log NHI & 20.3, and sub-Damped Lyman-α systems (sub-DLAs) with 19.0 . log NHI \u3c 20.3 are the highest NHI quasar absorbers and contain the most of the neutral gas available for star formation in the high-redshift Universe. These systems are believed to trace the progenitors of present-day galaxies and ac- curately probe chemical abundances in the ISM over ∼ 90% of the cosmic history. In contradiction with the cosmic chemical evolution models which predict the mean metallicity of galaxies to rise from low metallicities at high-z to a near-solar level at z ∼ 0, the DLAs are typically found to be metal-poor at all redshifts, showing little or no evolution. Interestingly, past work showed that the sub-DLAs at 0.6 . z . 1.5 are more metal-rich on average than DLAs, and evolve consistently with the chemical evolution models in this redshift range. This suggests that the DLAs and sub-DLAs may be tracing the progenitors of different populations of present-day galaxies. How- ever, chemical evolution of sub-DLAs is poorly constrained outside of the redshift range 0.6 \u3c z \u3c 1.5 which hinders a better understanding of galaxy evolution traced by DLAs and sub-DLAs. This dissertation presents chemical abundance measurements of sub-DLA quasar absorbers at z \u3c 0.6 and z \u3e 1.5. The low-z absorbers were studied using medium- resolution UV spectra from the Cosmic Origins Spectrograph on board the Hubble Space Telescope. The systems at z \u3e 1.5 were observed with the Magellan Inamori Kyocera Echelle spectrograph at the Magellan-Clay Telescope. Lines of various el- ements in several ionization stages, present in these spectra, were measured to de- termine the respective column densities. The metallicity of the absorbing gas was inferred from the nearly undepleted elements Zn or S, and several of the absorbers were found to be near-solar or super-solar in metallicity. We have also investigated the effect of ionization on the observed abundances using photoionization modelling. We find that some of the sub-DLAs have significant amounts of ionized gas, but the ionization corrections to metallicity for all of our sub-DLAs are relatively modest (.0.2 dex). Combining our data with other sub-DLA and DLA data from the lit- erature, we report the most complete existing determination of the metallicity vs. redshift relation for sub-DLAs and DLAs. This work confirms, over a larger redshift baseline, the suggestion from previous investigations that sub-DLAs are, on average, more metal-rich than DLAs and evolve marginally faster. We also find evidence for metallicity being anti-correlated with H I column density in DLAs and sub-DLAs. The relative abundances and abundance ratios seen in these absorbers are discussed in the context of the overall trends seen in quasar absorbers. We have explored the kinematic properties of DLAs and sub-DLAs determined via velocity width measure- ments of unsaturated absorption lines. We also present initial evidence for higher interstellar cooling rates in metal-rich sub-DLAs than those seen in DLAs. Our find- ings suggest that DLAs and sub-DLAs may trace different galaxy populations with sub-DLAs being the progenitors of more massive galaxies

Element Abundances in a Gas-rich Galaxy at z = 5: Clues to the Early Chemical Enrichment of Galaxies

Element abundances in high-redshift quasar absorbers offer excellent probes of the chemical enrichment of distant galaxies, and can constrain models for population III and early population II stars. Recent observations indicate that the sub-damped Lyman-alpha (sub-DLA) absorbers are more metal-rich than DLA absorbers at redshifts 0$<$$z$$<$3. It has also been suggested that the DLA metallicity drops suddenly at $z$$>$4.7. However, only 3 DLAs at $z$$>$4.5 and none at $z$$>$3.5 have "dust-free" metallicity measurements of undepleted elements. We report the first quasar sub-DLA metallicity measurement at $z$$>$3.5, from detections of undepleted elements in high-resolution data for a sub-DLA at $z$=5.0. We obtain fairly robust abundances of C, O, Si, and Fe, using lines outside the Lyman-alpha forest. This absorber is metal-poor, with O/H]=-2.00$\pm$0.12, which is $\gtrsim$4$\sigma$ below the level expected from extrapolation of the trend for $z$$<$3.5 sub-DLAs. The C/O ratio is 1.8$^{+0.4}_{-0.3}$ times lower than in the Sun. More strikingly, Si/O is 3.2$^{+0.6}_{-0.5}$ times lower than in the Sun, while Si/Fe is nearly (1.2$^{+0.4}_{-0.3}$ times) solar. This absorber does not display a clear alpha/Fe enhancement. Dust depletion may have removed more Si from the gas phase than is common in the Milky Way interstellar medium, which may be expected if high-redshift supernovae form more silicate-rich dust. C/O and Si/O vary substantially between different velocity components, indicating spatial variations in dust depletion and/or early stellar nucleosynethesis (e.g., population III star initial mass function). The higher velocity gas may trace an outflow enriched by early stars.Comment: 42 pages including 9 figures, accepted for publication in Ap

Keck and VLT Observations of Super-damped Lyman-alpha Absorbers at z=2=2.5: Constraints on Chemical Compositions and Physical Conditions

We report Keck/ESI and VLT/UVES observations of three super-damped Lyman-alpha quasar absorbers with H I column densities log N(HI) >= 21.7 at redshifts z=2-2.5. All three absorbers show similar metallicities (-1.3 to -1.5 dex), and dust depletion of Fe, Ni, and Mn. Two of the absorbers show supersolar [S/Zn] and [Si/Zn]. We combine our results with those for other DLAs to examine trends between N(HI), metallicity, dust depletion. A larger fraction of the super-DLAs lie close to or above the line [X/H]=20.59-log N(HI) in the metallicity vs. N(HI) plot, compared to the less gas-rich DLAs, suggesting that super-DLAs are more likely to be rich in molecules. Unfortunately, our data for Q0230-0334 and Q0743+1421 do not cover H2 absorption lines. For Q1418+0718, some H2 lines are covered, but not detected. CO is not detected in any of our absorbers. For DLAs with log N(HI) < 21.7, we confirm strong correlation between metallicity and Fe depletion, and find a correlation between metallicity and Si depletion. For super-DLAs, these correlations are weaker or absent. The absorbers toward Q0230-0334 and Q1418+0718 show potential detections of weak Ly-alpha emission, implying star formation rates of about 1.6 and 0.7 solar masses per year, respectively (ignoring dust extinction). Upper limits on the electron densities from C II*/C II or Si II*/Si II are low, but are higher than the median values in less gas-rich DLAs. Finally, systems with log N(HI) > 21.7 may have somewhat narrower velocity dispersions delta v_90 than the less gas-rich DLAs, and may arise in cooler and/or less turbulent gas.Comment: 57 pages, 15 figures. Accepted for publication in Ap

Atomic data for Zn II - Improving Spectral Diagnostics of Chemical Evolution in High-redshift Galaxies

Damped Lyman-alpha (DLA) and sub-DLA absorbers in quasar spectra provide the most sensitive tools for measuring element abundances of distant galaxies. Estimation of abundances from absorption lines depends sensitively on the accuracy of the atomic data used. We have started a project to produce new atomic spectroscopic parameters for optical/UV spectral lines using state-of-the-art computer codes employing very broad configuration interaction basis. Here we report our results for Zn II, an ion used widely in studies of the interstellar medium (ISM) as well as DLA/sub-DLAs. We report new calculations of many energy levels of Zn II, and the line strengths of the resulting radiative transitions. Our calculations use the configuration interaction approach within a numerical Hartree-Fock framework. We use both non-relativistic and quasi-relativistic one-electron radial orbitals. We have incorporated the results of these atomic calculations into the plasma simulation code Cloudy, and applied them to a lab plasma and examples of a DLA and a sub-DLA. Our values of the Zn II {\lambda}{\lambda} 2026, 2062 oscillator strengths are higher than previous values by 0.10 dex. Cloudy calculations for representative absorbers with the revised Zn atomic data imply ionization corrections lower than calculated before by 0.05 dex. The new results imply Zn metallicities should be lower by 0.1 dex for DLAs and by 0.13-0.15 dex for sub-DLAs than in past studies. Our results can be applied to other studies of Zn II in the Galactic and extragalactic ISM.Comment: accepted The Astrophysical Journa

Element Abundances at High-redshift: Magellan MIKE Observations of sub-Damped Lyman-alpha Absorbers at 1.7 < z <2.4

We present chemical abundance measurements from high-resolution observations of 5 sub-damped Lyman-alpha absorbers at 1.7 < z < 2.4 observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II Clay telescope. Lines of Zn II, Mg I, Mg II, Al II, Al III, S II, Si II, Si IV, C II, C II*, C IV, Ni II, Mn II and Fe II were detected and column densities were determined. The metallicity of the absorbing gas, inferred from the nearly undepleted element Zn, is in the range of < -0.95 to +0.25 dex for the five absorbers in our sample, with three of the systems being near-solar or super-solar. We also investigate the effect of ionisation on the observed abundances using photoionisation modelling. Combining our data with other sub-DLA and DLA data from the literature, we report the most complete existing determination of the metallicity vs. redshift relation for sub-DLAs and DLAs. We confirm the suggestion from previous investigations that sub-DLAs are, on average, more metal-rich than DLAs and evolve faster. We also discuss relative abundances and abundance ratios in these absorbers. The more metal-rich systems show significant dust depletion levels, as suggested by the ratios [Zn/Cr] and [Zn/Fe]. For the majority of the systems in our sample, the [Mn/Fe] vs. [Zn/H] trend is consistent with that seen previously for lower-redshift sub-DLAs. We also measure the velocity width values for the sub-DLAs in our sample from unsaturated absorption lines of Fe II 2344, 2374, 2600 A, and examine where these systems lie in a plot of metallicity vs. velocity dispersion. Finally, we examine cooling rate vs. H I column density in these sub-DLAs, and compare this with the data from DLAs and the Milky Way ISM. We find that most of the systems in our sample show higher cooling rate values compared to those seen in the DLAs.Comment: Accepted for publication in the Monthly Notices of The Royal Astronomical Societ

A Strong Blend in the Morning: Studying the Circumgalactic Medium Before Cosmic Noon with Strong, Blended Lyman-α\alpha Forest Systems

We study of the properties of a new class of circumgalactic medium absorbers identified in the Lyman-$\alpha$ forest: "Strong, Blended Lyman-$\alpha$" (or SBLA) absorption systems. We study SBLAs at $2.4<z<3.1$ in SDSS-IV/eBOSS spectra by their strong extended Lyman-$\alpha$ absorption complexes covering 138 km/s with an integrated $\log (N_{HI}/$cm$^{-2}) =16.04^{+0.05}_{-0.06}$ and Doppler parameter $b=18.1^{+0.7}_{-0.4}$ km/s. Clustering with the Lyman-$\alpha$ forest provides a large-scale structure bias of $b = 2.34\pm0.06$ and halo mass estimate of $M_h \approx 10^{12}{\rm h^{-1}M_{sol}}$ for our SBLA sample. We measure the ensemble mean column densities of 22 metal features in the SBLA composite spectrum and find that no single-population multiphase model for them is viable. We therefore explore the underlying SBLA population by forward modelling the SBLA absorption distribution. Based on covariance measurements and favoured populations we find that $\approx 25$% of our SBLAs have stronger metals. Using silicon only we find that our strong metal SBLAs trace gas with a $\log(n_H /$cm$^{-3}) > -2.45$ for $T=10^{3.5}$K and show gas clumping on $<255$ parsec scales. We fit multiphase models to this strong sub-population and find a low ionization phase with $n_H=1$cm$^{-3}$, $T=10^{3.5}$K and $[X/H]=0.8$, an intermediate ionization phase with $\log(n_H /$cm$^{-3}) = -3.35$, $T=10^{3.5}$K and $[X/H]=-1.1$, and a poorly constrained higher ionization phase. We find that the low ionization phase traces cold, dense super-solar metallicity gas with a clumping scale of just 0.009 parsecs.Comment: 28 pages, submitted to MNRA

Hubble Space Telescope Observations of Sub-Damped Lyman-alpha Absorbers at z < 0.5, and Implications for Galaxy Chemical Evolution

We report observations of four sub-damped Lyman-alpha (sub-DLA) quasar absorbers at z<0.5 obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. We measure the available neutrals or ions of C, N, O, Si, P, S, Ar, Mn, Fe, and/or Ni. Our data have doubled the sub-DLA metallicity samples at z<0.5 and improved constraints on sub-DLA chemical evolution. All four of our sub-DLAs are consistent with near-solar or super-solar metallicities and relatively modest ionization corrections; observations of more lines and detailed modeling will help to verify this. Combining our data with measurements from the literature, we confirm previous suggestions that the N(HI)-weighted mean metallicity of sub-DLAs exceeds that of DLAs at all redshifts studied, even after making ionization corrections for sub-DLAs. The absorber toward PHL 1598 shows significant dust depletion. The absorbers toward PHL 1226 and PKS 0439-433 show the S/P ratio consistent with solar, i.e., they lack a profound odd-even effect. The absorber toward Q0439-433 shows super-solar Mn/Fe. For several sub-DLAs at z<0.5, [N/S] is below the level expected for secondary N production, suggesting a delay in the release of the secondary N or a tertiary N production mechanism. We constrain the electron density using Si II* and C II* absorption. We also report different metallicity vs. Delta V_90 relations for sub-DLAs and DLAs. For two sub-DLAs with detections of emission lines from the underlying galaxies, our measurements of the absorption-line metallicities are consistent with the emission-line metallicities, suggesting that metallicity gradients are not significant in these galaxies.Comment: 77 pages, 13 figures; accepted for publication in the Astrophysical Journal. Submitted (in the original form) May 26, 2014; accepted Apr. 15, 201