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.

A Super-Damped Lyman-alpha QSO Absorber at z=2.2

We report the discovery of a super-damped Lyman-alpha absorber at $z_{abs}=2.2068$ toward QSO Q1135-0010 in the Sloan Digital Sky Survey, and follow-up VLT UVES spectroscopy. Voigt profile fit to the DLA line indicates log $N_{rm H I} = 22.05 pm 0.1$. This is the second QSO DLA discovered to date with such high $N_{rm H I}$. We derive element abundances [Si/H] = $-1.10 pm 0.10$, [Zn/H] = $-1.06 pm 0.10$, [Cr/H] = $-1.55 pm 0.10$, [Ni/H] = $-1.60 pm 0.10$, [Fe/H] = $-1.76 pm 0.10$, [Ti/H] = $-1.69 pm 0.11$, [P/H] = $-0.93 pm 0.23$, and [Cu/H] = $-0.75 pm 0.14$. Our data indicate detection of Ly-$alpha$ emission in the DLA trough, implying a star formation rate of $sim$10 $M_{odot}$ yr$^{-1}$ in the absence of dust attenuation. C II$^{*} , lambda 1336$ absorption is also detected, suggesting SFR surface density $-2 < {rm log} , dot{psi_{*}} < 0$ $M_{odot}$ yr$^{-1}$ kpc$^{-2}$. We estimate electron density in the range $3.5 times 10^{-4}$ to 24.7 cm$^{-3}$ from C II$^{*}$/C II, and $sim$0.5-0.9 cm$^{-3}$ from Si II$^{*}$/Si II. Overall, this is a robustly star-forming, moderately enriched absorber, but with relatively low dust depletion. Fitting of the SDSS spectrum yields low reddening for Milky Way, LMC, or SMC extinction curves. No CO absorption is detected, and C I absorption is weak. The low dust and molecular content, reminiscent of some SMC sight-lines, may result from the lower metallicity, and a stronger radiation field (due to higher SFR). Finally, we compare this absorber with other QSO and GRB DLAs.