The First Observations of Low-Redshift Damped Lyman-α Systems with the Cosmic Origins Spectrograph: Chemical Abundances and Affiliated Galaxies

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  • 20 October 2011
Abstract
We present Cosmic Origins Spectrograph (COS) measurements of metal abundances in eight 0.083 < zabs < 0.321 damped Lyman-{\alpha} (DLA) and sub-damped Ly{\alpha} absorption systems serendipitously discovered in the COS-Halos survey. We find that these systems show a large range in metallicities, with -1.10 < [Z/H] < 0.31, similar to the spread found at higher redshifts. These low-redshift systems on average have subsolar metallicities, but do show a rise in metallicity over cosmic time when compared to higher-redshift systems. Utilizing our sources and those in the literature, we find the average sub-DLA metallicity is higher than the average DLA metallicity at all redshifts. Nitrogen is underabundant with respect to {\alpha}-group elements in all but perhaps one of the absorbers. In some cases, [N/{\alpha}] is significantly below the lowest nitrogen measurements in nearby galaxies; the nitrogen abundances are more similar to those observed in high-redshift DLAs. Systems for which depletion patterns can be studied show clear evidence of depletion by dust at levels characteristic of Milky Way halo-type gas. We also identify affiliated galaxies for 3 of the sub-DLAs using spectra obtained from Keck/LRIS. None of these sub-DLAs arise in the stellar disks of luminous galaxies; instead, these absorbers may exist in galaxy halos at impact parameters ranging from 38 to 92 kpc. Multiple galaxies are present near two of the sub-DLAs, and galaxy interactions may play a role in the dispersal of the gas. Many of these low-redshift absorbers exhibit simple kinematics, but one sub-DLA has a complicated mix of at least 13 components spread over 150 km/s. We find three galaxies near this sub-DLA, which also suggests that galaxy interactions roil the gas. This study reinforces the view that DLAs have a variety of origins, and low-redshift studies are crucial for understanding absorber-galaxy connections.

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