2011ApJ...732...35M

We report on the first Cosmic Origins Spectrograph observations of damped Lyα systems (DLAs) and sub-damped Lyα systems (sub-DLAs) discovered in a new survey of the gaseous halos of low-redshift galaxies. From observations of 37 sightlines, we have discovered three DLAs and four sub-DLAs. We measure the neutral gas density Ω_ H I_, and redshift density d N/dz, of DLA and sub-DLA systems at z < 0.35. We find d N/dz = 0.25^+0.24_- 0.14_ and Ω_ H I_= 1.4^+1.3_- 0.7_x 10^–3 for DLAs, and d N/dz = 0.08^+0.19 _- 0.06_ with Ω_H I_= 4.2^+9.6_- 3.5_x 10^–5 for sub-DLAs over a redshift path Δz = 11.9. To demonstrate the scientific potential of such systems, we present a detailed analysis of the DLA at z_abs= 0.1140 in the spectrum of SDSS J1009+0713. Profile fits to the absorption lines determine log N(H I) = 20.68±0.10 with a metallicity determined from the undepleted element sulfur of [S/H] = -0.62±0.18. The abundance pattern of this DLA is similar to that of higher z DLAs, showing mild depletion of the refractory elements Fe and Ti with [S/Fe] = +0.24±0.22 and [S/Ti] = +0.28±0.15. Nitrogen is underabundant in this system with [N/H] = -1.40 ±0.14, placing this DLA below the plateau of the [N/α] measurements in the local universe at similar metallicities. This DLA has a simple kinematic structure with only two components required to fit the profiles and a kinematic width of Δv₉₀= 52 km/s. Imaging of the QSO field with the Hubble Space Telescope/Wide Field Camera 3 reveals a spiral galaxy at very small impact parameter to the QSO and several galaxies within 10'', or 20 comoving kpc at the redshift of the DLA. Follow-up spectra with the Low Resolution Imaging Spectrometer on the Keck telescope reveal that none of the nearby galaxies are at the redshift of the DLA. The spiral galaxy is identified as the host galaxy of the QSO based on the near perfect alignment of the nucleus and disk of the galaxy as well as spectra of an H II region showing emission lines at the QSO redshift. A small feature appears 0".70 from the nucleus of the QSO after point-spread function subtraction, providing another candidate for the host galaxy of the DLA system. Even with these supporting data, we are unable to unambiguously identify the host galaxy of the DLA, exemplifying some of the difficulties in determining DLA hosts even at low redshift.