Publication Date:
2015-10-09
Description:
We report the detection of a strong Milky Way-type 2175-Å extinction bump at z = 2.1166 in the quasar spectrum towards SDSS J121143.42+083349.7 from the Sloan Digital Sky Survey Data Release 10. We conduct follow up observations with the Echelle Spectrograph and Imager on-board the Keck II telescope and the Ultraviolet and Visual Echelle Spectrograph on the Very Large Telescope. This 2175-Å absorber is remarkable in that we simultaneously detect neutral carbon (C i ), neutral chlorine (Cl i ), and carbon monoxide (CO). It also qualifies as a damped Lyman α system. The J1211+0833 absorber is found to be metal rich and has a dust depletion pattern resembling that of the Milky Way disc clouds. We use the column densities of the C i fine structure states and the C ii /C i ratio (under the assumption of ionization equilibrium) to derive the temperature and volume density in the absorbing gas. A cloudy photoionization model is constructed, which utilizes additional atoms/ions to constrain the physical conditions. The inferred physical conditions are consistent with a canonical cold ( T ~ 100 K) neutral medium with a high density ( n (H i ) ~ 100 cm –3 ) and a slightly higher pressure than the local interstellar medium. Given the simultaneous presence of C i , CO, and the 2175-Å bump, combined with the high metallicity, high dust depletion level, and overall low ionization state of the gas, the absorber towards J1211+0833 supports the scenario that the presence of the bump requires an evolved stellar population.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
