Cold gas and a Milky Way-type 2175-Å bump in a metal-rich and highly depleted absorption system

We report the detection of a strong MilkyWay-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 followup 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 CI fine structure states and the CII/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.