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

Abstract

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 (SDSS) Data Release 10. We conduct follow up observations with the Echelle Spectrograph and Imager (ESI) onboard the Keck-II telescope and the Ultraviolet and Visual Echelle Spectrograph (UVES) on the VLT. 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 alpha system. The J1211+0833 absorber is found to be metal-rich and has a dust depletion pattern resembling that of the Milky Way disk 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 $\sim$ 100 K) neutral medium with a high density ($n$(H I) $\sim$ 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.

18 pages, 17 figures, to be published in MNRAS

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• Revealing the host galaxy of a quasar 2175 Å dust absorber at z = 2.12
• Newly discovered Ca II absorbers in the early universe: statistics, element abundances and dust
• Thermal Pressures in the Interstellar Medium away from Stellar Environments
• The 2175 Å~ Bump Features in FeLoBAL Quasars: One Indicator of MW-like Dust in the Nuclear Region of Quasar
• Studying Quasar Absorber Host Galaxy Properties Using Image Stacking Technique
• Probing the Interstellar Dust in Galaxies over > 10 Gyr of Cosmic History
• First detections of CO absorption in the Magellanic Clouds and direct measurement of the CO-to-H${}_2$ ratio