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arXiv · 1509.01593

Cold gas and a Milky Way-type 2175 {\AA} bump in a metal-rich and highly depleted absorption system

Jingzhe Ma·Paul Caucal·Pasquier Noterdaeme·Jian Ge·J. Xavier Prochaska·Tuo Ji·Shaohua Zhang·Hadi Rahmani·Peng Jiang·Donald P. Schneider·Britt Lundgren·Isabelle Pâris
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Abstract

We report the detection of a strong Milky Way-type 2175 \AA$ $ 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 \AA$ $ 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 \AA$ $ 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.

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Jingzhe Ma, Paul Caucal, Pasquier Noterdaeme, Jian Ge, J. Xavier Prochaska, Tuo Ji, Shaohua Zhang, Hadi Rahmani, Peng Jiang, Donald P. Schneider, Britt Lundgren, Isabelle Pâris. 2015-09-04. Cold gas and a Milky Way-type 2175 {\AA} bump in a metal-rich and highly depleted absorption system. https://doi.org/10.1093/mnras/stv2073

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