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

TL;DR

This paper reports the discovery of a Milky Way-like 2175 Å extinction bump at high redshift, associated with a metal-rich, dust-depleted, cold neutral medium containing molecules, indicating evolved stellar populations.

Contribution

First detection of a Milky Way-type 2175 Å bump at z=2.1166 with detailed physical characterization of the absorber's conditions.

Findings

Detection of a strong 2175 Å bump at z=2.1166

Presence of neutral carbon, chlorine, and CO molecules

Absorber is metal-rich with Milky Way-like dust depletion

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.