Carbonates and ices in the $z=0.89$ galaxy-absorber towards PKS 1830-211 and within star-forming regions of the Milky Way

TL;DR

This study analyzes the 6.0 and 6.9 micron absorption features in a galaxy at z=0.89, revealing the presence of water ice, methanol, and carbonates, and compares these findings with Milky Way molecular clouds and star-forming regions.

Contribution

It calibrates and applies $ ext{chi}^2$-fitting methods to identify and quantify ices and carbonates in a distant galaxy, providing new insights into its molecular composition.

Findings

Water ice dominates the 6.0 μm feature.

Methanol ice is present at high ratios, similar to MW YSOs.

Carbonate ratios are lower than in MW samples.

Abstract

A pair of 6.0 and 6.9 $\mu$m absorption features are frequently observed in Milky-Way (MW) molecular-clouds and YSOs; they also occur in the $z=0.886$ rest-frame of a molecule-rich spiral galaxy obscuring blazar PKS 1830-211. I calibrate $\chi^2$-fitting methods which match observations with two or three laboratory spectra. The 6.0-$\mu$m component is dominated by H$_2$O ice, as expected. Included MW sources were selected using opacity criteria which limit the range of explored H$_2$O-ice column densities to 1.6--$2.4 \times 10^{18}$ molecules cm$^{-2}$, while the H$_2$O-ice density in the galaxy absorber is $(2.7\pm 0.5)\times 10^{18}$ molecules cm$^{-2}$. CH$_3$OH ice and / or small (< 0.1-$\mu$m-sized) Ca- and Mg-bearing carbonates contribute at 6.9 $\mu$m. The 41 % CH$_3$OH : H$_2$O molecular ratio in the PKS 1830-211 absorber is significantly higher than in the molecular cloud towards Taurus-Elias 16 (<7.5 %) and similar to the highest value in MW YSOs (35 % in AFGL 989). Fitted carbonate (-CO$_3$) : H$_2$O ratios in the galaxy absorber of 0.091 % are low in comparison to most of the ratios detected in the MW sample (0.2 - 0.4 %; $\sim 0$ % in AFGL 989). Inorganic carbonates could explain the increased oxygen depletion at the diffuse-medium-to-molecular-cloud transition which Jones \& Ysard associated with unobserved organic carbonates or materials with a C:O ratio of 1:3.