2023MNRAS.524.2446B

A pair of 6.0 and 6.9 μm 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 χ²-fitting methods, which match observations with two or three laboratory spectra. The 6.0-μm component is dominated by H₂O ice, as expected. Included MW sources were selected using opacity criteria which limit the range of explored H₂O-ice column densities to 1.6-2.4 × 10¹⁸ molecules cm^–2, while the H₂O-ice density in the galaxy absorber is (2.7 ± 0.5) × 10¹⁸ molecules cm^–2. CH₃OH ice and / or small (< 0.1-μm-sized) Ca- and Mg-bearing carbonates contribute at 6.9 μm. The 41 per cent CH₃OH:H₂O molecular ratio in the PKS 1830-211 absorber is significantly higher than in the molecular cloud towards Taurus-Elias 16 (<7.5 per cent) and similar to the highest value in MW YSOs (35 per cent in AFGL 989). Fitted carbonate (-CO₃):H₂O ratios in the galaxy absorber of 0.091 per cent are low in comparison to most of the ratios detected in the MW sample (0.2 per cent-0.4 per cent; ∼0 per cent in AFGL 989). Inorganic carbonates could explain the increased oxygen depletion at the diffuse-medium-to-molecular-cloud transition, which Jones and Ysard associated with unobserved organic carbonates or materials with a C:O ratio of 1:3.