Halfway to the Peak: Ice Absorption Bands at z ≈ 0.5 with JWST MIRI/MRS

This paper presents the first combined detections of CO₂, CO, XCN, and water ices beyond the local Universe. We find gas-phase CO in addition to the solid phase CO. Our source, SSTXFLS J172458.3+591545, is a z = 0.494 star-forming galaxy that also hosts a deeply obscured AGN. The profiles of its ice features are consistent with those of other Galactic and local galaxy sources, and the implied ice mantle composition is similar to that of even more obscured sources. The ice features indicate the presence of a compact nucleus in our Galaxy and allow us to place constraints on its density and temperature (n > 10⁵ cm⁻³ and T = 20–90 K). We infer the visual extinction towards this nucleus to be A_V ≈ 6–7. An observed plot of τ_Si versus τ_CO2/τ_Si can be viewed as a probe for both the total dustiness of a system as well as the clumpiness of the dust along the line of sight. This paper highlights the potential of using JWST MIRI spectra to study the dust composition and geometric distribution of sources beyond the local Universe.