Halfway to the Peak: ice absorption bands at $z\approx0.5$ with JWST MIRI/MRS

Anna Sajina, Alexandra Pope, Henrik Spoon, Lee Armus, Miriam Eleazer, Duncan Farrah, Mark Lacy, Thomas Lai, Jed McKinney, Sylvain Veilleux, Lin Yan, Jason Young

First listed 2025-03-21 | Last updated 2025-03-21

Abstract

This paper presents the first combined detections of CO$_2$, 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 which 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 ($n>10^5$cm$^{-3}$ and $T=20-90K$). We infer the visual extinction towards this nucleus to be $A_V\approx6-7$. An observed plot of $τ_{Si}$ vs. $τ_{CO2}/τ_{Si}$ 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 {\sl JWST} MIRI spectra to study the dust composition and geometric distribution of sources beyond the local Universe.

Short digest

JWST/MIRI–MRS spectroscopy of the dusty starburst–AGN composite SSTXFLS J172458.3+591545 (z=0.494) yields the first combined extragalactic detections of CO2, CO, XCN, and H2O ices, with gas‑phase CO alongside solid CO. Ice-band profiles match Galactic/local templates, and their decomposition (polar‑dominated XCN and CO complexes) plus the silicate depth imply a compact, dense, cold nucleus with n>10^5 cm^-3, T≈20–90 K, and A_V≈6–7; [NeV]/[NeVI] confirm a deeply buried AGN. The authors introduce τ_Si versus τ_CO2/τ_Si as a diagnostic that jointly traces total dustiness and dust clumpiness along the line of sight. Together, these results showcase MIRI’s power to map dust composition and geometry well beyond the local Universe.

Key figures to inspect

• Figure 1: Check the collapsed channel‑1 cube and the 0.85″ extraction aperture versus the MIRI PSF; in the full spectrum, verify [NeVI] and [NeV] alongside the red‑shaded ice bands and the 12 sub‑band overlap regions.
• Figure 2: Inspect the channel‑1 spectrum with the Savitzky–Golay smoothing; confirm the optical z≈0.494 via the alignment of the narrow Br line and locate the rest‑frame positions of H2O, CO2, XCN, and CO absorption.
• Figure 3: Compare the target to two nearby-galaxy NIRSpec+MIRI spectra to see how silicate strengths bracket this source and how the stronger hot‑dust continuum signals a more buried nucleus.
• Figure 4: Use the zooms on CO2 and XCN+CO to follow the continuum placements, optical‑depth fits, and polar vs. apolar sub‑components—evidence that the polar components dominate and anchor the τ measurements used for density/temperature inferences.