Beyond the dust: JWST reveals extinction and ice content in local (U)LIRGs

Abstract

We investigate the spatially resolved extinction and ice column densities in four nearby interacting (U)LIRGs using JWST/NIRSpec integral field spectroscopy (R$\sim$2700; 0.97--5.14 $\mu$m). This wide range encompasses numerous (>25) H recombination and H${}_2$ ro-vibrational emission lines, as well as the prominent H${}_2$O ($\sim$3.02 $\mu$m) and CO${}_2$ ($\sim$4.26 $\mu$m) ice absorption features. We develop three methods to derive phase-specific extinction for ionized (A${}_{\text{K}}^{\text{H}II}$) and hot molecular gas (A${}_{\text{K}}^{{\text{H}}_2}$), and H${}_2$O and CO${}_2$ ice column densities (N${}_{{\text{H}}_{2}O}$ and N${}_{\text{C}{O}_2}$), exploiting H and H${}_2$ emission lines and ice absorption features. Nuclear regions (0.2" radius) exhibit higher extinction and ice column densities than extended regions, with median A${}_{\text{K}}^{\text{H}II}$ and A${}_{\text{K}}^{{\text{H}}_2}$ of 1.25 and 0.75 in the nuclei, compared to 0.61 and 0.50 in the extended regions. The ionized gas is more extincted than the hot molecular gas, with A${}_{\text{K}}^{\text{H}II}\approx 1.5$ A${}_{\text{K}}^{{\text{H}}_2}$, while H${}_2$O ice column densities are comparable between phases. In some nuclei, continuum-derived N${}_{{\text{H}}_{2}O}$ reaches $\sim 10^{18.81}$ cm${}^{-2}$, likely due to compact obscured components. These results suggest that ionized gas originates in more deeply embedded regions, whereas water ice envelopes both gas phases, with additional ice-rich material around the most obscured nuclei. The median N${}_{\text{C}{O}_2}$/N${}_{{\text{H}}_{2}O}$ abundance ratio is $\sim 0.10$, a factor of 2--3 below typical Galactic values.

arXiv commentAccepted for publication in A&A; 21 pages, 24 figures, 3 tables