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 $μ$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 $μ$m) and CO$_2$ ($\sim$4.26 $μ$m) ice absorption features. We develop three methods to derive phase-specific extinction for ionized (A$_{\text K}^{\text HII}$) and hot molecular gas (A$_{\text K}^{\text H_2}$), and H$_2$O and CO$_2$ ice column densities (N$_{\text H_2O}$ and N$_{\text CO_2}$), 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$_{\text K}^{\text HII}$ 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$_{\text K}^{\text HII}\approx1.5$ A$_{\text K}^{\text H_2}$, while H$_2$O ice column densities are comparable between phases. In some nuclei, continuum-derived N$_{\text H_2O}$ reaches $\sim10^{18.81}$ 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$_{\text CO_2}$/N$_{\text H_2O}$ abundance ratio is $\sim0.10$, a factor of 2--3 below typical Galactic values.