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

We investigate the spatially resolved extinction levels and ice column densities in the deeply obscured environments of four nearby interacting (ultra)luminous infrared galaxies ((U)LIRGs; L_ ̊m IR >, 11.6 L_⊙ and D_ ̊m L ∼ 39 -- 107 Mpc). Accurate extinction corrections are essential to constraining the dominant power source and unveiling the most heavily obscured structures. We used the three high-resolution (R∼2700) gratings of the JWST/NIRSpec integral field spectrograph with spectral coverage of (∼ 0.97 -- 5.14) μm. This wide range encompasses numerous ($>25$) H recombination and H_2 ro-vibrational emission lines as well as the prominent H_2O (∼3.02 μm) and CO_2 (∼4.26 μm) ice absorption features. We present three methods to derive phase-specific extinction affecting the ionized gas (A_ ̊m K ^ ̊m HII ), the hot molecular gas (A_ ̊m K ^ ̊m H_2 ), and the H_2O and CO_2 ice column densities (N_̊m H_2O and N_̊m CO_2). Using H and H_2 lines detected within the H_2O absorption feature, we also derived phase-specific water ice column densities (N_ ̊m H_2O ^ ̊m HII and N_ ̊m H_2O ^ ̊m H_2 ). We compared results from these methods with classical extinction diagnostics and provide the corresponding correlations. We find that nuclear regions (0.2" radius) exhibit extinction and ice column densities typically about factor of two higher than the extended regions, with A_ ̊m K ^ ̊m HII = (1.25 ± 0.02) and A_ ̊m K ^ ̊m H_2 = (0.75 ± 0.10) in the nuclei, compared to A_ ̊m K ^ ̊m HII = (0.61 ± 0.04) and A_ ̊m K ^ ̊m H_2 = (0.50 ± 0.07) in the extended regions. The ionized gas is more extincted than the hot molecular counterpart, on average with A_ ̊m K ^ ̊m HII ≈ 1.5 A_ ̊m K ^ ̊m H_2 , whereas the H_2O ice column densities are comparable between the two phases, with median values of ∼1018.11 cm^-2 in the nuclei and ∼10^17.89 cm^-2 in the extended regions. The latter value is consistent, within the uncertainties, with that derived from the continuum, though not in the case of some nuclear regions, where N_̊m H_2O increases up to ∼10^18.81 cm^-2 due to the contribution of compact obscured nuclei. These results suggest that the ionized gas originates from more deeply embedded regions than the hot H_2$ emission, while the shell of dust grains, where water ice accumulates, envelopes both gas phases, with an additional ice-rich component surrounding the most obscured nuclei. Finally, from the continuum ice absorption features we derived a median relative abundance of N_̊m CO_2/N$_ ̊m H_2O ,∼ ,0.10, a factor of around two to three lower than typical Galactic values.