2019ApJ...882..153G

Recent interferometric observations have shown bright HCN emission from the ν₂ = 1 vibrational state arising in buried nuclear regions of galaxies, indicating an efficient pumping of the ν₂ = 1 state through the absorption of 14 µm continuum photons. We modeled the continuum and HCN vibrational line emission in these regions, characterized by high column densities of dust and high luminosities, using a spherically symmetric approach, simulating both a central heating source (active galactic nucleus, AGN) and a compact nuclear starburst (SB). We find that when the H₂ columns become very high, N_H2 >= 10²⁵ cm^–2, trapping of continuum photons within the nuclear region dramatically enhances the dust temperature (T_dust) in the inner regions, even though the predicted spectral energy distribution as seen from the outside becomes relatively cold. The models thus predict a bright continuum at millimeter wavelengths for a luminosity surface brightness (averaged over the model source) of ∼10⁸ L_☉ pc^–2. This greenhouse effect significantly enhances the mean mid-infrared intensity within the dusty volume, populating the ν₂ = 1 state to the extent that the HCN vibrational lines become optically thick. AGN models yield higher T_dust in the inner regions and higher peak (sub)millimeter continuum brightness than SB models, but similar HCN vibrational J = 3-2 and 4-3 emission owing to both optical depth effects and a moderate impact of high T_dust on these low-J lines. The observed HCN vibrational emission in several galaxies can be accounted for with an HCN abundance of ∼10^–6 (relative to H₂) and luminosity surface brightness in the range (0.5-2) x 10⁸ L_☉ pc^–2, predicting a far-infrared photosphere with T_dust ∼ 80-150 K, in agreement with the values inferred from far-infrared molecular absorption.