The Greenhouse Effect in Buried Galactic Nuclei and the Resonant HCN Vibrational Emission

Abstract

Recent interferometric observations have shown bright HCN emission from the ν 2  = 1 vibrational state arising in buried nuclear regions of galaxies, indicating an efficient pumping of the ν 2  = 1 state through the absorption of 14 μ m 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 H 2 columns become very high, N H2  ≳ 10 25 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 ∼10 8 L ⊙ pc −2 . This greenhouse effect significantly enhances the mean mid-infrared intensity within the dusty volume, populating the ν 2  = 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 H 2 ) and luminosity surface brightness in the range (0.5–2) × 10 8 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.