Recent interferometric observations have shown bright HCN emission from the \nu _ { 2 } = 1 vibrational state arising in buried nuclear regions of galaxies , indicating an efficient pumping of the \nu _ { 2 } = 1 state through absorption of 14 \mu m continuum photons .
We have modeled the continuum and HCN vibrational line emission in these regions , characterized by high column densities of dust and high luminosities , with a spherically symmetric approach , simulating both a central heating source ( AGN ) and a compact nuclear starburst ( SB ) .
We find that when the H _ { 2 } columns become very high , N _ { \mathrm { H 2 } } \gtrsim 10 ^ { 25 } cm ^ { -2 } , trapping of continuum photons within the nuclear region dramatically enhances the dust temperature ( T _ { \mathrm { dust } } ) in the inner regions , even though the predicted spectral energy distribution as seen from outside becomes relatively cold .
The models thus predict bright continuum at millimeter wavelengths for luminosity surface brightness ( averaged over the model source ) of \sim 10 ^ { 8 } L _ { \odot } pc ^ { -2 } .
This greenhouse effect significantly enhances the mean mid-infrared intensity within the dusty volume , populating the \nu _ { 2 } = 1 state to the extent that the HCN vibrational lines become optically thick .
AGN models yield higher T _ { \mathrm { 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 _ { \mathrm { dust } } on these low- J lines .
The observed HCN vibrational emission in several galaxies can be accounted for with a HCN abundance of \sim 10 ^ { -6 } ( relative to H _ { 2 } ) and luminosity surface brightness in the range ( 0.5 - 2 ) \times 10 ^ { 8 } L _ { \odot } pc ^ { -2 } , predicting a far-infrared photosphere with T _ { \mathrm { dust } } \sim 80 - 150 K –in agreement with the values inferred from far-infrared molecular absorption .