2004MNRAS.348.1065C

We discuss the different physical processes contributing to the infrared continuum of active galactic nuclei (AGNs), assuming that both photoionization from the active centre and shocks ionize and heat the gas and dust contained in an ensemble of clouds surrounding the nucleus. In our model, radiation transfer of primary and secondary radiation throughout a cloud is calculated consistently with collisional processes due to the shock. We consider that the observed continuum corresponds to reprocessed radiation from both dust and gas in the clouds. Collisional processes are important in the presence of shocks. The grains are sputtered crossing the shock front. The models are constrained by sputtering as well as by the far-infrared data. The model is applied to the continuum of Seyfert galaxies from which the best estimate of the nuclear, stellar subtracted, emission is available. The results show that radiation-dominated high-velocity clouds are more numerous in Seyfert 1-1.5 whereas shock-dominated low-velocity clouds are dominant in Seyfert type 2. This result is in full agreement with the unified model for AGNs, by which high-velocity clouds, placed deeper into the central region and therefore reached by a more intense radiation, should play a more significant role in the spectra of broad-line objects. We could therefore conclude that in type 2 objects, radiation is partly suppressed by a central dusty medium with a high dust-to-gas ratio. Once the model approach is tested, a grid of models is used to provide a phenomenological analysis of the observed infrared spectral energy distribution. This empirical method is a useful tool to rapidly access the physical conditions of the AGN emitting clouds. For this, analytical forms are derived for the two processes contributing to the infrared emission: dust emission and thermal bremsstrahlung produced by the narrow-line region clouds. Their relative contribution provides a measurement of the dust-to-gas ratio.