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Ju n 20 05 DUST EMISSION BY AGN AND STARBURSTS

Present AGN and starburst models aiming to account for the observed infrared SEDs consider a physical description of the dust and a solution of the radiative transfer problem. Mid infrared spectra obtained at different spatial scales (SST-IRS, ISO and Timmi2) are presented. They show that PAH bands are detected in starburst regions but significantly reduced near the centre of AGN. This is explained by examining the heating mechanism of PAHs after hard (FUV, X-ray) photon interactions. Most economic radiative transfer models of starbursts and AGN are presented where only key parameters such as luminosity, dust mass or size of the nucleus are varied. For both activity types syntetic spectra are made available. The successful application of the starburst model is demonstrated by fitting broad band data and detailed Spitzer IRS spectra of NGC7714. The AGN model is applied to ISOCAM and ISOPHT broad band data of a sample of 68 radio galaxies and quasars of the 3CR catalogue. Radiative transfer models of galaxies with Hidden Broad Line Regions (HBLR) are presented. Their SED enable us to separate the contributions from the dusty disc of the AGN and the starbursts. We find that the combination of AGN heated discs and starbursts provide good fits to the data. The composite model is consistent with the unified scheme and the idea that the infrared emission of AGN is dominated by a dusty disc in the mid-infrared and starbursts in the far-infrared. According to the unified scheme, AGN are surrounded by a dust-torus, and the observed diversity of AGN properties results from the different orientations relative to our line of sight. The strong resonance of silicate dust at 10 µm is therefore, as expected, seen in absorption towards many type-2 AGN. In type-1 AGN, it should be seen in emission because the hot inner surface of the dust torus becomes visible. However, this has not been observed so far, thus challenging the unification scheme or leading to exotic modifications of the dust-torus model. Here the recent discovery of the 10 µm silicate feature in emission in luminous quasar spectra observed with the SST/IRS is reported.