ISOCAM survey and dust models of 3CR radio galaxies and quasars

Abstract

We present a survey of all 3CR sources imaged with ISOCAM onboard the Infrared Space Observatory (ISO) . The sample consists mostly of radio-loud active galactic nuclei (AGN). For each source, we present spatially integrated mid-infrared (MIR, $5{-}18~\mu$m) fluxes measured from newly calibrated ISOCAM images. In total, we detected 68 objects of the 3CR catalogue, at redshifts $z \le 2.5$, and obtained upper limits for 17 objects. In addition, we detected 10 galaxies not listed in the 3CR catalogue. The one with the highest redshift is 4C+72.26 at $z = 3.53$. ISOCAM data are combined with other photometric measurements to construct the spectral energy distribution (SED) from optical to radio wavelengths. The MIR emission may include synchrotron radiation of the AGN, stars of the host galaxy or dust. Extrapolation of radio core fluxes to the MIR show that the synchrotron contribution is in most cases negligible. In order to describe dust emission we apply new radiative transfer models. In the models the dust is heated by a central source which emits photons up to energies of 1 keV. By varying three parameters, luminosity, effective size and extinction, we obtain a fit to the SED for our objects. Our models contain also dust at large (several kpc) distances from the AGN. Such a cold dust component was neglected in previous computations which therefore underestimated the AGN contribution to the far infrared (FIR). In 53 cases (~$ 75$% of our detected 3CR sources), the MIR emission can be attributed to dust. The hot dust component is mainly due to small grains and PAHs. The modelling demonstrates that AGN heating suffices to explain the ISO broad band data, starburst activity is not necessary. In the models, a type 1 AGN is represented by a compact dust distribution, the dust is therefore very warm and emission of PAHs is weak because of photo-destruction. In AGNs of type 2, the dust is relatively colder but PAH bands are strong.