Hot Dust in Radio-loud Active Galactic Nuclei

We have measured mid-infrared (MIR) fluxes of 3C sources on images taken with ISOCAM on board the Infrared Space Observatory (ISO). The photometric data were combined with existing photometry at other wavelengths to assemble the spectral energy distribution (SED) for each galaxy from infrared to radio wavelengths. In addition, we used ISOPHOT spectra to compute average MIR spectra for different types of active galactic nuclei (AGNs). The MIR emission of 53 of our sources shows evidence of dust. We find a clear correlation between the SEDs, emission from polycyclic aromatic hydrocarbons (PAHs), and the type of AGN. Specifically, we find that in broad emission line radio galaxies (BLRGs), the dust emission peaks at λ ≈ 40 μm and PAH bands are weak, whereas emission in comparable narrow-line radio galaxies (NLRGs) peaks at a longer wavelength of λ ≈ 100 μm and the PAH bands are much stronger. Although less pronounced, a similar trend is seen when comparing QSOs with high-luminosity NLRGs. We used dust radiative transfer models with a central heating source to describe the SEDs. The difference in the dust emission appears to be the effect of heating by radiation from the central engine. In type 1 AGNs (BLRGs and QSOs), the broad-line region (BLR) is directly visible, and therefore hot (T ≳ 300 K) dust dominates the emission. In this region, PAHs are destroyed, which explains the weak PAH emission. On the other hand, the BLR is hidden in type 2 AGNs (NLRGs). Their SEDs are therefore dominated by cooler dust, including PAHs.