THE FAR-INFRARED PROPERTIES OF 3CR QUASARS AND RADIO GALAXIES

Measurements at 60 mu wavelength of 3CR quasars and powerful radio galaxies in the redshift range 0.3 InfraRed Astronomical Satellite (IRAS). On the basis of the detection rates, the data on this sample are consistent with quasars being more luminous at this wavelength than radio galaxies, as claimed previously by Heckman et al. (1994). Assuming that all far infrared emission represents isotropic thermal re-radiation from a circumnuclear dust torus, this result is in conflict with the unified model for quasars and powerful radio galaxies. Supplementing our data with IRAS measurements of 3CR sources from the literature, we attempt to identify other processes that determine the infrared output. We find evidence for the effect of relativistic beaming on the infrared emission in those quasars that were known to be beamed from measurements at other wavelengths. Adding IRAS data on additional radio loud quasars, we find continuity of quasar 60 mu luminosities with increasing relative strength of the radio core. This suggests that lobe dominated steep-spectrum quasars also display some level of beaming in the far infrared. Moderately different far infrared spectral indices for these quasars in comparison to radio galaxies are consistent with this picture. Modeling the infrared radiation of powerful radio galaxies and quasars as the sum of isotropic thermal and aspect-dependent nonthermal radiation, the inconsistency of the data with the unified model may be less severe. The probability of detecting a radio galaxy in the far infrared is enhanced by N-type morphology, in connection with either broad permitted lines or the combination of compact radio structure and blue optical colours. Broad line galaxies furthermore have infrared spectra with warmer infrared emission than both quasars and narrow line galaxies. Some of the strongest infrared emitters among the narrow line radio galaxies have disturbed optical morphology and blue colours indicative of massive star formation and dust heating. Compact steep-spectrum radio morphologies are frequent in these galaxies, suggesting that the dust is heated by stars formed through the interaction of the radio source with the interstellar matter. In an Appendix we assess for 20 3CR N-type radio galaxies their classification as narrow or broad lined object and we propose several updated classifications.