The Origin of the Infrared Emission in Radio Galaxies:III. Analysis of 3CRR Objects

We present Spitzer photometric data for a complete sample of 19 low-redshift (z <0.1) 3CRR radio galaxies as part of our efforts to understand the origin of the prodigious mid-to far-infrared (MFIR) emission from radio-loud active galactic nuclei (AGNs). Our results show a correlation between AGN power (indicated by [O III]lambda 5007 emission line luminosity) and 24 mu m luminosity. This result is consistent with the 24 mu m thermal emission originating from warm dust heated directly by AGN illumination. Applying the same correlation test for 70 mu m luminosity against [O III] luminosity we find this relation to suffer from increased scatter compared to that of 24 mu m. In line with our results for the higher-radio-frequency-selected 2 Jy sample, we are able to show that much of this increased scatter is due to heating by starbursts that boost the far-infrared emission at 70 mu m in a minority of objects (17%-35%). Overall this study supports previous work indicating AGN illumination as the dominant heating mechanism for MFIR emitting dust in the majority of low-to-intermediate redshift radio galaxies (0.03 <z <0.7), with the advantage of strong statistical evidence. However, we find evidence that the low-redshift broad-line objects (z <0.1) are distinct in terms of their positions on the MFIR versus [O III] correlations.