The origin of the infrared emission in radio galaxies. III. Analysis of 3CRR objects

TL;DR

This study uses Spitzer data to analyze the infrared emission origins in low-redshift radio galaxies, showing AGN illumination as the main heating source for dust, with starbursts contributing in some cases.

Contribution

It provides strong statistical evidence linking AGN power to infrared emission and distinguishes properties of broad-line objects at low redshift.

Findings

24 micron emission correlates with AGN power, indicating dust heating by AGN.

70 micron emission shows more scatter due to starburst contributions.

Low-redshift broad-line objects differ in their infrared and [OIII] emission properties.

Abstract

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 AGN. Our results show a correlation between AGN power (indicated by [OIII] 5007 emission line luminosity) and 24 micron luminosity. This result is consistent with the 24 micron thermal emission originating from warm dust heated directly by AGN illumination. Applying the same correlation test for 70 micron luminosity against [OIII] luminosity we find this relation to suffer from increased scatter compared to that of 24 micron. In line with our results for the higher-radio-frequency-selected 2Jy sample, we are able to show that much of this increased scatter is due to heating by starbursts which boost the far-infrared emission at 70 micron 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 vs. [OIII] correlations.