Deep Herschel observations of the 2Jy sample: assessing the non-thermal and AGN contributions to the far-IR continuum

TL;DR

This study uses Herschel observations of 46 radio AGN to evaluate the impact of non-thermal emission on far-IR measurements, revealing significant contamination that affects star formation rate estimates in host galaxies.

Contribution

It provides the first comprehensive analysis of non-thermal contamination in far-IR data of a complete radio AGN sample, highlighting its dependence on morphology and optical spectra.

Findings

Non-thermal contamination ranges from 28% to 72% depending on wavelength.

Strong correlations between far-IR luminosities and AGN power suggest AGN heating of dust.

Contamination is more prevalent in compact and type 1 AGN objects.

Abstract

The far-IR/sub-mm wavelength range contains a wealth of diagnostic information that is important for understanding the role of radio AGN in galaxy evolution. Here we present the results of Herschel PACS and SPIRE observations of a complete sample of 46 powerful 2Jy radio AGN at intermediate redshifts (0.05 < z < 0.7), which represent the deepest pointed observations of a major sample of radio AGN undertaken by Herschel. In order to assess the importance of non-thermal synchrotron emission at far-IR wavelengths, we also present new APEX sub-mm and ALMA mm data. We find that the overall incidence of non-thermal contamination in the PACS bands ($<$200$\mu$m) is in the range 28 -- 43%; however, this rises to 30 -- 72% for wavelengths ($> $200$\mu$m) sampled by the SPIRE instrument. Non-thermal contamination is strongest in objects with compact CSS/GPS or extended FRI radio morphologies, and in those with type 1 optical spectra. Considering thermal dust emission, we find strong correlations between the 100 and 160$\mu$m monochromatic luminosities and AGN power indicators, providing further evidence that radiation from the AGN may be an important heating source for the far-IR emitting dust. Clearly, AGN contamination -- whether by the direct emission from synchrotron-emitting lobes and cores, or via radiative heating of the cool dust -- needs to be carefully considered when using the far-IR continuum to measure the star formation rates in the host galaxies of radio AGN.