HerMES: Disentangling active galactic nuclei and star formation in the radio source population

TL;DR

This study disentangles active galactic nuclei and star formation in faint radio sources using infrared spectral energy distributions and multi-wavelength diagnostics, revealing a high prevalence of hybrid systems influencing radio source counts.

Contribution

The paper introduces a method combining infrared SED fitting with multi-wavelength diagnostics to distinguish AGN and star formation contributions in radio sources.

Findings

60% of radio sources show AGN signatures at some wavelength.

58% of AGN-signature sources are hybrid systems with star formation-powered radio emission.

Hybrid sources constitute 20-65% of sources below 1 mJy flux density.

Abstract

We separate the extragalactic radio source population above ~50 uJy into active galactic nuclei (AGN) and star-forming sources. The primary method of our approach is to fit the infrared spectral energy distributions (SEDs), constructed using Spitzer/IRAC and MIPS and Herschel/SPIRE photometry, of 380 radio sources in the Extended Chandra Deep Field-South. From the fitted SEDs, we determine the relative AGN and star-forming contributions to their infrared emission. With the inclusion of other AGN diagnostics such as X-ray luminosity, Spitzer/IRAC colours, radio spectral index and the ratio of star-forming total infrared flux to k-corrected 1.4 GHz flux density, qIR, we determine whether the radio emission in these sources is powered by star formation or by an AGN. The majority of these radio sources (60 per cent) show the signature of an AGN at some wavelength. Of the sources with AGN signatures, 58 per cent are hybrid systems for which the radio emission is being powered by star formation. This implies that radio sources which have likely been selected on their star formation have a high AGN fraction. Below a 1.4 GHz flux density of 1 mJy, along with finding a strong contribution to the source counts from pure star-forming sources, we find that hybrid sources constitute 20-65 per cent of the sources. This result suggests that hybrid sources have a significant contribution, along with sources that do not host a detectable AGN, to the observed flattening of the source counts at ~1mJy for the extragalactic radio source population.