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