We separate the extragalactic radio source population above ˜50
μJy 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 (Infrared Array
Camera) and Multiband Imaging Photometer for Spitzer (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
˜1 mJy for the extragalactic radio source population.