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Published October 1, 2015 | Submitted + Published
Journal Article Open

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


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, q_(IR), 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.

Additional Information

© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 July 11. Received 2015 June 28. In original form 2014 April 18. First published online August 10, 2015. We thank the anonymous referee for their comments which improved the manuscript. We also thank Neal Miller for useful discussions and James Mullaney for help regarding DecompIR. JIR acknowledges the support of a Science and Technologies Facilities Council studentship. MJP acknowledges support from the Science and Technology Facilities Council (STFC) (grant number ST/K000977/1). SJO and MS acknowledge support from the STFC (grant number ST/I000976/1). NS is the recipient of an Australian Research Council Future Fellowship. MV acknowledges support from the South African Department of Science and Technology (DST/CON 0134/2014), the European Commission Research Executive Agency (FP7-SPACE-2013-1 GA 607254) and the Italian Ministry for Foreign Affairs and International Cooperation (PGR GA ZA14GR02). The Dark Cosmology Centre is funded by the Danish National Research Foundation. SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). This work is also based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory (JPL), California Institute of Technology (Caltech) under contract with NASA. This work benefitted from the NASA/IPAC Extragalactic Database (NED), which is operated by the JPL, Caltech, under contract with NASA.

Attached Files

Published - MNRAS-2015-Rawlings-4111-27.pdf

Submitted - 1507.07044v1.pdf


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