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

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.

21 pages, 11 figures

References in corpus (19)

• The 0.4 < z < 1.3 star formation history of the Universe as viewed in the far-infrared
• The Multiwavelength Survey by Yale-Chile (MUSYC): Deep Medium-Band optical imaging and high quality 32-band photometric redshifts in the ECDF-S
• Magnetic Fields in Starburst Galaxies and The Origin of the FIR-Radio Correlation
• Radio imaging of the Subaru/XMM-Newton Deep Field - I. The 100-microJy catalogue, optical identifications, and the nature of the faint radio source population
• Deep ATLAS radio observations of the CDFS-SWIRE field
• The Star Formation History of the Universe as Revealed by Deep Radio Observations
• Dust in starburst nuclei and ULIRGs: SED models for observers
• A new method to separate star forming from AGN galaxies at intermediate redshift: The submillijansky radio population in the VLA-COSMOS survey
• Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 II: Diagnostics
• A Public, K-Selected, Optical-to-Near-Infrared Catalog of the Extended Chandra Deep Field South (ECDFS) from the MUltiwavelength Survey by Yale-Chile (MUSYC)
• The Deep SWIRE Field I. 20cm Continuum Radio Observations: A Crowded Sky
• The VLA 1.4GHz Survey of the Extended Chandra Deep Field South: First Data Release
• Radio Jets in Galaxies with Actively Accreting Black Holes: new insights from the SDSS
• Exploring the infrared/radio correlation at high redshift
• The VLA Survey of the Chandra Deep Field South: I. Overview of the Radio Data
• Large amounts of optically-obscured star formation in the host galaxies of some type-2 quasars
• The properties of 70micron selected high-redshift galaxies in the Extended Groth Strip
• HerMES: Spectral Energy Distributions of Submillimeter Galaxies at z >4
• ESO Imaging Survey: Infrared Deep Public Survey

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• The KMOS AGN Survey at High redshift (KASHz): the prevalence and drivers of ionised outflows in the host galaxies of X-ray AGN
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• Winds as the origin of radio emission in $z=2.5$ radio-quiet extremely red quasars
• The XXL Survey: XXXI. Classification and host galaxy properties of 2.1 GHz ATCA XXL-S radio sources
• AGN and star formation across cosmic time
• The Nature of the Faint Low-Frequency Radio Source Population
• SPARCS-North Wide-field VLBI Survey: Exploring the resolved MicroJy extra-galactic radio source population with EVN+e-MERLIN
• Cosmology from LOFAR Two-metre Sky Survey Data Release 2: Counts-in-Cells Statistics