Journal Article

Exploring the infrared/radio correlation at high redshift

Edo Ibar, Michele Cirasuolo, Rob Ivison, Philip Best, Ian Smail, Andy Biggs, Chris Simpson, Jim Dunlop, Omar Almaini, Ross McLure, Sebastien Foucaud and Steve Rawlings

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 386, issue 2, pages 953-962
Published in print May 2008 | ISSN: 0035-8711
Published online April 2008 | e-ISSN: 1365-2966 | DOI:
Exploring the infrared/radio correlation at high redshift

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We have analysed the 24-μm properties of a radio-selected sample in the Subaru–XMM–Newton Deep Field in order to explore the behaviour of the far-infrared/radio (FIR/radio) relation at high redshifts. Statistically, the correlation is described by q24, the ratio between the observed flux densities at 24 μm and 1.4 GHz, respectively. Using 24-μm data results in considerably more scatter in the correlation than previous work using data at 60–70 μm. Nevertheless, we do observe a steady correlation as a function of redshift, up to z≈ 3.5, suggesting its validity back to primeval times. We find q24= 0.30 ± 0.56 for the observed and q24= 0.71 ± 0.47 for the k-corrected radio sample, based on sources with 300 μJy < S1.4 GHz < 3.2 mJy and 24-μm detections. A suitable k-correction given by a M82-like mid-infrared (mid-IR) template suggests no extreme silicate absorption in the bulk of our radio sample. Using thresholds in q24 to identify radio-excess sources, we have been able to characterize the transition from radio-loud active galactic nuclei (AGN) to star-forming galaxies and radio-quiet AGN at faint (≲1 mJy) radio-flux densities. Our results are in broad agreement with previous studies which show a dominant radio-loud AGN population at >1 mJy. The rest-frame UB colours of the expected radio-excess population have a redder distribution than those that follow the correlation. This is therefore a promising way to select obscured type 2 AGN, with a radio-loud nature, missed by deep X-ray observations. Spectroscopic follow-up of these sources is required to fully test this method.

Keywords: galaxies: active; galaxies: fundamental parameters; galaxies: high-redshift; galaxies: starburst

Journal Article.  8203 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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