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Published in

Oxford University Press, Monthly Notices of the Royal Astronomical Society, 2(383), p. 479-496, 2007

DOI: 10.1111/j.1365-2966.2007.12576.x

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The radio properties of optically obscured Spitzer sources

Journal article published in 2007 by Manuela Magliocchetti ORCID, Paola Andreani, Martin A. Zwaan
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Preprint: archiving allowed
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Postprint: archiving allowed
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Published version: archiving allowed
Data provided by SHERPA/RoMEO

Abstract

Abstract This paper analyses the radio properties of a subsample of optically obscured (R≥ 25.5) galaxies observed at 24 μm by the Spitzer Space Telescope within the First Look Survey. Ninety-six F24 μm≥ 0.35 mJy objects out of 510 are found to have a radio counterpart at 1.4 GHz, 610 MHz or at both frequencies, respectively, down to ∼40 and ∼200 μJy. IRAC photometry sets the majority of them in the redshift interval z≃[1–3] and allows for a broad distinction between active galactic nucleus (AGN) dominated galaxies (∼47 per cent of the radio-identified sample) and systems powered by intense star formation (∼13 per cent), the remaining objects being impossible to classify. The percentage of radio identifications is a strong function of 24-μm flux: almost all sources brighter than F24 μm∼ 2 mJy are endowed with a radio flux at both 1.4 GHz and 610 MHz, while this fraction drastically decreases by lowering the 24-μm flux level. The radio number counts at both radio frequencies suggest that the physical process(es) responsible for radio activity in these objects have a common origin regardless of whether the source shows mid-infrared emission compatible with being an obscured AGN or a star-forming galaxy. We also find that both candidate AGN and star-forming systems follow (although with a large scatter) the relationship between 1.4-GHz and 24-μm fluxes reported by Appleton et al. which identifies sources undergoing intense star formation activity. However, a more scattered relation is observed between 24-μm and 610-MHz fluxes. On the other hand, the inferred radio spectral indices α indicate that a large fraction of objects in our sample (∼60 per cent of all galaxies with estimated α) may belong to the population of ultrasteep spectrum sources, typically ‘frustrated’ radio-loud AGN. We interpret our findings as a strong indication for concurrent AGN and star-forming activity, whereby the 1.4-GHz flux is of thermal origin, while that at 610 GHz mainly stems from the nuclear source.