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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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.