2008MNRAS.383..479M

This paper analyses the radio properties of a subsample of optically obscured (R ≥ 25.5) galaxies observed at 24µm by the Spitzer Space Telescope within the First Look Survey. Ninety-six F_24µm ≥ 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 ∼200µJy. 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 F_24µm∼ 2 mJy are endowed with a radio flux at both 1.4GHz and 610MHz, 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 610GHz mainly stems from the nuclear source.