Radio galaxies and type-2 quasars in the Spitzer Extragalactic First Look Survey

Abstract

We use mid-infrared colour-colour plots made with data from the Spitzer First Look Survey to explore the emission mechanisms of faint radio sources in that field. Using mid-infrared colours alone, we are able to see the transition from a starburst-dominated radio source population at sub-mJy levels to the radiogalaxy/quasar population at mJy levels. Both FR I and some FR II galaxies show little or no mid-infrared excess, suggesting they either lack dusty tori, or a strong continuum source to heat them, or both. Objects with hot, dusty tori, most likely radio-intermediate and radio-quiet AGN, make up a significant fraction of the faint radio source population. We also discuss the use of mid-infrared colour-colour plots to select radio quiet obscured AGN. Early results from imaging the host galaxies of these type-2 quasars at high spatial resolution with adaptive optics indicates that they, like type-1 quasars and radio galaxies, are often found to be hosted by giant elliptical galaxies.