VLA survey of the CDFS: the nature of faint radio sources

We present the multiwavelength properties of 266 cataloged radio sources identified with 20 and 6 cm VLA deep observations of the CDFS at a flux density limit of 42 μJy at the field centre at 1.4 GHz. These new observations probe the faint end of both the star formation and radio galaxy/AGN population. X–ray data, including upper limits, turn out to be a key factor in establishing the nature of faint radio sources. We find that, while the well–known flattening of the radio number counts below 1 mJy is mostly due to star forming galaxies, these sources and AGN make up an approximately equal fraction of the sub– millijansky sky, contrary to some previous results. We have also uncovered a population of distant AGN systematically missing from many previous studies of sub–millijansky radio source identifications. The AGN include radio galaxies, mostly of the low–power, Fanaroff–Riley I type, and a significant radio–quiet component, which amounts to approximately one fifth of the total sample. We also find that radio detected, X–ray AGN are not more heavily obscured than the X–ray detected AGN. This argues against the use of radio surveys as an efficient way to search for the missing population of strongly absorbed AGN.