The micro - Jy Radio Source Population : the VLA - CDFS View

We analyse the 267 radio sources from our deep (flux limit of 42 µJy at the field center at 1.4 GHz) Chandra Deep Field South 1.4 and 5 GHz VLA survey. The radio population is studied by using a wealth of multi-wavelength information, including morphology and spectral types, in the radio, optical, and X-ray bands. The availability of redshifts for ∼ 70% of our sources allows us to derive reliable luminosity estimates for the majority of the objects. Contrary to some previous results, we find that star-forming galaxies make up only a minority (≈ 1/3) of sub-mJy sources, the bulk of which are faint radio galaxies, mostly of the Fanaroff-Riley I type. The extragalactic radio source population ranges from normal galaxies with lu-minosities ∼ 10 20 W/Hz to galaxies whose radio emission is as much as 10 4−7 times greater owing to regions of massive star formation or to an active galactic nucleus (AGN). The population of radio sources in the sky with flux densities > 1 mJy is dominated by AGN driven emission generated from the gravita-tional potential associated with a supermassive black-hole in the nucleus. For these sources, the observed radio emission includes the classical extended jet and double lobe radio source as well as compact radio components more directly associated with the energy generation and collimation near the central engine of the AGN. Below 1 mJy there is an increasing contribution to the radio source population from synchrotron emission resulting from relativistic plasma ejected from supernovae associated with massive star formation in galaxies or groups of galaxies, often associated with mergers or interactions (e.g., Windhorst et al.