The AGN content of deep radio surveys and radio emission in radio-quiet
  AGN. Why every astronomer should care about deep radio fields
  We present our very recent results on the sub-mJy radio source populations at
1.4 GHz based on the Extended Chandra Deep Field South VLA survey, which
reaches ~ 30 {\mu}Jy, with details on their number counts, evolution, and
luminosity functions. The sub-mJy radio sky turns out to be a complex mix of
star-forming galaxies and radio-quiet AGN evolving at a similar, strong rate
and declining radio-loud AGN. 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-mJy sky. Our results
shed also light on a fifty-year-old issue, namely radio emission from
radio-quiet AGN, and suggest that it is closely related to star formation, at
least at z ~ 1.5 - 2. The implications of our findings for future, deeper radio
surveys, including those with the Square Kilometre Array, are also discussed.
One of the main messages, especially to non-radio astronomers, is that radio
surveys are reaching such faint limits that, while previously they were mainly
useful for radio quasars and radio galaxies, they are now detecting mostly
star-forming galaxies and radio-quiet AGN, i.e., the bulk of the extragalactic
sources studied in the infrared, optical, and X-ray bands.
