The radio emission from Active Galactic Nuclei
  For nearly seven decades astronomers have been studying active galaxies, that
is to say galaxies with actively accreting central supermassive black holes,
AGN. A small fraction of these are characterized by luminous, powerful radio
emission: this class is known as radio-loud. A substantial fraction, the
so-called radio-quiet AGN population, displays intermediate or weak radio
emission. However, an appreciable fraction of strong X-rays emitting AGN are
characterized by the absence of radio emission, down to an upper limit of about
$10^{-7}$ times the luminosity of the most powerful radio-loud AGN.
  We wish to address the nature of these - seemingly radio-silent -
X-ray-luminous AGN and their host galaxies: is there any radio emission, and if
so, where does it originate? Focusing on the GOODS-N field, we examine the
nature of these objects employing stacking techniques on ultra-deep radio data
obtained with the JVLA. We combine these radio data with Spitzer far-infrared
data. We establish the absence, or totally insignificant contribution of
jet-driven radio-emission in roughly half of the otherwise normal population of
X-ray luminous AGN, which appear to reside in normal star-forming galaxies. We
conclude that AGN- or jet-driven radio emission is simply a mechanism that may
be at work or may be dormant in galaxies with actively accreting black holes.
The latter can be classified as radio-silent AGN.
