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.