The radio emission from Active Galactic Nuclei

TL;DR

This study investigates the radio emission properties of X-ray luminous AGN in the GOODS-N field, revealing that about half lack significant jet-driven radio emission, suggesting many AGN are radio-silent or dormant.

Contribution

It provides new insights into the radio emission mechanisms of X-ray luminous AGN, especially the prevalence of radio-silent AGN in star-forming galaxies, using stacking techniques on deep radio data.

Findings

Approximately half of X-ray luminous AGN show no significant radio emission.

Radio-silent AGN are common among actively accreting black holes.

Radio emission may be dormant or absent in many AGN, indicating diverse activity states.

Abstract

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.