Letter to the Editor

The radio emission from active galactic nuclei

¹ Kapteyn Astronomical Institute, University of Groningen, 9747 AD Groningen, The Netherlands
e-mail: pdb@astro.rug.nl
² Department of Physics, University of Pretoria, Lynnwood Road, Hatfield, Pretoria 0083, South Africa
³ Jodrell Bank Centre for Astrophysics, School of Physics & Astronomy, The University of Manchester, Alan Turing Building, Oxford Road, Manchester M13 9PL, UK
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 12 March 2021
Accepted: 8 April 2021

Abstract

Context. For nearly seven decades, astronomers have been studying active galaxies, that is to say, galaxies with actively accreting central supermassive black holes: active galactic nuclei (AGN). A small fraction are characterized by luminous, powerful radio emission: This class is known as radio-loud AGN. A substantial fraction, the so-called radio-quiet AGN population, display intermediate or weak radio emission. However, an appreciable fraction of strong X-ray-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.

Aims. We wish to address the nature of these – seemingly radio-silent – X-ray-luminous AGN and their host galaxies to determine if there is any radio emission, and, if so, where it originates.

Methods. 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.

Results. 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.

Conclusions. 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 cases can be classified as radio-silent AGN.