Radio galaxy and their relatives, radio-loud quasars and blazars, are active galactic nuclei that are extremely luminous at radio wavelengths, with luminosities ranging from 10 MHz to 100 GHz. The synchrotron process is responsible for radio emissions.
The interaction between twin jets and the external medium determines the observed structure in radio emission, which is modified by the effects of relativistic beaming.
Almost all of the host galaxies are large elliptical galaxies. Radio-loud active galaxies can be detected over long distances, making them useful for observational cosmology.
Much research has recently been conducted into the effects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.
In radio maps, radio galaxies and, to a lesser extent, radio-loud quasars exhibit a wide range of structures. The most common large-scale structures are lobes, which are double, often symmetrical, roughly ellipsoidal structures that are placed on either side of the active nucleus. A significant minority of low-luminosity sources have structures that are much more elongated than plumes.
Because of its very smooth, broad-band nature and strong polarisation, radio emission from radio-loud active galaxies is synchrotron emission. This implies that the radio-emitting plasma contains at least relativistic electrons and magnetic fields. Because the plasma must be neutral, it must also contain protons or positrons.
Radio galaxies and radio-loud quasars were widely used to find distant galaxies, particularly in the 1980s and 1990s by selecting based on radio spectrum and then observing the host galaxy, it was possible to find objects at high redshift at a low cost in telescope time.