Introductory Astronomy: Double Lobed Radio Galaxies

A double-lobed radio galaxy gets its name from the fact that it emits radio energy from two regions, one on each side of the galaxy. These regions of strong radio emission are called "lobes." Although galaxies are enormous, the radio lobes of these AGN are generally much larger than the galaxy in which they originate. Many are as large as 60 kpc in diameter, which is twice the size of the Milky Way Galaxy.

Radio lobes produce extremely intense radiation called synchrotron radiation. Synchrotron radiation is produced when electrons which are moving at nearly the speed of light spiral through a magnetic field. The total energy in a single radio lobe is about 10^53 Joules. This is an enormous amount! As a comparison, a typical supernova explosion puts out approximately 10^44 Joules of energy.

Generally, the radio lobes are thought to be caused by the interaction of a jet of high energy gas and the surrounding medium. As these jets stream out of the nucleus of the galaxy, they must move through the intergalactic medium. The intergalactic medium tries to slow the jet, just like water slows a swimmer or a boat; this causes material to be dragged along with the jet, just like the wake of a boat. Because the jet is moving with a much higher speed than the medium, shocks form at the ends of the jet. These shocks create a "Hot Spot," which is a site of intense emission. The radio lobe includes the hot spot and the material in the wake of the jet.

As they travel through the intergalactic medium, these jets may become twisted by the motion of the galaxy itself or even be made to change direction due to the streaming motions of the medium. These effects make each double-lobed radio galaxy unique and interesting.

According to the unified theory, these radio lobes are the ends of jets produced by a supermassive black hole at the center of the galaxy. The jets extend on each side of the galaxy perpendicular to the central accretion disk which surrounds the black hole because it is easier for the jet to move perpendicular to the disk (where there is not much material) than through the thick disk. This is why we see two jets moving in opposite directions.

An HST picture of three different radio lobed galaxies. Notice that some of the lobes appear twisted.