What Are Active Galaxies?

While it may sound like an Active Galaxy is one that likes to run for fun, these high-luminosity galaxies are so named because of the active accretion going on to the super massive black hole at the centre. Despite a simple definition at its core, “Active Galaxy” is actually a very broad term that covers a lot of conditions.

The Active Galactic Nucleus

The main reason a galaxy is called “active” is because it contains an Active Galactic Nucleus. This AGN is a highly compact region in the host galaxy that provides a level of luminosity that is very high—higher, in fact, than the majority of the electromagnetic spectrum, which includes infrared, optical, ultra-violet, X-ray, gamma ray and radio wave bands. When a galaxy has an Active Galactic Nucleus at its centre, it is dubbed an Active Galaxy.

Why Are Active Galaxies So Luminous?

The radiation that comes from an active galactic nucleus is thought to be caused by accretion to the super massive black hole at the centre of the galaxy. This belief has lead to a lot of research and study as scientists struggled to understand the impact of this data, should it be true. As a result it was discovered that there are many different features of an active galaxy, causing the label to be a very broad one.

How to Label an Active Galaxy

There is no one feature that, upon observation, would lead an astronomer to say “this is an active galaxy.” There are plenty that would lead that astronomer to suggest that it might be such—such as a jet off an accretion disc with nuclear optical continuum emission. Nuclear infra-red emission, X-ray line emission and radio continuum emission are other clues. A galaxy with observable broad optical emission lines might be an active galaxy—as might a galaxy with observable narrow optical emission lines.

Categorising Active Galaxies

There are two broad categories that an active galaxy could fall into- radio loud, and radio quiet. With the former radiation coming off the jet from the accretion disk is the dominant factor in the luminosity, and in the latter the jet contributions are not significant enough to be counted.

In the radio loud bucket of active galaxies there are a number of sub-types. Radio galaxies are objects that have extended radio emissions, while Blazars have no optical emission lines but very active, variable radio and x-ray emissions. Final there are radio-loud galaxies, similar to radio-loud quasars, but with a jet of emissions that spands the electromagnetic spectrum.

In the radio quite bucket are the radio quiet quasars, quasar 2’s, and Seyfert galaxies, which were among the first active galactic nuclei to be discovered. There are also LINERs, or Low- ionization nuclear emission-line regions. The LINERs have weak nuclearn emissions, and in fact some debate whether or not the LINERs are true active galaxies with true AGNs.

Time Table

Active galaxies have always been a useful tool in helping to determine the age of the universe and its state of being in the “early days”. It has been determined that there used to be more AGNs than there are now, suggesting that there were more super massive black holes, and that the conditions of the universe in the past were more conducive to the forming of AGNs than now.