Active Galaxies and Quasars: Monsters of the Cosmos

An artistic look at what an early distant quasar might look like. ESO/M. Kornmesser

Once upon a time, not too long ago, no one knew much about supermassive black holes at their hearts. After several decades of observations and study, astronomers now have more insight into these hidden behemoths and the role they play in their galactic hosts.  For one thing, very active black holes are like beacons, streaming massive amounts of radiation out to space. These "active galactic nuclei" (AGN) are most commonly seen in radio wavelengths of light, with jets of plasma streaming hundreds of thousands of light-years away from the galactic core. They're also very bright in x-rays and also give off visible light. The very brightest are called "quasars" (which is short for "quasi-stellar radio sources") and can be seen across the cosmos. So, where did these behemoths come from and why are they so active? 

The Sources of Supermassive Black Holes

The monster black holes at the hearts of galaxies are most likely created a dense region of stars in the inner part of a forming galaxy merge to form an increasingly larger black hole. It's also very possible that the most massive ones formed during galaxy collisions when the black holes of two galaxies merged into one. The specifics are a little fuzzy, but ultimately the supermassive black hole will find itself in the middle of an enormous galaxy surrounded by stars, gas, and dust.

And it is the gas and dust in the immediate vicinity around the supermassive black hole that plays a key role in producing the incredible emission seen from some galaxies. The material that does not get swept out into the outer part of the galaxy during the formation of the supermassive black hole, will begin to circle the core in an accretion disk. As the material gets closer to the core it will heat up (and eventually fall into the black hole).

This process of heating up causes the gas to emit brightly in x-rays, as well as a host of wavelengths from infrared to gamma ray. Some of these objects have readily identifiable structures known as jets that spill forth high-energy particles from either pole of the supermassive black hole. An intense magnetic field from the black hole contains the particles in a narrow beam, constraining their path out of the galactic plane. As the particles stream out, traveling at nearly the speed of light, they interact with intergalactic gas and dust. Again, this process produces electromagnetic radiation at radio frequencies.

It is this combination of an accretion disk, core black hole and possibly jet structure that comprise the aptly named objects active galactic nuclei. Since this model relies on the existence of surrounding gas and dust in order to create the disk (and jet) structures, it is concluded that perhaps all galaxies have the potential to have AGN, but have depleted the gas and dust reserves in their cores.

Not all AGN are the same, however. The type of black hole, as well as the jet structure and orientation, lead to a unique categorization of these objects.

Seyfert Galaxies

Seyfert galaxies are those which contain AGN characterized by a medium-mass black hole at their core. They were also the first galaxies to exhibit radio jets.

Seyfert galaxies are seen edge on, meaning that the radio jets are clearly visible. The jets terminate in hugh plumes called radio lobes, and these structures can sometimes be larger than the entire host galaxy.

It was these giant radio structures that first caught the eye of radio astronomer Carl Seyfert in the 1940s. Subsequent studies revealed the morphology of these jets. A spectral analysis of these jets reveal that the material must be traveling and interacting at nearly the speed of light.

Blazars and Radio Galaxies

Traditionally blazars and radio galaxies were considered two different classes of objects. However, more recent study has suggested that they may actually be the same class of galaxy and that we are simply viewing them at different angles.

In both cases, these galaxies exhibit incredibly strong jets. And, while they can exhibit radiation signatures across the entire electromagnetic spectrum, they are typically extremely bright in the radio band.

The difference between these objects lies in the fact that blazars are observed by looking directly down the jet, while radio galaxies are viewed at some angle of inclination. This gives a different perspective of the galaxies which can lead to their radiation signatures looking completely different.

Because of this angle of inclination some of the wavelengths are weaker in radio galaxies, where as blazars are bright in virtually all bands. In fact, it was not until 2009 that a radio galaxy was even detected in the very high energy gamma-ray band.


In the 1960s it was noticed that some radio sources exhibited spectral information like that of the Seyfert galaxies, but appeared to be point-like sources, as if they were stars. That's how they got the name "quasars". 

In reality, these objects were not stars at all, but instead giant galaxies, many of which reside near the edge of the known universe. So distant where most of these quasars that their galaxy structure was not evident, again causing scientists to believe they were stars.

Like Blazars, these active galaxies appear face on, with their jets beamed directly at us. Therefore they can appear bright in all wavelengths. Interestingly, these objects also exhibit spectra similar to that of Seyfert galaxies.

These galaxies are of particular interest as they may hold the key to the behavior of galaxies in the early ​universe.

Updated and edited by Carolyn Collins Petersen.