Active Galaxies

15.9 - Know that some galaxies emit large quantities of radiation in addition to visible light
15.10 - Know that an Active Galactic Nucleus (AGN) is powered by matter falling onto a super-massive black hole
15.11 - Know types of active galaxies, including:
a) Seyfert galaxies
b) quasars
c) blazars
15.12 - Know that information about AGNs can be obtained from many regions of the electromagnetic spectrum

Like stars and nebula, galaxies emit radiation that can be seen in the electromagnetic spectrum. This includes visible light, radio waves, ultra-violet, infrared, x-rays and gamma rays.

An Active Galactic Nucleus (AGN) is an area at the centre of a galaxy that has an above average brightness (luminosity) over the spectrum. These galaxies are called active galaxies.

The area at the centre is powered by a supermassive black hole. The amount of material spinning around the black hole forms an accretion disc. The heat caused by its speed and the effect of it falling into the black hole produces enormous radiation across the electromagnetic spectrum.

Scientists cannot see the black hole but they can sometimes see radiation at different wavelengths forming an accretion disc, or by looking for jets of material thrown out of the galaxy by the gravity of the black hole.

There are 3 types of active galaxy we shall look at:


These have bright spectral emission lines caused by either the accretion disc, or from highly ionized gas around the nucleus. Gas that rotates the black hole faster shows a broader emission line. Typically, serfert galaxies are spiral or irregular.


Blazers are active galaxies that have jets pointing towards Earth. This is the effect of 'looking down' the jet of a blazer. It is difficult to make comparisons between blazers because of the angle from which we view them. When they are angled at 90 to 35° we see them differently to when they're angled between 0 and 35° to our line of sight. Because of this they are quite variable, emitting radio waves and x-rays.


Quasars are active galaxies emitting radio and x-rays. They have extremely large black holes at their centre and are moving away from us at extremely fast rates. They are the furthest and oldest objects we know of; some are as far as 11 billion light years away.


Mix & Match
  • What powers an AGN?
  • How do astronomers find evidence for AGNs?
  • What types of radiation can some galaxies emit?
  • Briefly describe these galaxies...
    - Seyfert
    - Blazers
    - Quasars

Cosmos Active Galactic Nuclei