Active Galaxies

demonstrate an understanding that some galaxies emit large quantities of radiation in addition to visible light (for example radio waves, X-rays)
demonstrate an understanding that an Active Galactic Nucleus (AGN) is powered by matter falling onto a super-massive black hole
recall the types of active galaxies, including Seyfert galaxies, blazers, quasars
recall that quasars are distant galaxies with high redshifts
describe the discovery of quasars by astronomers
demonstrate an understanding that astronomers use many regions of the electromagnetic spectrum to obtain evidence for the existence and properties of AGNs

An active galactic nucleus, AGN, is a region in the centre of some galaxies which emits huge amounts of radiation. Not just visible light but many waves from the rest of the electromagnetic spectrum including radio, infra-red, ultra violet, x-ray and gamma rays.

It is thought that AGN's have super massive black holes at their centre. Matter close to this forms an accretion disk, a bit like a whirlpool, with matter spiralling inwards at incredible speeds. This is what causes radiation to be emitted. As the gas spirals into the black hole a huge amount of energy is generated, a trillion times more than our Sun emits. Not all the gas is sucked in, some of it escapes as a hot wind which is blown away from the disc at an incredible speed. These are known as galactic jets.

Quasars

A quasar is an extremely bright and distant active galactic nucleus. They consist of a compact halo of matter surrounding a super massive black hole at the centre of a young galaxy.

The term quasar comes from "quasi-stellar radio source" which means star-like emitter of radio waves. We now know that most quasars don't emit a lot of radio waves but do emit light, infra-red, ultra-violet and gamma rays, nevertheless the name has stuck. They are one of the brightest objects in the Universe, so bright we don't see the light from other stars in the galaxy they are part of. We can see quasars 12 billion light years away they are so bright.

At the centre of the quasar is a very large black hole sucking in super hot gas at an incredible rate.

Quasars are amongst the earliest known objects in the Universe. the light from some of them shows a massive red shift, sometimes suggesting that they are around 12 billion light years away. When we look at quasars we are looking at the very young Universe.

pics courtesy of NASA

The Hubble image above shows the quasar 3C273, firstly as a brilliant point of light and then with the centre covered, so that the surrounding host galaxy can be observed.

Seyfert galaxies

These have very bright nuclei. They emit lots or radiation (emission lines) which shows the presence of hydrogen, helium, nitrogen and oxygen. It is thought that this radiation does not come from the accretion disk itself but from a cloud of gas surrounding the accretion disk which is excited by radiation from the disk.

Blazers

The amount of radiation we detect from these galaxies varies greatly with time. It is thought that it is because their galactic jets are pointing almost directly towards Earth so any variation, e.g. due to rotation, is exaggerated.