MSSL Astrophysics Group Beginner's Guides

The images shown here are simulations of the inner regions of a quasar nucleus, based upon the most popular and/or successful models.

GIF animations illustrating the structure of a quasar nucleus are now available!

Fly through the stars of a distant galaxy and into the heart of the quasar within, with this computer simulation of the quasar tour (183kb)

At the very heart of a quasar lies a supermassive black hole, about a thousand, million times more massive than our Sun. The gravitational field around a black hole is so strong that even light cannot escape, if it falls within the `event horizon'. Surrounding the black hole is an `accretion disc' (shown blue); a plate-shaped region of gas, which spirals inwards and fuels the `monster' at the centre. Hurled away from the plane of the disc are jets of matter (shown white), moving at velocities close to the speed of light. This region is typically a few light-days across, several times the size of our Solar System. Take a trip around an accretion disc and into the black hole with the computer animation of the disc, black hole and jets (229kb).

Moving out to about 100 light days and the accretion disc is now too small to see, although the jets may still be visible (in some cases, the jets reach distances of hundreds of thousands of light years across). This is an ensemble of gas clouds and is known as the `broad line region', because of the spectral lines which the gas in the clouds emits. There are two parts to the broad line region; the inner zone (coloured purple/yellow) has a relatively high density, it's probably spherical and the atoms in the gas have been stripped of many of their electrons (ie., the gas is `highly ionized'). The outer zone (shown green) is less dense, the clouds move more slowly and it's flattened rather than spherical. Here, the atoms retain more of their electrons, ie. the gas is said to be in a state of `low-ionization'. Take a look at this computer simulation of a quasar broad line region (136kB).

Now you are looking at a region which is about 100 light years across. This `doughnut' is made up of many clouds of dusty gas - it is known as the `molecular torus'. A similar torus has been imaged directly by the Hubble Space Telescope in the Seyfert galaxy NGC4261 and you can see this, and other AGN images, at the Space Telescope Science Institute's (STScI) Pictures page. The torus is `optically thick', ie. light cannot pass through it, so if the torus is edge-on as we see it, the central regions (the accretion disc, the region around the black hole and the broad line region) will be blocked out. This means that AGN will look different according to the angle at which it lies - this is the principle behind the Unified Model of AGN.

These are the very outer regions of an active nucleus, before the surrounding galaxy is reached. The molecular torus can be seen in the centre, surrounded by small clouds of gas (shown blue). This is the `narrow line region', which is similar to the broad line region, but it has a lower density and the clouds move with a lower velocity (so the spectral lines it emits are narrow). Because of its low density, this gas can produce strong `forbidden' lines; these are spectral lines which are suppressed in higher density gas. Notice that the only clouds which are lit up (light blue) are those which lie in the `cone' of light from the inner nucleus. Clouds that lie in the shadow of the torus (dark blue) are not lit up. These `ionization cones' are seen in many nearby galaxies and a good example, NGC5728 (a caption for the image is also available), can be seen at STScI. This computer simulation of the narrow line region (123kb) will give you another visualization.

Other Extragalactic Astronomy pages:

• Extragalactic Astronomy home page
• Population Studies of AGN
• A Beginner's Guide to Active Galactic Nuclei
• Unified Models of AGN

Also - why not try the public pages of the Hubble Space Telescope for more pictures and information?