There are many different types of AGN and broadly speaking, can be divided into groups according to the following properties:
Radio power -
Radio-loud' AGN have very strong radio emission when compared to the optical, whereas `radio-quiet' objects have relatively little or no measurable radio flux.
In low-luminosity objects, which include Starburst Galaxies and Seyferts, the host galaxy can be seen around the active nucleus. The more distant and energetic Quasars lie so far away that the surrounding galaxy can no longer be seen, yet the nucleus is so powerful that it outshines the galaxy and appears `star-like' on the sky.
Optical spectral lines -
If `broad' spectral lines are seen in the optical spectrum of an AGN, these indicate that we can see high-velocity gas (moving at velocities between 1,000 and 10,000 km per second), presumably from the broad line region. These are known as `Type 1' AGN (eg Seyfert 1s and type 1 quasars). However if only `narrow' lines are observed, we are only seeing low velocity gas from the narrow line region (between about 100 and 1,000 km per second), either because the high velocity gas is obscured (perhaps by the molecular torus) or because it is simply not there. These are called `Type 2' AGN.
The Unified Model -
Orientation of the Nucleus
The basic idea behind Unified Model of AGN is that all of the AGN which we observe have the same internal structure in their nucleus. All the differences that we see are caused by the angle of the system (ie. the plane in which the accretion disc , the outer parts of the broad line region and the molecular torus all lie).
2 objects - Seyfert 2s, Narrow Line Radio Galaxies and Type 2 Quasars:
When we are looking at the system edge-on (ie. along the plane of the disc), the central regions, including the black hole, accretion disc and broad line region, are obscured. All that we can see directly from the nucleus, are the molecular torus (which radiates predominantly in the infra-red) and the emission lines from the narrow line region. Thus when we look at the optical spectra, only narrow lines are seen; there is no broad line emission. Light from the nucleus can be reflected towards us by hot gas which lies above and below the torus, and which acts as a kind of mirror. Observations like this have demonstrated that `type 1' nuclei are hidden in `type 2' objects, at least in some cases.
1 objects - Seyfert 1s, Broad Line Radio Galaxies and (Type 1) Quasars:
As the nucleus `tips' over towards us, we are able to see directly into the central regions. Now we can see both broad and narrow lines in the optical spectrum, and the direct emission from the accretion disc.
- BL Lac Objects and Optically-Violent Variables:
These are a special case of unobscured (type 1) objects where we are looking face-on at the nucleus and directly into the jet. Matter in the jet is moving at velocities close to the speed of light, so that the radiation it emits is beamed and can vary wildly in very short periods of time (hours to days). Although both broad and narrow lines are visible, the emission from the jet can be so strong that it completely swamps any spectral lines: these are the BL Lac objects. Optically-violent variables are similar to BL Lacs, but emission lines are sometimes observed in their spectra.
This is a description of the basic idea behind the Unified Model. It has been proposed that there are two parallel sequences, one for radio-loud AGN where the radio jets are very strong and extend into the surrounding galaxy, and another for the radio-quiet AGN (where the radio emission is relatively weak). The pictures above illustrate the radio-quiet case, principally because these dominate the AGN population by a ratio of about 10 to 1, and because at present, they are the objects on which we do the most research here at MSSL.
A description of some of our work which is relevant to the Unified Model may be found in the section on the Population Studies of AGN.
Other Extragalactic Astronomy pages:
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