When we examine certain galaxies, especially in other parts of the spectrum than visible light, we find some astonishing characteristics, particularly a higher activity than other normal galaxies.
In the 1940's, Karl Seyfert, by studying galaxies, discovered that
about 2% of spiral galaxies have a very bright core, with a strange spectrum
: he found emission lines, corresponding to highly ionized gas, with a high
velocity around 10,000 km/s.
The core of such a galaxy - excluding the disk - is able to transmit 10 times more light than the whole Milky Way. What's more, this brightness can vary on a time scale of about one month, what implies that the size of the active part must be less than one light-month wide.
NGC3393, a Seyfert galaxy (type 2).
It looks like an ordinary barred spiral galaxy.
However, the curvature of the central bar is quite spectacular.
We can tell the difference between type 1 Seyfert galaxies, with
wide emission lines, and type 2, where the emission lines are narrow.
The type 2 Seyfert galaxies are also powerful infra-red transmitters.
Some galaxies are not only very bright in the visible, but they are also powerful transmitters at radio-frequency wavelenghts.
Most of these galaxies are elliptical. The radio emission, which exhibits a synchrotron spectrum, arises from the central core and from two nearly symmetrical lobes, which can be spread over millions of light years away from the core.
These radio lobes are produced by the encouter between high velocity electron beams ejected off the core, with gas clouds.
Notice the size of the lobes, with respect to the size of the galaxy itself.
Source : NRAO.
By studying some powerful radio-sources, Alan Sandage, around 1960,
discovered that some of them looked like very blue stars, but their spectra
exhibited strange properties : emission lines that were strongly redshifted.
And if we calculate the distance to these stars, with their redshift and the Hubble law, we must conclude that these stars are billions of light years away from us.
According to their visual magnitude, we must conclude that these "stars" are as bright as 10,000 times our whole galaxy. They cannot be isolated stars, but in fact, exceptionally bright and active galaxies. These objects are so called quasars, summary of QUAsi-StellAR Sources (they are often called QSO).
Source NASA / HST
Some of these quasars show brightness variations, whose periods can take a few days, even a few hours. This means that the size of the quasar itself must be about the same size as our Solar system.
The blazars, or 'BL Lac objects' are objects which are as powerful as the quasars, but whose spectrum does not show emission lines.
These objects were controversial : some blazars seemed to emit jets of matter with at speeds greater than the speed of light.
According to current opinion, they are simply radio galaxies with
one of the matter jets, which produce the radio lobes, directed toward us.
The lack of emission lines can be simply explained because we can not see the accretion disk. This one is hidden by the radio lobes.
The visible speed of the jet is only an "optical illusion", and is in accordance with the laws of the theory of relativity.
The figure below compares the spectra of the different types of active galaxies, against a "usual" galaxy, in visible wavelenghts.
Click on the image to get a larger one.
At visible wavelenghts, you can notice that the spectra of a Seyfert galaxy and of a radio-galaxy are similar :
Because the spectrum of the blazars is nearly flat, it is very difficult to get a precise estimation of the distance to these objects with their redshift, as there is no emission line.