Although there are dissenting voices, Active Galaxies are assumed to be very distant objects, containing a highly energetic object at its center.
Typically, an active galaxy is emitting an enormous amount of energy from a small region. Their spectra show strong broadening indicating gas speeds of several thousand kilometers per second.
It is assumed that the only object capable of producing enough energy within such a small volume will be a black hole, the energy actually coming from an accretion disk circling the black hole. Obviously no energy can be emitted from within the black hole itself, but its enormous gravitational effect influences the said accretion disk.
I will make a brief mention of Starburst Galaxies - these are more energetic galaxies but not what we actually understand as an active galaxy. They are spirals but have an excess of Infra Red - some event has accelerated starbirth. An example is M82 in Ursa Major.
A typical Radio Galaxy shows two lobes either side of the galaxy.
The radio waves don't appear to be coming from the galaxy itself but from regions of space that lie millions of light years on either side.
The images below show Centaurus A, the nearest active galaxy at a distance of 16 million light years. There is a galaxy that can be seen at the center of the radio emission - a peculiar giant elliptical galaxy NGC5128. This galaxy is surrounded by faint shells of material of the type produced when galaxies merge. In the photo to the left, a lane of dust crosses the central region. The image to the right shows its radio emission at right angles to the dust lane. Radio emission from Centaurus A is about a thousand times less than its light emission, but this is still about a thousand times greater than expected from a spiral galaxy like M31.
One of the first radio galaxies detected, Cygnus A, has about 10 million times the radio output
of an ordinary galaxy and much more than Centaurus A. In fact, it is outshing Centaurus A despite being 40 times further away. In the image of Cygnus A at the left, it can be seen that the radio lobes are being produced by beams of high-speed particles emanating from the center.
Seyferts look like ordinary spirals when viewed optically. However their nuclei, of about 10 light years diameter (about 3 parsecs), emits radiation comparable to the total radiation output of the entire Milky Way. This emission is mostly in the Infra Red and is noted to fluctuate.
Most Seyferts are a radio source although not a very powerful one.About 2% of spirals are Seyferts making them the most common type of active galaxy (actually 10% of large spiral galaxies are active as Seyferts).
Carl Seyfert investigated these galaxies originally in the 1940s.
In 1962, the radio souce 3C-273 was matched with a visual star. Investigations found this star to be at an enormous distance of 2 billion light-years, where no normal star would be visible. This was a quasar, not actually a star but now believed to be the active center of a galaxy. The name is derived ultimately from Quasi-Stellar Radio Source, reflecting this fact that they were first assumed to be stars (this original name did undergo a change to Quasi Stellar Object when it was realized that not all quasars emit strong radio waves - in fact only about 1 in 200 quasars is also a strong radio source).
It was emitting more than 100 times the flux of the Milky Way but from a very compact source. They emit strongly in X-rays.
The only telescope capable of analyzing the spectrum of this star at the time was Palomar and this was carried out by Maarten Schmidt in 1960. The spectrum lay unanalyzed for three years before it was realized that actually the spectrum was well-known but had not been recognized because it was red-shifted by an unbelievable amount. The 'star' was actually traveling away at 16% of the speed of light. It is said that the fact that all the glory for this discovery fell to the Americans despite the pioneering work in Britain and Australia led to the building of the joint British-Australian Telescope (the so-called Anglo(sic)-Australian Telescope in Siding Spring).
Incidentally, 3C-273 is item number 273 in the Third Cambridge Catalog of Radio Stars. The optical 'star' had been identified by virtue of the fact that this object lies in the elliptical plane and is often occulted by the Moon, allowing accurate positioning - this was done for the first time at the Parkes telescope in Australia. 3C-273 can be seen by amateurs but a 20cm telescope is required as a minimum - it is 13th magnitude and lies 3 billion light years away. The furthest quasar now known is receeding at 90% of the speed of light and is seen in light emitted when the Universe was less than 10% of its present age.
BL Lacertae was considerd to be a star for many years but in 1968 it was found to be a strong emitter of radio waves. It is known known to be an extremely powerful soure at the center of a (elliptical) galaxy. About 100 others are now known.
They are extremely variable, being capable of brightening by a factor of 100 in a few weeks and then fading back to its original strength. Unlike other active nuclei, the spectrum is featureless. This is now assumed to be because we are loking at the galaxy 'end-on' - by that I mean the view of the galactic core is unimpeded by the surrounding torus or any of the high velocity clouds that affect emissions in other directions.
BLacs appear to lie at the center of elliptical galaxies.