References and Links
What is an Active Galaxy?
The phrase "active galaxy" is used to describe galaxies which
contain uncharacteristically bright central sources of energy (one
which emits more light than ordinary stars would). The
existence of these galaxies is an indication that galaxies contain
large central black holes.
The "bright central source of energy" of an active galaxy is referred
to as an Active Galactic Nucleus (or AGN).
Types of AGN's:
- Quasars
-
Quasars, also known as QSOs (quasi-stellar objects), are
galaxy-like objects found at the far reaches of the universe. At
their centers, they contain an AGN which emits many hundreds of
times more radiation than all of the rest of the stars in the
galaxy. In fact, they were called "quasi-stellar" because the
center is so bright that it is hard to see the edges, and so they
look like stars. Quasars are the largest sustained power source
in the universe!
Quasars are typically found at very high red-shifts (z>1), meaning that they were more prevalent in the early universe. Quasars have been recently detected as far out as z>5, making them the most distant objects we can see.
- Seyfert Galaxies
-
Seyfert Galaxies are similar to quasars in that they have a very
bright core - the difference is that Seyfert cores are about as
bright as the entire remaining part of the galaxy, rather than a
100 times more bright. They were first classified by Carl
Seyfert in 1943, but their underlying physics wasn't studied in
detail until 1959 (by Woltjer). About 1 in every hundred spiral
galaxy is in a "Seyfert phase".
- Blazars
- Blazars are not really another type of AGN - they are actually AGN's viewed from a special angle. Because AGNs contain black holes, they emit jets of plasma outside of the plane of the galaxy. These jets push out against the interstellar medium and through inverse Compton-scattering can accelerate photons to very high energies. If the galaxy happens to be angled such that one of the jets is pointing directly at us, the nucleus can appear a very bright gamma-ray source - more bright than quasars. Such galaxies are called Blazars.
How are we involved?
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