10% of all galaxies show some
level of "activity".
Generally, active galaxies have spectra that look rather different from that to be expected
from a collection of billions of stars. They are believed to ALL have
ONE THING IN COMMON: an ACTIVE CENTRAL NUCLEI.
Active galaxies exhibit some combination of the following
characteristics:
- Unusual appearance, particularly of the nucleus
- Jets emanating from the nucleus
- High luminosity
- Non-thermal continuum emission, often polarized,
with excess radiation at RF, IR, UV, and X-Ray wavelengths
- Rapid variability from compact energy source in the nucleus