Observationally,
what are Active Galactic Nuclei (AGNs)?
Luminous,
non-thermal emission from core of a galaxy
Generally
unresolved except in radio
Covers
entire electromagnetic spectrum (radio to X-rays, Gamma rays)
Luminosity ranges from 100x their host galaxies to undetectable (hard to define therefore on the faint end, see work by
Luis Ho of Carnegie Observatories);
MB = -23 to beyond -29 defines Quasars
Varaiability
(Very rapid in X-rays, less so in optical, UV)
Non-thermal
continuum, generally blue in optical
Broad
Emission Lines (up to 10,000s of km/s)
Can look
like a bright galactic core, or a point source
Above list fairly
comprehensive, but any particular sub-class may not have some of these properties. There is NOT a single property that is guaranteed in an AGN. Will discuss when discussing the AGN ZOO