active galaxy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A galaxy that is many times brighter than other galaxies of the same type. Active galaxies commonly emit most of their excess luminosity from a compact active galactic nucleus.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy A galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy.