Active Galactic Nuclei
Galaxies are large systems of stars, and most of the light we see comes from the stars. In addition to stars, many galaxies, perhaps most, contain a super-massive black hole in their center. As gas (and an occasional star) falls into the black hole, the falling matter is heated up and emits infrared and visible light and X-rays, often in jet-like features. This process is known as an "Active Galactic Nucleus" or AGN. About 10% of AGN are also strong radio emitters or "radio galaxies." Some AGN are so bright that the host galaxy is barely visible or not visible at all; these are known as "quasi-stellar objects" or "quasars." A few AGN are also known to emit light at highest enegy levels, in the form of Gamma Rays. The Whipple Observatory 10m Gamma Ray telescope has observed such emission from Mrk 421, Mrk 501, 1ES 2344+514 and H1426+428, which have become the benchmarks for researching how the AGN produce their light and how their properties and numbers have changed over cosmic time.
Anna Barnacka, Wystan Benbow, Matteo Cerruti, Giovanni Fazio,Steven Willner