Active Galactic Nuclei

Galaxies are groupings of a large number of stars (roughly a million or so), with some dust and gas thrown in, held together by their mutual gravity. A special class of galaxies, know as Active Galaxies have centers (or nuclei) that are the source of tremendous energy, shining with power equivalent to trillions of suns. It is believed that at the center of these objects there lies a supermassive black hole, which ejects jets of matter in opposite directions at nearly the speed of light. An accretion disk around the black hole lies in a plane perpendicular to the jets (as shown in diagram below). In addition, surrounding the accretion disk is a torus (donut) of molecular material, which can obscure observations of the black hole and accretion disk if viewed through the donut.

Hubble image of M87 Schematic of an AGN

The first image is a Hubble Heritage image of M87, while the second one is a schematic diagram of an AGN

Active Galactic Nuclei (AGNs) represent the largest identified class of high-energy gamma-ray sources and are generally only detected as gamma-ray sources when one of the jets is directed toward us. In this orientation the AGN is referred to as a Blazar. This is also the orientation in which the torus and material accreting around the black hole at the center does not actually obscure the black hole or the inner part of the jet. Think of looking at a donut from the side and the top, where the donut itself represents the material around the black hole and the donut hole is the black hole.

The Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory (CGRO) detected about 66 gamma-ray sources that were determined to be blazars; GLAST will detect thousands. Blazars are key to understanding the AGN phenomenon, and GLAST will help to answer some of the most important questions about them: How are the jets formed? How is the matter in the jets accelerated to such fantastic speeds? Is a billion-solar-mass black hole really the central power source?