Black Holes in the Universe

In the early sixties, a number of strong radio sources were detected that were associated with optical point sources. Although they appear stellar on photographic plates, they had very strange optical emission lines which were deferent from anything seen before in stars.

This unusual high red shift leads to another problem. The optical brightness of corresponds to a luminosity. Since the sources show variability on timescales of less than a year, the size of the emitting region is restricted to be smaller than one light year. Although conned into a fraction of the volume of a galaxy, the luminosities of these quasars exceed the luminosity of an entire galaxy.
The most way to release energy is accretion in a deep, relativistic gravitational potential. Only super massive black holes have survived today as major paradigm to explain the huge energy output of the central engine of active galactic nuclei.

The strong gravity of the black hole will attract gas clouds in the central region of its host galaxy. This gas clouds will collide with each other in the vicinity of the black hole and transform kinetic energy into frictional heating. Usually the gas clouds will have a certain amount of angular momentum which has to be conserved. This will lead to the formation of an accretion disk. 

The particles in the disk will continue to interact with each other and heat the disk even more. In this process the particles will lose kinetic energy and move inwards while angular momentum is transported outwards, until the gas reaches the last stable orbit and falls into the black hole. The heated accretion disk will radiate predominantly in the optical and UV and produce the enormous luminosities observed in quasars.

A further hint for massive matter concentrations comes from the kinematics behavior of the interstellar gas close to the core of galaxies. The Hubble Space Telescope is able to resolve the dynamics of stars and the gas in nearby galaxies. A prime example is the nearby giant elliptical galaxy M87 which contains a hot rotating disk in the center.