It is tantalizing that the peak of AGN activity coincides with the epoch at which most of the stars in the Universe were being formed in galaxies. Another important piece of evidence linking the growth of supermassive black holes to the formation of galaxies was the discovery of a linear correlation between the masses of these black holes and the masses of the galactic bulges that harbour them. At average about 0.3 percent of the mass of the bulge is contained in the central black hole. This suggested that the formation of stars in bulges and the assembly of supermassive black holes at their centres are closely linked. This suggestion is strongly supported by the recently reported much tighter correlation between black hole mass and velocity dispersion of the host galaxy shown in figure 1.
Many astronomers now believe that mergers between galaxies are the prime suspect.
According to the standard theoretical paradigm for the formation of structure in the Universe, small density inhomogeneities in the dark matter were generated shortly after the Big Bang by quantum fluctuations during a period of accelerated expansion, usually termed inflation. These early inhomogeneities were then gravitationally amplified as the Universe expanded. Eventually, material contained in initially overdense regions of the Universe stopped expanding and began to collapse. The smallest objects formed first in this way, and these later merged together to form larger and larger structures. Galaxies formed in regions where the overdensity of matter was high enough to allow gas to cool, condense and form stars.