A simple model for quasar density evolution
  It is widely agreed upon that AGN and Quasars are driven by gas accretion onto a supermassive black hole. The origin of the latter however still remains an open question. In this work we present the results of an extremely simple cosmological model combined with an evolutionary scenario in which both the formation of the black hole as well as the gas accretion onto it are triggered by major mergers of gas-rich galaxies. Despite its very generous approximations our model reproduces the quasar density evolution in remarkable agreement with observations. 
Hannes Horst (1,2) and Wolfgang J. Duschl (2,3) ((1) ESO, Santiago,   Chile, (2) Institut fuer Theoretische Astrophysik, Hei...