The formation of nuclei in newly formed galaxies and the evolution of the quasar population

A model to explain the evolution of the quasar luminosity function is presented. At intermediate and high redshifts, the formation of ‘new’ black holes at each step in the hierarchical growth of structure is invoked as the driving force of the strong evolution of the quasar population. Quasars are assumed to be short-lived, constituting the first phase of the formation of a galaxy in the potential well of a dark matter halo. The timelag between halo virialization and the birth of the quasar is expected to be short compared to the cosmological time-scale, even at high redshifts. Simple assumptions are made to relate the luminosity of a quasar to the mass of its central black hole and to the mass of its corresponding host object. The Press-Schechter formalism is used within the CDM scenario (q0 = 0.5, h100 = 0.5) to estimate the number of newly forming dark matter haloes at successive cosmic epochs. A rapid rise in the number density of newly forming massive black holes is followed by a steep fall; this can explain the observed dependence of the quasar population on redshift. Model luminosity functions are calculated and found to be in good agreement with the observational data. By assuming that slingshot ejection is unimportant, a mass function of remnant black holes is calculated. Remnant black holes are predicted for virtually all galaxies, but would be less massive in late-forming galaxies and in galaxies with shallower potential wells.