Cosmological Evolution Models for QSO/AGN Luminosity Functions: Effects of Spectrum-Luminosity Correlation and Massive Black Hole Remnants

Abstract

We investigate a large number of cosmological evolution models for QSOs and Active Galactic Nuclei (AGN). We introduce a spectrum-luminosity correlation as a new input parameter and adopt the estimated mass function (MF) of massive black holes in centers of nearby galactic nuclei as a constraint to distinguish among different QSO/AGN models. We explore three basic types of phenomenological scenarios; (i) Models with multiple short-lived ($\sim$ a few $10^{6-8}$ yrs) populations, (ii) Models with a single long-lived ($\sim 10^9$ yr) QSO population, and (iii) Models with recurrent QSO/AGN activities which are driven by long-term variabilities of the disk instability type. In each model, we derive the expected theoretical luminosity function (LF) and the MF of black holes which grow through mass accretion. We assess the plausibility of each model based on whether each model's LF and MF are compatible with the observed data. We find that the best fits to the observed LFs are obtained in the model with multiple short-lived populations and without any significant spectral evolution. This finding suggests that the QSO populations may be composed of many short-lived generations ($\sim$ a few 10$^8$ yrs) and that there is no significant spectral evolution within each generation. On the other hand, we also show that there is no satisfactory model which can simultaneously account for the observed LF and the estimated MF. We speculate that some of the present-day black holes (BHs) found in galactic nuclei may have formed without undergoing the QSO/AGN phase.