Supermassive Black Hole Mass Functions at Intermediate Redshifts from Spheroid and AGN Luminosity Functions

Abstract

Redshift evolution of supermassive black hole mass functions (BHMFs) is investigated up to z ~ 1. BHMFs at intermediate redshifts are calculated in two ways. One way is from early-type galaxy luminosity functions (LFs); we assume an M_BH - L_sph correlation at a redshift by considering a passive evolution of L_sph in the local relationship. The resultant BHMFs (spheroid-BHMFs) from LFs of red sequence galaxies indicates a slight decrease of number density with increasing redshift at M_BH > 10^{7.5-8} M_solar. Since a redshift evolution in slope and zeropoint of the M_BH - L_sph relation is unlikely to be capable of making such an evolution in BHMF, the evolution of the spheroid-BHMFs is perhaps due mainly to the decreasing normalization in the galaxy LFs. We also investigate how spheroid-BHMFs are affected by uncertainties existing in the derivation in detail. The other way of deriving a BHMF is based on the continuity equation for number density of SMBHs and LFs of active galactic nucleus (AGN). The resultant BHMFs (AGN-BHMFs) show no clear evolution out to z = 1 at M_BH > 10^8 M_solar, but exhibit a significant decrease with redshift in the lower mass range. Comparison of the spheroid-BHMFs with the AGN-BHMFs suggests that at M_BH > 10^{8} M_solar, the spheroid-BHMFs are broadly consistent with the AGN-BHMFs out to z ~ 1. The agreement between the spheroid-BHMFs and the AGN-BHMFs appears to support that most of the SMBHs are already hosted by massive spheroids at z ~ 1 and they evolve without significant mass growth since then.

13 pages, accepted for publication in MNRAS