Multiplicity functions of quasars: Predictions from the MassiveBlackII simulation

Abstract

We examine multiple active galactic nucleus (AGN) systems (triples and quadruples, in particular) in the MassiveBlackII simulation over a redshift range of 0.06 ≲ z ≲ 4. We identify AGN systems (with bolometric luminosity Lbol > 1042 erg s-1) at different scales (defined by the maximum distance between member AGNs) to determine the AGN multiplicity functions. This is defined as the volume/surface density of AGN systems per unit richness R, the number of AGNs in a system. We find that gravitationally bound multiple AGN systems tend to populate scales of ≲0.7 cMpch -1; this corresponds to angular separations of ≲100 arcsec and a line-of-sight velocity difference ≲200 km s-1. The simulation contains ∼10 and ∼100 triples/quadruples per deg2 up to depths of DESI (g ≲ 24) and LSST (g ≲ 26) imaging, respectively; at least 20 per cent of these should be detectable in spectroscopic surveys. The simulated quasar (Lbol > 1044 erg s-1) triples and quadruples predominantly exist at 1.5 ≲ z ≲ 3. Their members have black hole masses 106.5 ≲ Mbh ≲ 109 M⊙ h -1 and live in separate (one central and multiple satellite) galaxies with stellar masses 1010 ≲ M∗≲ 1012 M⊙ h -1. They live in the most massive haloes (e.g. ∼1013 M⊙ h -1 at z = 2.5; ∼1014 M⊙ h -1 at z = 1) in the simulation. Their detections provide an exciting prospect for understanding massive black hole growth and their merger rates in galaxies in the era of multimessenger astronomy.