Multiplicity functions of quasars: predictions from the MassiveBlackII simulation

We examine multiple active galactic nucleus (AGN) systems (triples and quadruples, in particular) in the MassiveBlackil simulation over a redshift range of 0.06 <, z <, 4. We identify AGN systems (with bolometric luminosity L1,01 > 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 cMpc 11-1; this corresponds to angular separations of <100 arcsec and a line-of-sight velocity difference 2200 km s-1. The simulation contains 10 and 100 triples/quadruples per deg2 up to depths of DESI (g < 24) and LSST (g r<1--- 26) imaging, respectively; at least 20 per cent of these should be detectable in spectroscopic surveys. The simulated quasar (Lb01 > 1044 erg s-1) triples and quadruples predominantly exist at 1.5 < z < 3. Their members have black hole masses 106'5< Met, < 109 Mc h-1 and live in separate (one central and multiple satellite) galaxies with stellar masses 1010 < M, < 1012 Mo It I. They live in the most massive haloes (e.g. 1013 h at z = 2,5; 1014 Mc 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.