Multiplicity functions of quasars: Predictions from the MassiveBlackII simulation

Abstract

We examine multiple AGN systems (triples and quadruples, in particular) in the \texttt{MassiveBlackII} simulation over a redshift range of $0.06\lesssim z\lesssim 4$. We identify AGN systems (with bolometric luminosity $L_{\mathrm{bol}}>10^{42}ergs/sec$) 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 \textit{richness} $R$, the number of AGNs in a system. We find that gravitationally bound multiple AGN systems tend to populate scales of $\lesssim 0.7\mathrm{cMpc}/h$; this corresponds to angular separations of $\lesssim 100\mathrm{arcsec}$ and a line of sight velocity difference $\lesssim 200km/sec$. The simulation contains $\sim 10$ and $\sim 100$ triples/quadruples per ${\mathrm{deg}}^2$ up to depths of DESI ($g\lesssim 24$) and LSST ($g\lesssim 26$) imaging respectively; at least $20\%$ of these should be detectable in spectroscopic surveys. The simulated quasar ($L_{\mathrm{bol}}>10^{44}ergs/sec$) triples and quadruples predominantly exist at $1.5\lesssim z\lesssim 3$. Their members have black hole masses $10^{6.5}\lesssim M_{bh}\lesssim 10^9{M}_{\odot }/h$ and live in separate (one central and multiple satellite) galaxies with stellar masses $10^{10}\lesssim M_{\ast }\lesssim 10^{12}{M}_{\odot }/h$. They live in the most massive haloes (for e.g. $\sim 10^{13}{M}_{\odot }/h$ at $z=2.5$; $\sim 10^{14}{M}_{\odot }/h$ 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 multi-messenger astronomy.

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