On the small-scale clustering of quasars: constraints from the MassiveBlack II simulation

Abstract

Abstract We examine recent high-precision measurements of small-scale quasar clustering (at z ∼ 0.5–2 on scales of ${\sim }25~\mathrm{kpc}\, h^{-1}$) from the SDSS in the context of the MassiveBlack II (MBII) cosmological hydrodynamic simulation and conditional luminosity function (CLF) modelling. At these high luminosities (g < 20.85 quasars), the MBII simulation volume ($100~\mathrm{cMpc}\, h^{-1}$ comoving boxsize) has only three quasar pairs at distances of 1–4 Mpc. The black hole masses for the pairs range between $M_{\rm bh}\sim 1{\, \rm and\, }3\times 10^{9}~\mathrm{M}_{\odot }\, h^{-1}$ and the quasar hosts are haloes of $M_{\rm h}\sim 1\hbox{--}3\times 10^{14}~\mathrm{M}_{\odot }\, h^{-1}$. Such pairs show signs of recent major mergers in the MBII simulation. By modelling the central and satellite AGN CLFs as lognormal and Schechter distributions, respectively (as seen in MBII AGNs), we arrive at CLF models which fit the simulation predictions and observed luminosity function and the small-scale clustering measured for the SDSS sample. The small-scale clustering of our mock quasars is well-explained by central--satellite quasar pairs that reside in $M_{\rm h}\gt 10^{14}~\mathrm{M}_{\odot }\, h^{-1}$ dark matter haloes. For these pairs, satellite quasar luminosity is similar to that of central quasars. Our CLF models imply a relatively steep increase in the maximum satellite luminosity, $L^*_{\mathrm{sat}}$, in haloes of $M_{\rm h}\gt 10^{14}~\mathrm{M}_{\odot }\, h^{-1}$ with associated larger values of $L^*_{\mathrm{sat}}$ at higher redshift. This leads to increase in the satellite fraction that manifests itself in an enhanced clustering signal at ≲1 Mpc h−1. For the ongoing eBOSS-CORE sample, we predict ∼200–500 quasar pairs at z ∼ 1.5 (with $M_{\rm h} \gtrsim 10^{13}~\mathrm{M}_{\odot }\, h^{-1}$ and $M_{\rm bh} \gtrsim 10^{8}~\mathrm{M}_{\odot }\, h^{-1}$) at ∼25 kpc scales. Such a sample would be ≳ 10 times larger than current pair samples.