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

We examine recent high-precision measurements of small-scale quasar clustering (at $z\sim0.5-2$ on scales of $\sim25~\mathrm{kpc/h}$) from the SDSS in the context of the MassiveBlackII (MBII) cosmological hydrodynamic simulation and conditional luminosity function (CLF) modeling. At these high luminosities ($g < 20.85$ quasars), the MBII simulation volume ($100~\mathrm{cMpc}/h$ comoving boxsize) has only 3 quasar pairs at distances of $1-4$ Mpc. The black-hole masses for the pairs range between $M_{bh}\sim1-3\times 10^{9}~M_{\odot}/h$ and the quasar hosts are haloes of $M_h\sim1-3\times10^{14}~M_{\odot}/h$. Such pairs show signs of recent major mergers in the MBII simulation. By modeling the central and satellite AGN CLFs as log-normal 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 $M_h>10^{14}~M_{\odot}/h$ 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 $M_h>10^{14}~M_{\odot}/h$ 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 $\lesssim$ 1 Mpc/h. For the ongoing eBOSS-CORE sample, we predict $\sim 200-500$ quasar pairs at $z\sim1.5$ (with $M_h \gtrsim10^{13}~M_{\odot}/h$ and $M_{bh} \gtrsim10^{8}~M_{\odot}/h$) at $\sim25~\mathrm{kpc}$ scales. Such a sample would be $\gtrsim10$ times larger than current pair samples.