Black hole clustering in cosmological hydrodynamic simulations: evidence for mergers

We examine the clustering properties of a population of quasars drawn from fully hydrodynamic cosmological simulations that directly follow black hole (BH) growth. We find that the BH correlation function is best described by two distinct components: contributions from BH pairs occupying the same dark matter halo ('1-halo term', xi(BH,1h)) which dominate at scales below similar to 300 kpc h-1, and contributions from BHs occupying separate haloes ('2-halo term', xi(BH,2h)) which dominate at larger scales. From the 2-halo BH term, we find a typical host halo mass for faint-end quasars (those probed in our simulation volumes) ranging from M similar to 1011 to a few 1012 M-circle dot from z = 5 to 1 respectively. The BH correlation function shows a luminosity dependence as a function of redshift, though weak enough to be consistent with observational constraints. At small scales, the high resolution of our simulations allows us to probe the 1-halo clustering in detail, finding that xi(BH,1h) follows an approximate power law, lacking the characteristic decrease in slope at small scales found in 1-halo terms for galaxies and dark matter. We show that this difference is a direct result of a boost in the small-scale quasar bias caused by galaxies hosting multiple quasars (1-subhalo term) following a merger event, typically between a large central subgroup and a smaller, satellite subgroup hosting a relatively small BH. We show that our predicted small-scale excess caused by such mergers is in good agreement with both the slope and amplitude indicated by recent small-scale measurements. Finally, we note the excess to be a strong function of halo mass, such that the observed excess is well matched by the multiple BHs of intermediate mass (107-108 M-circle dot) found in hosts of M similar to 4-8 x 1011 M-circle dot, a range well probed by our simulations.