Black hole clustering in cosmological hydrodynamic simulations: evidence for mergers

Abstract

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’, ξBH,1h) which dominate at scales below ∼300 kpc h−1, and contributions from BHs occupying separate haloes (‘2-halo term’, ξBH,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∼ 1011 to a few 1012 M⊙ 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 ξBH,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⊙) found in hosts of M∼ 4–8 × 1011 M⊙, a range well probed by our simulations.