Quasar 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
growth. We find that the black hole correlation function is best described by
two distinct components: contributions from BH pairs occupying the same dark
matter halo ('1-halo term') which dominate at scales below 300 kpc/h, and
contributions from BHs occupying separate halos ('2-halo term') 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 10^11
to a few 10^12 solar masses from z=5 to z=1 respectively (consistent with the
mean halo host mass). 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
the 1-halo term 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 black hole. 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 black holes of
intermediate mass (10^7-10^8 solar masses) found in hosts of 4-8*10^11 solar
masses, a range well probed by our simulations.
