Modelling the stochasticity of high-redshift halo bias
  A very large dynamic range with simultaneous capture of both large- and
small-scales in the simulations of cosmic structures is required for correct
modelling of many cosmological phenomena, particularly at high redshift. This
is not always available, or when it is, it makes such simulations very
expensive. We present a novel sub-grid method for modelling low-mass
($10^5\,M_\odot\leq M_{\rm halo}\leq 10^9\,M_\odot$) haloes, which are
otherwise unresolved in large-volume cosmological simulations limited in
numerical resolution. In addition to the deterministic halo bias that captures
the average property, we model its stochasticity that is correlated in time. We
find that the instantaneous binned distribution of the number of haloes is well
approximated by a log-normal distribution, with overall amplitude modulated by
this "temporal correlation bias". The robustness of our new scheme is tested
against various statistical measures, and we find that temporally correlated
stochasticity generates mock halo data that is significantly more reliable than
that from temporally uncorrelated stochasticity. Our method can be applied for
simulating processes that depend on both the small- and large-scale structures,
especially for those that are sensitive to the evolution history of structure
formation such as the process of cosmic reionization. As a sample application,
we generate a mock distribution of medium-mass ($ 10^{8} \leq M/M_{\odot} \leq
10^{9}$) haloes inside a 500 Mpc$\,h^{-1}$, $300^3$ grid simulation box. This
mock halo catalogue bears a reasonable statistical agreement with a halo
catalogue from numerically-resolved haloes in a smaller box, and therefore will
allow a very self-consistent sets of cosmic reionization simulations in a box
large enough to generate statistically reliable data.
