Testing the Accuracy of Halo Occupation Distribution Modelling using
  Hydrodynamic Simulations
  Halo models provide a simple and computationally inexpensive way to
investigate the connection between galaxies and their dark matter haloes.
However, these models rely on the assumption that the role of baryons can be
easily parametrized in the modelling procedure. We aim to examine the ability
of halo occupation distribution (HOD) modelling to reproduce the galaxy
clustering found in two different hydrodynamic simulations, Illustris and
EAGLE. For each simulation, we measure several galaxy clustering statistics on
two different luminosity threshold samples. We then apply a simple five
parameter HOD, which was fit to each simulation separately, to the
corresponding dark matter only simulations, and measure the same clustering
statistics. We find that the halo mass function is shifted to lower masses in
the hydrodynamic simulations, resulting in a galaxy number density that is too
high when an HOD is applied to the dark matter only simulation. However, the
exact way in which baryons alter the mass function is remarkably different in
the two simulations. After applying a correction to the halo mass function in
each simulation, the HOD is able to accurately reproduce all clustering
statistics for the high luminosity sample of galaxies. For the low luminosity
sample, we find evidence that in addition to correcting the halo mass function,
including spatial, velocity, and assembly bias parameters in the HOD is
necessary to accurately reproduce clustering statistics.
