Dark matter halo occupation: environment and clustering
  We use a large dark matter simulation of a LambdaCDM model to investigate the
clustering and environmental dependence of the number of substructures in a
halo. Focusing on redshift z=1, we find that the halo occupation distribution
is sensitive at the tens of percent level to the surrounding density and to a
lesser extent to asymmetry of the surrounding density distribution. We compute
the autocorrelation function of halos as a function of occupation, building on
the finding of Wechsler et al. (2006) and Gao and White (2007) that halos (at
fixed mass) with more substructure are more clustered. We compute the relative
bias as a function of occupation number at fixed mass, finding a strong
relationship. At fixed mass, halos in the top 5% of occupation can have an
autocorrelation function ~ 1.5-2 times higher than the mean. We also compute
the bias as a function of halo mass, for fixed halo occupation. We find that
for group and cluster sized halos, when the number of subhalos is held fixed,
there is a strong anticorrelation between bias and halo mass. Such a
relationship represents an additional challenge to the halo model.
