Halo intrinsic alignment: dependence on mass, formation time and
  environment
  In this paper we use high-resolution cosmological simulations to study halo
intrinsic alignment and its dependence on mass, formation time and large-scale
environment. In agreement with previous studies using N-body simulations, it is
found that massive halos have stronger alignment. For given mass, older halos
have stronger alignment than younger ones. By identifying the cosmic
environment of halo using Hessian matrix, we find that for given mass, halos in
cluster regions also have stronger alignment than those in filament. The
existing theory has not addressed these dependencies explicitly. In this work
we extend the linear alignment model with inclusion of halo bias and find that
the halo alignment with its mass and formation time dependence can be explained
by halo bias. However, the model can not account for the environment
dependence, as it is found that halo bias is lower in cluster and higher in
filament. Our results suggest that halo bias and environment are independent
factors in determining halo alignment. We also study the halo alignment
correlation function and find that halos are strongly clustered along their
major axes and less clustered along the minor axes. The correlated halo
alignment can extend to scale as large as $100h^{-1}$Mpc where its feature is
mainly driven by the baryon acoustic oscillation effect.
