# Halo intrinsic alignment: dependence on mass, formation time and environment

## TL;DR

This study uses high-resolution simulations to analyze how halo intrinsic alignments depend on mass, formation time, and environment, revealing that bias and environment independently influence alignment.

## Contribution

It extends the linear alignment model to include halo bias and explores the environment's role, providing new insights into factors affecting halo alignment.

## Key findings

- Massive halos have stronger alignment.
- Older halos exhibit stronger alignment than younger ones.
- Halo bias explains mass and formation time dependence, but not environment dependence.

## Abstract

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.