# The Secondary Spin Bias of Dark Matter Haloes

## TL;DR

This study examines how the angular momentum of dark matter haloes influences their clustering, finding that secondary spin bias is mainly driven by correlations with halo properties and the presence of nearby similar-mass haloes.

## Contribution

It reveals that the secondary spin bias is primarily explained by halo property correlations and the twin bias effect, rather than large-scale matter distribution.

## Key findings

- Halo spin correlates with age, concentration, and accretion rate.
- Removing property correlations reduces spin bias at low masses.
- High spin haloes tend to be near similar-mass haloes, explaining the bias.

## Abstract

We investigate the role of angular momentum in the clustering of dark matter haloes. We make use of data from two high-resolution N-body simulations spanning over four orders of magnitude in halo mass, from $10^{9.8}$ to $10^{14}\ h^{-1}\ \text{M}_\odot$. We explore the hypothesis that mass accretion in filamentary environments alters the angular momentum of a halo, thereby driving a correlation between the spin parameter $\lambda$ and the strength of clustering. However, we do not find evidence that the distribution of matter on large scales is related to the spin of haloes. We find that a halo's spin is correlated with its age, concentration, sphericity, and mass accretion rate. Removing these correlations strongly affects the strength of secondary spin bias at low halo masses. We also find that high spin haloes are slightly more likely to be found near another halo of comparable mass. These haloes that are found near a comparable mass neighbour - a \textit{twin} - are strongly spatially biased. We demonstrate that this \textit{twin bias}, along with the relationship between spin and mass accretion rates, statistically accounts for halo spin secondary bias.