# Angular momentum evolution in Dark Matter haloes: a study of the Bolshoi and Millennium simulations

## TL;DR

This study investigates the evolution of angular momentum in dark matter haloes using cosmological simulations, identifying artificial effects, and developing Monte Carlo models to accurately reproduce angular momentum changes over cosmic time.

## Contribution

The paper introduces a Monte Carlo simulation method to model angular momentum evolution in dark matter haloes, correcting for artificial effects and improving analysis in low-resolution simulations.

## Key findings

- Artificial mass changes affect angular momentum orientation.
- Angular momentum change correlates with mass change, time, and halo mass.
- Monte Carlo models accurately reproduce angular momentum evolution.

## Abstract

We use three different cosmological dark matter simulations to study how the orientation of the angular momentum vector (AM) in dark matter haloes evolve with time. We find that haloes in this kind of simulations are constantly affected by a spurious change of mass, which translates into an artificial change in the orientation of the AM. After removing the haloes affected by artificial mass change, we found that the change in the orientation of the AM vector is correlated with time. The change in its angle and direction (i.e. the angle subtended by the AM vector in two consecutive timesteps) that affect the AM vector has a dependence on the change of mass that affects a halo, the time elapsed in which the change of mass occurs and the halo mass. We create a Monte-Carlo simulation that reproduces the change of angle and direction of the AM vector. We reproduce the angular separation of the AM vector since a look back time of 8.5 Gyrs to today ( $\rm \alpha$) with an accuracy of approximately 0.05 in $\rm cos(\alpha)$. We are releasing this Monte-Carlo simulation together with this publication. We also create a Monte Carlo simulation that reproduces the change of the AM modulus. We find that haloes in denser environments display the most dramatic evolution in their AM direction, as well as haloes with a lower specific AM modulus. These relations could be used to improve the way we follow the AM vector in low-resolution simulations.