Angular momentum evolution can be predicted from cosmological initial conditions

Corentin Cadiou,Hiranya V Peiris,Andrew Pontzen

doi:10.1093/mnras/stab440

Abstract

ABSTRACT The angular momentum of dark matter haloes controls their spin magnitude and orientation, which in turn influences the galaxies therein. However, the process by which dark matter haloes acquire angular momentum is not fully understood; in particular, it is unclear whether angular momentum growth is stochastic. To address this question, we extend the genetic modification technique to allow control over the angular momentum of any region in the initial conditions. Using this technique to produce a sequence of modified simulations, we can then investigate whether changes to the angular momentum of a specified region in the evolved universe can be accurately predicted from changes in the initial conditions alone. We find that the angular momentum in regions with modified initial conditions can be predicted between 2 and 4 times more accurately than expected from applying tidal torque theory. This result is masked when analysing the angular momentum of haloes, because particles in the outskirts of haloes dominate the angular momentum budget. We conclude that the angular momentum of Lagrangian patches is highly predictable from the initial conditions, with apparent chaotic behaviour being driven by stochastic changes to the arbitrary boundary defining the halo.

Highlights

A pressing question in galaxy formation theory is the origin of the angular momentum of dark matter haloes and of their host galaxies
Our results demonstrate that the angular momentum of Lagrangian patches can be accurately predicted from the initial conditions, and allow us to rule out scenarios where it would be driven by chaotic or stochastic processes
Using the ansatz (2) we have shown that, for individual Lagrangian patches, the angular momentum can be predicted from the product of its initial value and a growth rate

Summary

Introduction

A pressing question in galaxy formation theory is the origin of the angular momentum of dark matter haloes and of their host galaxies. Angular momentum controls the spin magnitude and orientation of dark matter haloes, which in turn influences the galaxies they host (Fall & Efstathiou 1980; Mo et al 1998; Bullock et al 2001; Burkert et al 2016). Due to its common cosmological origin, the angular momentum accreted by neighbouring galaxies tends to be aligned, which in turn contributes to the alignment of galaxy spins. This effect is degenerate with the lensing signal, and needs to be mitigated to extract cosmological information from weak lensing surveys The theory states that in the linear regime of structure formation, angular momentum is generated from slight misalignments of the inertia tensor of the Lagrangian patch with the tidal shear tensor

Methods

Results

Conclusion