The build up of the correlation between halo spin and the large scale structure

TL;DR

This study uses N-body simulations to investigate how the correlation between halo spin and large-scale structure develops over time, revealing the influence of environmental anisotropy and mass accretion patterns.

Contribution

It provides a detailed evolutionary picture of the spin-LSS correlation and introduces the fractional anisotropy parameter to quantify environmental effects.

Findings

Early halo progenitors have spins parallel to LSS.

Mass accretion along slowest collapsing directions causes spin reorientation.

Higher environmental anisotropy correlates with aligned halo spins.

Abstract

Both simulations and observations have confirmed that the spin of haloes/galaxies is correlated with the large scale structure (LSS) with a mass dependence such that the spin of low-mass haloes/galaxies tend to be parallel with the LSS, while that of massive haloes/galaxies tend to be perpendicular with the LSS. It is still unclear how this mass dependence is built up over time. We use N-body simulations to trace the evolution of the halo spin-LSS correlation and find that at early times the spin of all halo progenitors is parallel with the LSS. As time goes on, mass collapsing around massive halo is more isotropic, especially the recent mass accretion along the slowest collapsing direction is significant and it brings the halo spin to be perpendicular with the LSS. Adopting the $fractional$ $anisotropy$ (FA) parameter to describe the degree of anisotropy of the large-scale environment, we find that the spin-LSS correlation is a strong function of the environment such that a higher FA (more anisotropic environment) leads to an aligned signal, and a lower anisotropy leads to a misaligned signal. In general, our results show that the spin-LSS correlation is a combined consequence of mass flow and halo growth within the cosmic web. Our predicted environmental dependence between spin and large-scale structure can be further tested using galaxy surveys.