The Evolution of the Spin Alignments of Dark Matter Halos in the Cosmic Web

TL;DR

This study examines how dark matter halo spins align with cosmic filaments over time, revealing mass-dependent alignment patterns and the impact of major mergers on spin reorientation.

Contribution

It provides new insights into the evolution of halo spin alignments with filaments, emphasizing the role of halo mass and mergers in shaping orientations.

Findings

Low-mass halos tend to align parallel to filaments.

High-mass halos show a preference for perpendicular orientations.

Major mergers can cause significant spin reorientations and shape changes.

Abstract

We investigate the evolution of dark matter halo spin alignments with respect to cosmic filaments, exploring how halo mass, proximity to filaments, and major mergers influence their orientation over time. We perform a suite of dark matter-only zoom-in N-body simulations centered on ten filaments extracted from a cosmological box using the 1DREAM structure finder. This approach allows us to resolve low-mass halos within filaments while preserving the large-scale environment. Halos are identified with the Amiga Halo Finder (AHF), and their evolutionary histories are reconstructed to trace the spin, shape, and distance to the filament from redshift $z = 1$ to $z = 0$. We confirm a strong mass-dependent alignment signal: low-mass halos tend to align parallel to the filament, while high-mass halos preferentially exhibit perpendicular orientations, despite limited statistics. Perpendicular alignments become dominant at the highest halo masses in our sample, around $\log_{10}(M_\mathrm{h}/h^{-1}\mathrm{M_\odot}) \sim 12$. We also find that major mergers can induce sharp spin reorientations and temporary transitions toward more prolate halo shapes, particularly in massive halos located near the filament core, suggesting a preferential merger direction within filaments. Overall, halo mass emerges as the primary factor governing spin-filament alignments in our sample. By analyzing the global evolution, we find that the average orientations at z = 0 do not differ significantly from those at $z = 1$, indicating that the present-day spin configuration is largely established at earlier stages of halo evolution. Major mergers, although relatively rare, represent one of the few mechanisms capable of disrupting this initial alignment.