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

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.

Figures (9)

• Figure 1: Side view of the zoomed-in region corresponding to the 10 filaments at redshift 0 colored in blue. Gray represents the particles of the level-7 initial simulation.
• Figure 2: Side view of the zoomed-in region corresponding to filament 4 at redshift 1. Gray represents the particles of the level-11 zoom-in region, red shows the filament body detected by Crawling, and blue indicates the particles collapsed to the filament center by MBMS.
• Figure 3: Stack of the spin-filament alignment distributions for the sample of 10 filaments. The panel shows histograms normalized to unity, separated into different mass bins as indicated by the legend. The shaded regions represent the 1$\sigma$ uncertainty estimated via bootstrap resampling.
• Figure 4: Similar to Figure \ref{['global_spin_mass_S']}, but for the alignments related to the components of the inertia tensor. The top three panels show the alignment between the major axis, the second major axis and the minor axis, denoted by $\textbf{E}_\mathrm{a}, \textbf{E}_\mathrm{b}, \textbf{E}_\mathrm{c}$ respectively, and the filament direction. In each panel, halos are separated into different mass bins. The bottom three panels display the dot product between the spin vector $\textbf{J}$ and each of the inertia tensor axes, using the same mass binning as in the top panels. Again the shaded regions represent the 1$\sigma$ uncertainty estimated via bootstrap resampling.
• Figure 5: Average evolution of the halos belonging to the filaments at z=0 ($r \leq 3 ~h^{-1}\mathrm{Mpc}$), colored according to the final mass at $z=0$, with shaded regions indicating the standard error of the mean. From top to bottom, the panels show the spin alignment with respect to the filament, the shape alignment of the major and minor axes with respect to the filament, the distance from the filament, and the triaxiality parameter $T$, respectively.