The evolution of the spin alignments of dark matter halos in the cosmic web

We investigate the evolution of dark matter halo spin alignments with respect to cosmic filaments and explore how halo mass, proximity to filaments, and major mergers affect their orientation over time. We performed 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. The zoom-in technique allowed us to resolve low-mass dark matter halos within each filament while preserving the large-scale environment. Halos were identified with the AMIGA Halo Finder, and their evolutionary histories were reconstructed to trace the spin, shape, and distance to the filament from redshift z = 1 to z=0. Despite limited statistics, we confirm a strong mass-dependent alignment signal: low-mass halos tend to align parallel to the filament, while high-mass halos favor perpendicular orientations. The perpendicular alignments become dominant toward the highest halo masses in our sample, around log_ 10 (M_ h /h^ -1 M_⊙ ) ∼ 12. In addition, we identify major mergers as events capable of producing sharp spin reorientations and temporary transitions toward more prolate halo shapes, particularly for massive halos located near the filament core; this suggests 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 measured at z=0 do not differ significantly from those at z = 1, indicating that the bulk of the present-day spin configuration was 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.