Influence of the large-scale structure velocity field on halo mass assembly within cosmic filaments

Abstract

The cosmic web governs the growth of dark matter halos through anisotropic matter flows, yet the role of local velocity-field dynamics in halo mass assembly remains poorly understood. The velocity field carries non-linear information about matter transport that is not fully captured by density alone, particularly within filaments. We investigate this connection through velocity divergence and vorticity using dark matter halos in the IllustrisTNG TNG50-1-Dark simulation at $z=0$. Cosmic filaments are extracted with the 1-DREAM framework, and we statistically study how halo mass relates to divergence, vorticity, and radial distance from filament spines across three overdensity regimes. The velocity field correlates strongly with the cosmic web, with converging flows tracing filaments and vorticity arising primarily in overdense, non-linear regions. Within filaments, both divergence and vorticity show enhanced kinematic activity at intermediate distances from the spine, with amplitudes increasing with overdensity. We confirm a clear mass segregation, with more massive halos preferentially located near filament centers. Most notably, halos with masses $M \geq 10^{12} M_\odot$ are found almost exclusively in regions of low absolute divergence and low vorticity, while highly dynamical environments are dominated by low-mass halos. We further find that halos residing in kinematically calm regions assembled later and maintain higher recent accretion rates than those in dynamically active environments, linking present-day velocity-field properties to halo growth histories. These results demonstrate that the local kinematic state of the cosmic flow encodes information about halo mass and assembly beyond that contained in density alone, providing a complementary dynamical description of halo growth within the cosmic web.