Channelled versus Isotropic Accretion of Galaxy Groups onto Clusters: Kinematics, Spin Alignment, and Assembly Bias

Galaxy clusters grow by accreting groups from the cosmic web, but whether the route of infall (along a cluster-connected filament or from the open field) imprints a group’s kinematics, spin, and orbit remains unmeasured at scale. We classify 212, 704 group-cluster infall pairs across 27, 029 clusters in the (1 h−1 Gpc)3 MDPL2 simulation into primary (cluster-connected), secondary (tributary), primary-outskirt, and isotropic channels using DisPerSE filaments. (i) In both channelled classes, group spin axes lie mainly parallel to the filament below a ‘spin-flip’ mass and perpendicular above it, log10Mflip [h−1 M⊙] = 13.07 and 13.12, a 0.05-dex offset consistent with zero, lying 1.3–1.4 dex above the field, while isotropic groups show no flip; Mflip stays radially flat out to 7 R200. (ii) Within the primary class Mflip rises with the linear mass density of the host filament, from 12.96 (thin tendrils) to 13.44 (dense bridges); matching group mass, cluster mass and distance keeps the rise positive but leaves its size uncertain (0.07–0.58 dex). (iii) Isotropic groups fall below random alignment at 2 R200 (41 per cent within 60○, a 4.3σ deficit). Primary groups show 85 per cent velocity-filament alignment at 2 R200, and isotropic groups infall 150–180 km s−1 faster, and more radially, out to 5 R200. At matched distance isotropic groups are therefore predicted to reach orbital pericentres up to 10 per cent smaller than primary groups, whose smaller pooled pericentres reflect only their proximity to the cluster. The topological route of accretion, and within it the host filament’s linear density, shapes how groups enter clusters, testable with wide-area spectroscopic surveys.