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

Abstract

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 $(1\,h^{-1}\,\mathrm{Gpc})^3$ 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, $\log_{10} M_\mathrm{flip}\;[h^{-1}\,M_{\odot}] = 13.07$ 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; $M_\mathrm{flip}$ stays radially flat out to $7\,R_{200}$. (ii) Within the primary class $M_\mathrm{flip}$ 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 $2\,R_{200}$ ($41$ per cent within $60^\circ$, a $4.3σ$ deficit). Primary groups show $85$ per cent velocity-filament alignment at $2\,R_{200}$, and isotropic groups infall $150-180\,\mathrm{km\,s^{-1}}$ faster, and more radially, out to $5\,R_{200}$. 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.