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 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, log10 Mflip [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.
Versions
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BibTeX
@misc{singh2026channelled,
title = {{Channelled versus Isotropic Accretion of Galaxy Groups onto Clusters: Kinematics, Spin Alignment, and Assembly Bias}},
author = {Ankit Singh and Frazer Pearce and Meghan Gray and Ulrike Kuchner and Julian Onions and Gustavo Yepes and Alexander Knebe},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.39987},
eprint = {2610.39987},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.39987},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Channelled versus Isotropic Accretion of Galaxy Groups onto Clusters: Kinematics, Spin Alignment, and Assembly Bias
AU - Ankit Singh
AU - Frazer Pearce
AU - Meghan Gray
AU - Ulrike Kuchner
AU - Julian Onions
AU - Gustavo Yepes
AU - Alexander Knebe
PY - 2026
DA - 2026/10/29
PB - arXiv
DO - 10.48550/arxiv.2610.39987
UR - https://arxiv.org/abs/2610.39987
AB - 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, log_10 M_flip [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; M_flip stays radially flat out to 7 R_200. (ii) Within the primary class M_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^∘, a 4.3σ deficit). Primary groups show $85$ per cent velocity-filament alignment at 2 R_200, and isotropic groups infall 150−180 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.
ER -