The Broken Similarity: Sinking and Merging of Dark Matter Subhalos Across Hierarchical Levels

The Broken Similarity: Sinking and Merging of Dark Matter Subhalos Across Hierarchical Levels
Wenkang Jiang, Jiaxin Han, Kun Xu, Victor J. Forouhar Moreno, Feihong He, Zhaozhou Li, Chunyan Jiang, Yipeng Jing, Xiaohu Yang
arXiv:2604.05580v2 Announce Type: replace
Abstract: We investigate hierarchical mergers among subhalos within a $Lambda$CDM simulation using the HBT+ subhalo finder. Unlike previous methods, HBT+ tracks subhalo evolution across hierarchy levels, identifying the coalescence of subhalo cores in phase-space as a ”sinking” event. This coalescence marks a distinct stalled phase in orbital decay, providing a physically motivated and natural definition of a resolved merger. Our main findings include: 1) Over 90% of sinking events occur between adjacent subhalo levels, while cross-level pathways arise from tidal stripping, group infall, and numerical constraints. 2) Resolved mergers are predominantly major mergers (mass ratios > 1:10), while the occurrence of minor mergers decreases with the dynamical age of the host halo. 3) Although deep-level subhalos have low mass ratios relative to the host halo, their high mass ratios relative to direct parents significantly boost merger statistics. Consequently, the satellite-satellite merger rate can rival or exceed the central-satellite rate at lower mass thresholds. 4) Satellite-satellite mergers are spatially biased toward the outer regions of the host, suggesting that the central tidal field suppresses their orbital decay. 5) A bidirectional sinking detection recovers 32% more sinking events than the original algorithm, revealing that child-dispersion-driven mergers are dominated by tidal heating at the final stage of sinking, while parent-dispersion-driven and doubly-identified events proceed primarily via orbital decay. Altogether, these results reveal a complex landscape of hierarchical satellite mergers that deviate from the self-similarity of host halo mergers, due to additional physical processes including dynamical friction and the scale-dependent halo growth history.arXiv:2604.05580v2 Announce Type: replace
Abstract: We investigate hierarchical mergers among subhalos within a $Lambda$CDM simulation using the HBT+ subhalo finder. Unlike previous methods, HBT+ tracks subhalo evolution across hierarchy levels, identifying the coalescence of subhalo cores in phase-space as a ”sinking” event. This coalescence marks a distinct stalled phase in orbital decay, providing a physically motivated and natural definition of a resolved merger. Our main findings include: 1) Over 90% of sinking events occur between adjacent subhalo levels, while cross-level pathways arise from tidal stripping, group infall, and numerical constraints. 2) Resolved mergers are predominantly major mergers (mass ratios > 1:10), while the occurrence of minor mergers decreases with the dynamical age of the host halo. 3) Although deep-level subhalos have low mass ratios relative to the host halo, their high mass ratios relative to direct parents significantly boost merger statistics. Consequently, the satellite-satellite merger rate can rival or exceed the central-satellite rate at lower mass thresholds. 4) Satellite-satellite mergers are spatially biased toward the outer regions of the host, suggesting that the central tidal field suppresses their orbital decay. 5) A bidirectional sinking detection recovers 32% more sinking events than the original algorithm, revealing that child-dispersion-driven mergers are dominated by tidal heating at the final stage of sinking, while parent-dispersion-driven and doubly-identified events proceed primarily via orbital decay. Altogether, these results reveal a complex landscape of hierarchical satellite mergers that deviate from the self-similarity of host halo mergers, due to additional physical processes including dynamical friction and the scale-dependent halo growth history.