Mass Accretion Scatter

The rate at which galaxy clusters grow through accretion of surrounding matter — the mass accretion rate dM/dt — shows substantial scatter at fixed cluster mass in both observations and simulations, but the observed scatter in several cluster properties linked to recent accretion (BCG growth rate, radio relic occurrence, substructure fraction, and X-ray morphology indicators) is larger than CDM N-body simulations predict. The most massive clusters in flux-limited surveys also show a higher fraction of merging and disturbed morphologies than simulations predict at the same mass and redshift, suggesting either that the merger rate is higher than CDM hierarchical assembly produces, or that the observational signatures of merging persist longer than simulations indicate. Both effects would inflate the observed scatter in accretion-related cluster properties beyond the intrinsic scatter from CDM mass accretion histories.

Successive Collision Theory explains the excess scatter in cluster mass accretion through two mechanisms. First, the pre-existing massive structures from the colliding pockets that seed the most massive clusters do not arrive as smooth NFW halos from random field accretion but as already-assembled proto-cluster cores with their own substructure, companion groups, and organized velocity fields. These pre-existing components merge with the forming cluster over timescales set by their pre-collision orbital configurations rather than by the random infall timescales of CDM subhalos, contributing a bursty, non-Gaussian accretion history that increases the scatter in present-day morphological indicators. The accretion rate at any given epoch can be either anomalously high (when a pre-existing massive companion merges) or anomalously low (between such events), producing wider scatter than smooth CDM accretion histories generate.

Second, the angular momentum organization of the large-scale environment around each cluster determines how efficiently surrounding material is funneled into the cluster along filaments versus arriving isotropically. Clusters embedded in dense filament networks — where the inherited angular momentum field channels matter along preferred infall directions — experience episodic high accretion rate events when filamentary gas clumps arrive, separated by lower accretion rate periods when the filamentary supply is interrupted. Clusters in lower-density environments with weaker angular momentum organization experience smoother, lower-amplitude accretion. This environmental variation in accretion mode produces a correlation between cluster accretion scatter and large-scale environment that CDM simulations with random initial conditions only partially reproduce, and the excess scatter is predicted by SCT to be most pronounced for clusters at the intersections of multiple filaments — precisely the environments where angular momentum inheritance most strongly organizes the infall geometry.