Hierarchical structure formation theory is based on the notion that mergers drive galaxy evolution, so a considerable framework of semi-analytic models and N-body simulations has been constructed to calculate how mergers transform a growing galaxy. However, galaxy mergers are only one type of major dynamical interaction between halos—another class of encounter, a close flyby, has been largely ignored. We use cosmological N-body simulations to reconstruct the entire dynamical interaction history of dark matter halos. We present a careful method of identifying and tracking a dark matter halo which resolves the typical classes of anomalies that occur in N-body data. This technique allows us to robustly follow halos and several hierarchical levels of subhalos as they grow, dissolve, merge, and flyby one another—thereby constructing both a census of the dynamical interactions in a volume and an archive of the dynamical evolution of an individual halo. In addition to a census of mergers, our tool characterizes the frequency of close flyby interactions in the universe. We find that the number of close flyby interactions is comparable to, or even surpasses, the number of mergers for halo masses 1011 M ☉ h –1 at z 2. Halo flybys occur so frequently to high-mass halos that they are continually perturbed, unable to reach a dynamical equilibrium. In particular, we find that Milky Way type halos undergo a similar number of flybys as mergers irrespective of mass ratio for z 2. We also find tentative evidence that at high redshift, z 14, flybys are as frequent as mergers. Our results suggest that close halo flybys can play an important role in the evolution of the earliest dark matter halos and their galaxies, and can still influence galaxy evolution at the present epoch. Our companion paper quantifies the effect of close flyby interactions on galaxies and their dark matter hosts.
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