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We present a statistical analysis of flybys of dark matter halos compared tomergers using cosmological $N$-body simulations. We mainly focus ongravitationally interacting target halos with mass of$10^{10.8}-10^{13.0}h^{-1}M_{\odot}$, and their neighbors are counted only whenthe mass ratio is 1:3$-$3:1 and the distance is less than the sum of the virialradii of target and neighbor. The neighbors are divided into the flyby ormerger samples if the pair's total energy is greater or smaller, respectively,than the capture criterion with consideration of dynamical friction. The mainresults are as follows: (a) The flyby fraction increases by up to a factor of50 with decreasing halo mass and by up to a factor of 400 with increasinglarge-scale density, while the merger fraction does not show any significantdependencies on these two parameters; (b) The redshift evolution of the flybyfraction is twofold, increasing with redshift at $0<z<1$ and remaining constantat $z>1$, while the merger fraction increases monotonically with redshift at$z=0\sim4$; (c) The multiple interactions with two or more neighbors are onaverage flyby-dominated, and their fraction has a mass and environmentdependence similar to that for the flyby fraction; (d) Given that flybyssubstantially outnumber mergers toward $z=0$ (by a factor of five) and themultiple interactions are flyby-dominated, the flyby's contribution to galacticevolution is stronger than ever at the present epoch, especially for lessmassive halos and in the higher density environment. We propose a scenario thatconnects the evolution of the flyby and merger fractions to the hierarchicalstructure formation process.

Living with Neighbors. II. Statistical Analysis of Flybys and Mergers of Dark Matter Halos in Cosmological Simulations