Statistical Properties of Flyby Encounters of Galaxies in Cosmological N-body Simulations

Using cosmological N-body simulations we investigate statistical properties of flyby encounters between halos in comparison with mergers. We classify halo pairs into two groups based on the total energy (E12); flybys (E12 > 0) and mergers (E12 < 0). By measuring the flyby and merger fractions, we assess their dependencies on redshift (0 < z < 4), halo mass (10.8 < log Mhalo/Msun < 13.0), and large-scale environment (from field to cluster). We find that the flyby and merger fractions similarly increase with redshift until z = 1, and that the flyby fraction at higher redshift (1 < z < 4) slightly decreases in contrast to the continuously increasing merger fraction. While the merger fraction has little or no dependence on the mass and environment, the flyby fraction correlates negatively with mass and positively with environment. The flyby fraction exceeds the merger fraction in filaments and clusters; even 10 times greater in the densest environment. Our results suggest that the flyby makes a substantial contribution to the observed pair fraction, thus heavily influencing galactic evolution across the cosmic time.