The ecology of dark matter haloes
Interactions such as mergers and flybys play a fundamental role in shaping galaxy morphology. Using the Horizon Run 4 cosmological N--body simulation, we studied the frequency and type of halo interactions, and their redshift evolution as a function of the environment defined by the large-‐scale density, pair separation, mass ratio, and target halo mass. Most interactions happen at large-‐scale density contrast delta~20, regardless of the redshift corresponding to haloes in filaments . However, the fraction of interacting target is maximum at delta~1000. We provide a new empirical fitting form for the interaction rate as a function of the halo mass, large-‐scale density, and redshift. We also report the existence of two modes of interactions from the distributions of mass ratio and relative distance, probing two different stages of the interaction. Satellite targets lose their mass as they proceed deeper into the host halo. The relative importance of these two trends strongly depends on the large-‐scale density, target mass, and redshift.
I then discuss the alignments of the spins and main axes of the interacting pairs and their evolutions with redshift, mass, and large--scale density.