The Cosmic Evolution of Halo Pairs: I. Global Trends
  Accumulating evidence suggests that galaxy interactions play an important role in shaping the properties of galaxies. For this reason, cosmological studies focused on the evolution of halo/subhalo pairs are vital. In this paper I describe a large catalogue of halo pairs extracted from the publicly available Millennium Simulation (Springel et. al. 2005). (Throughout this work I use the term `halo' to refer both to individual haloes in the field and subhaloes embedded in larger structures.) Pairs are selected according to whether or not they come within a given critical (comoving) distance dcrit, without the prerequisite that they must merge. Also, a condition requiring haloes to surpass a critical mass Mcrit during their history is imposed. The primary catalogue, consisting of 502,705 pairs, is selected by setting dcrit=1 Mpc/h and Mcrit = 8.6 10^10 Msun/h (equivalent to 100 simulation particles). One of the central goals of this paper is to evaluate the effects of modifying these criteria. For this purpose, additional subcatalogues with more stringent proximity and mass conditions are constructed (i.e., dcrit=200 kpc/h or/and Mcrit=8.6 10^11 Msun/h =1,000 simulation particles). I use a simple five stage picture to perform statistical analyses of their separations, redshifts, masses, mass ratios, and relevant lifetimes. The fraction of pairs that never merge (because one of the members is absorbed by an external halo or both members survive until today) is accounted for. These results provide a broad picture that captures the essential characteristics behind the evolution of halo pairs. This is the first of a series of papers aimed to explore the huge wealth of information encoded in this catalogue. Such investigations will play a fundamental role in future cosmological studies of interacting galaxies and binary (and multiple) quasars. 
Jorge Moreno
astro-ph.CO