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, the largest of its kind to date. (Throughout this work I use the term halo to refer both to individual haloes in the field and to subhaloes embedded in larger structures.) Pairs are selected according to whether or not they come within a given critical (comoving) distance d(crit), without the pre-requisite that they must merge. Moreover, a condition requiring haloes to surpass a critical mass M-crit during their history is imposed. The primary catalogue, consisting of 502 705 pairs, is selected by setting d(crit) = 1 Mpc h(-1) and M-crit = 8.6 x 10(10) M-circle dot h(-1) (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. d(crit) = 200 kpc h(-1) or/and M-crit = 8.6 x 10(11) M-circle dot h(-1) = 1000 simulation particles see Table 1 for a summary). 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 in the pair is absorbed by a third halo or both members survive until the present time) is accounted for. These results provide a broad picture that captures the essential characteristics behind the evolution of these 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.