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.
