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 dcrit, without the pre-requisite that they must merge. Moreover, 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−1 and Mcrit= 8.6 × 1010 M 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. dcrit= 200 kpc h−1 or/and Mcrit= 8.6 × 1011 M 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.

Table 1.  Halo pair sets shown in the panels of all figures in Section 3. The abbreviations are as follows: C – Close, VC – Very Close, MC – Massive Close and MVC – Massive Very Close. Distances are in comoving coordinates. Recall that mp= 8.6 × 108 M h−1. For reference, C and MC are the Close Sets, VC and MVC are the Very Close Sets, C and VC are the non-Massive Sets, and MC and MVC are the Massive Sets.
SetdcritMcrit (M h−1)Mcrit/mpNumber
C1 Mpc h−18.6 × 1010100502 705
VC200 kpc h−18.6 × 101010032 582
MC1 Mpc h−18.6 × 1011100013 637
MVC200 kpc h−18.6 × 101110002067