The cosmic evolution of halo pairs - I. Global trends

Abstract

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 fieldandto subhaloes embedded in larger structures.) Pairs are selected according to whether or not they come within a given critical (comoving) distancedcrit,withoutthe pre-requisite that they must merge. Moreover, a condition requiring haloes to surpass a critical massMcrit during their history is imposed. The primary catalogue, consisting of 502705 pairs, is selected by settingdcrit= 1Mpch-1 andMcrit= 8.6 × 1010M⊙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= 200kpch-1 or/andMcrit= 8.6 × 1011M⊙h-1= 1000 simulation particles - see Table 1 for a summary). I use a simplefive-stagepicture to perform statistical analyses of their separations, redshifts, masses, mass ratios and relevant lifetimes. The fraction of pairs thatnevermerge (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. 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 thatmp= 8.6 × 108M⊙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. Set d crit M crit(M⊙h-1) M crit/mp Number C 1Mpch-1 8.6 × 1010 100 502705 VC 200kpch-1 8.6 × 1010 100 32582 MC 1Mpch-1 8.6 × 1011 1000 13637 MVC 200kpch-1 8.6 × 1011 1000 2067 © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.