Universality of dark matter haloes shape over six decades in mass: Insights from the Millennium XXL and SBARBINE simulations

Abstract

For the last 30 yr many observational and theoretical evidences have shown that galaxyclusters are not spherical objects, and that their shape is much better described by a triaxialgeometry.With the advent of multiwavelength data of increasing quality, triaxial investigationsof galaxy clusters is gathering a growing interest from the community, especially in the timeof 'precision cosmology'. In this work, we aim to provide the first statistically significantpredictions in the unexplored mass range above 3 × 1014 Mo˙h-1, using haloes from tworedshift snapshots (z = 0 and z = 1) of the Millennium XXL simulation. The size of thiscosmological dark matter-only simulation (4.1 Gpc) allows the formation of a statisticallysignificant number of massive cluster scale haloes (≈500 with M > 2× 1015 Mo˙ h-1, and780 000 withM > 1014 Mo˙ h-1). Besides, we aim to extend this investigation to lower massesin order to look for universal predictions across nearly six orders of magnitude in mass, from1010 to almost 1016 Mo˙ h-1. For this purpose we use the SBARBINE simulations, allowingus to model haloes of masses starting from ≈1010 Mo˙ h-1. We use an elliptical overdensitymethod to select haloes and compute the shapes of the unimodal ones (approximately 50 percent), while we discard the more unrelaxed. The minor to major and intermediate to major axisratio distributions are found to be well described by simple universal functional forms thatdo not depend on cosmology or redshift. Our results extend the findings of Jing & Suto to ahigher precision and a wider range of mass. This 'recipe' is made available to the communityin this paper and in a dedicated web page.