On the relation between radial alignment of dark matter subhalos and host mass in cosmological simulations

Abstract

We explore the dependence of the radial alignment of subhalos on the mass of the host halo they orbit in. As the effect is seen on a broad range of scales including massive clusters as well as galactic systems it only appears natural to explore this phenomenon by means of cosmological simulations covering the same range in masses. We have 25 well resolved host dark matter halos at our disposal ranging from 10^15 Msun/h down to 10^12 Msun/h each consisting of order of a couple of million particles within the virial radius. We observe that subhalos tend to be more spherical than isolated objects. Both the distributions of sphericity and triaxiality of subhalos are Gaussian distributed with peak values of s approx. 0.80 and T approx. 0.56, irrespective of host mass. Interestingly we note that the radial alignment is independent of host halo mass and the distribution of \cosθ(i.e. the angle between the major axis E_a of each subhalo and the radius vector of the subhalo in the reference frame of the host) is well fitted by a simple power law P(\cosθ) proportional to \cos^4θwith the same fitting parameters for all host halos.