Early Supersymmetric Cold Dark Matter Substructure

Abstract

Earth-mass ''microhalos'' may be the first objects to virialize in the early universe. Their ability to survive the hierarchical clustering process as substructure in the larger halos that form subsequently has implications for dark matter detection experiments. We present a large N-body simulation of early substructure in a supersymmetric cold dark matter (SUSY-CDM) scenario characterized by an exponential cutoff in the power spectrum at M c ¼ 10 À6 M . The simulation resolves a 0.014 M parent SUSY halo at z ¼ 75 with 14 million particles. On these scales, the effective index of the power spectrum approaches À3, and a range of mass scales collapses almost simultaneously. Compared to a z ¼ 0 galaxy cluster, substructure within our SUSY host is less evident both in phase-space and in physical space, and it is less resistant against tidal disruption. As the universe expands by a factor of 1.3, we find that between 20% and 40% of well-resolved SUSY substructure is destroyed, compared to only $1% in the low-redshift cluster. Nevertheless, SUSY substructure is just as abundant as in z ¼ 0 galaxy clusters; i.e., the normalized mass and circular velocity functions are very similar. The DM self-annihilation -ray luminosity from bound subhalos and other deviations from a smooth spherical configuration is at least comparable to the spherically averaged signal in the SUSY host, and at least 3 times larger than the spherically averaged signal in the cluster host. Such components must be taken into account when estimating the total cosmological extragalactic -ray annihilation background. The relative contribution of bound substructure alone to the total annihilation luminosity is about 4 times smaller in the SUSY host than in the z ¼ 0 cluster because of the smaller density contrast of micro-subhalos. Subject headingg s: early universe -large-scale structure of universe -methods: n-body simulationsmethods: numerical

Building merger trees from cosmologicalN-body simulations

“…decide if a particle is gravitationally bound to it or not). Ultimately, to obtain the most reliable results, one would want to define haloes as structures detected directly in the 6 dimensional (6D) phase space (for a review and extensive comparison of the methods which have been proposed to do that see Maciejewski et al 2009a), but the developments in that direction are pretty recent (Diemand et al 2006;Maciejewski et al 2009b) so our approach in this paper remains three dimensional. Moreover, in practise, the bound structures detected in 6D space are not very different from the 3D ones, except that they tend to be systematically (albeit slightly) more massive (Maciejewski et al 2009b).…”

Precise constraints on the dark matter content of Milky Way dwarf galaxies for gamma-ray experiments

“…Past work in the literature considered detecting γ-rays from dark matter annihilation in Milky Way-bound dark matter halos: dSphs were studied in [14,21,22,23,24], more massive galaxies in the local group were considered in [25], potentially dark subhalos were studied in [26,27,28,29,30,31], and the prospects of detecting microhalos were explored in [32,33].…”

Detecting the dark matter via the proper motion of γ-rays from microhalos

“…The subhalo mass function has been studied in numerical simulations and found to be described by dN/d ln M ∼ M −1 , normalized in a way such that for a Milky Way size halo, 10% of the mass of the halo is in subhalos of mass greater than 10 7 M ⊙ . Preliminary results from N-body simulations [10,11], as well as approximate analytical arguments find that this mass function is preserved down to microhalo scales, with the exception that on sub-solar mass scales the survival probability is reduced to only [10 − 20]% due to early rapid merging processes as well as potential interactions with stars [14,15,16]. In this case, the value of ξ as defined above reduces to ξ ≈ 0.002.…”

“…For the particular case of supersymmetric (SUSY) dark matter, this scale is M min ≈ 10 −4 (T d /10MeV) [9]. The abundance of microhalos in the solar neighborhood is still under debate [10,11,12,13,14,15,16]. As such, it can be paremetrized by assuming that a certain fraction of the local dark matter density (ρ ≈ 10 −2 M ⊙ pc −3 ) is in microhalos in a logarithmic mass interval [1],…”