Early Supersymmetric Cold Dark Matter Substructure

Earth-mass dark matter “microhalos” may be the first objects to collapse and 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 direct and indirect dark matter detection experiments. We present here the large N -body simulation of early substructure in a supersymmetric cold dark matter (SUSY-CDM) scenario characterized by an exponential cutoff in the matter power spectrum at Mc = 10 −6 M⊙. The simulation resolves a 0.014 M⊙ parent “SUSY” halo at z = 75 with 12 million particles within its virial radius. On these scales the effective index of the power spectrum approaches −3, a range of mass scales collapse almost simultaneously, and the formation history of the early SUSY host appears very different from that of a low-redshift massive halo. Compared to a z = 0 galaxy cluster with similar “concentration” parameter, substructure within our SUSY host is less evident both in phase-space and in physical space, and it is less resistant against tidal stripping and disruption. As the scale factor of the universe increases by a factor of 1.3, we find that between 20 and 40 percent of well-resolved SUSY substructure is destroyed, compared to only ∼ 1 percent in the low-redshift cluster. Despite the lower contrast and higher disruption probability, SUSY substructure is just as abundant as in z = 0 galaxy clusters, i.e. the substructure normalized mass and circular velocity functions are very similar. The dark matter self-annihilation γ-ray signal from resolved sub-microhalos is comparable to the spherically-averaged SUSY host signal, and should be included in estimates of the cosmological extragalactic γ-ray background. The fractional contribution of substructure to the total annihilation luminosity is smaller in the z = 75 host than in the z = 0 counterpart because of the smaller density contrast of sub-microhalos. Subject headings: methods: N-body simulations – methods: numerical – early universe – structure formation