Dark matter in Draco and the Local Group: Implications for direct detection experiments

Abstract

We use a cosmological simulation of the Local Group to make quantitative and speculative predictions for direct detection experiments. Cold dark matter (CDM) halos form via a complex series of mergers, accretion events and violent relaxation which precludes the formation of significant caustic features predicted by axially symmetric collapse. The halo density profiles are combined with observational constraints on the galactic mass distribution to constrain the local density of cold dark matter to lie in the range 0.18 ∼ < ρ CDM (R⊙)/(GeV cm −3 ) ∼ < 0.30. In velocity space, coherent streams of dark matter from tidally disrupted halos fill the halo and provide a tracer of the merging hierarchy. The particle velocities within triaxial CDM halos cannot be approximated by a simple Maxwellian distribution and is radially biased at the solar position. The detailed phase space structure within the solar system will depend on the early merger history of the progenitor halos and the importance of major mergers over accretion dominated growth. We follow the formation of a "Draco" sized dSph halo of mass 10 8 M⊙ with several million particles and high force accuracy. Its internal structure and substructure resembles that of galactic or cluster mass halos: the density profile has a singular central cusp and it contains thousands of sub-halos orbiting within its virial radius demonstrating a self-similar nature to collisionless dark matter sub-clustering. The singular cores of substructure halos always survive complete tidal disruption although mass loss is continuous and rapid. Extrapolating wildly to earth mass halos with velocity dispersion of 1 m s −1 (roughly equal to the free streaming scale for neutralinos) we find that most of the dark matter may remain attached to bound subhalos. Further numerical and analytic work is required to confirm the existence of a detectable smooth component. PACS number(s): 95.35.+d, 98.35.Gi, 98.35.Df, 98.35.Mp, 95.75.Pq

An all–sky study of compact, isolated high–velocity clouds

“…The Blitz et al (1999) model also predicts an enhancement of high-velocity clouds here, albeit a stronger one than observed. Likewise the mini-halo simulations of Klypin et al (1999), Moore et al (1999Moore et al ( , 2001, and Putman & Moore (2002) predict a strongly enhanced density of low mass objects around the major galaxies of the Local Group, in particular toward M 31, which lies close to the barycenter direction. We comment further on the expected strength of such an enhancement in the observed distribution below.…”

“…Putman & Moore (2002) compared the results of the full N-body simulation described by Moore et al (2001) with various spatial and kinematic properties of the CHVC distribution, as well as with properties of the more general HVC phenomenon, without regard to object size and degree of isolation. Putman & Moore were led to reject the Local Group deployment of CHVCs, for reasons which we debate below.…”

Structure in the motions of the fastest halo stars

“…Analytic arguments and high-resolution cosmological simulations of the formation of dark matter halos (Helmi & White 1999;Helmi et al 2002;Moore et al 2001) suggest that the halo should be spatially smooth in the Solar neighborhood, although see some recent discussion in Diemand et al (2005) and Zhao et al (2005). This is also supported by measurements of the degree of lumpiness in the angular distribution of halo stars (e.g.…”

The local dark matter phase-space density and impact on WIMP direct detection

“…In particular cosmological N-body simulations find that DM halos have density profiles falling more rapidly at large radii, as r −3 , and velocity distributions showing significant departures from the Maxwell-Boltzmann shape, see, e.g., the results from the high-resolution simulations Via Lactea [10] and Aquarius [11]. A few analyses have discussed the impact on direct detection results when using DM phasespace distribution function as directly read out from the numerical simulations or from other specialized approaches devised to describe in detail the fine-grained structure of the DM velocity distribution, see, e.g., [12][13][14][15][16][17][18][19][20][21]. The main shortcoming of these approaches is that, since it is highly non-trivial to include baryons and baryonic feedback in the simulations, these analyses treat the Galaxy as if made of DM only (in some cases, normalizing the circular velocity obtained in the model for the DM particles to the locally measured circular velocity).…”