Modelling ultrafine structure in dark matter haloes

Abstract

Various laboratory‐based experiments are underway attempting to detect dark matter directly. The event rates and detailed signals expected in these experiments depend on the dark matter phase‐space distribution on submilliparsec scales. These scales are many orders of magnitude smaller than those that can be resolved by conventional N‐body simulations, so one cannot hope to use such tools to investigate the effect of mergers in the history of the Milky Way on the detailed phase‐space structure probed by the current experiments. In this paper, we present an alternative approach to investigate the results of such mergers, by studying a simplified model for a merger of a subhalo with a larger parent halo. With an appropriate choice of parent halo potential, the evolution of material from the subhalo can be expressed analytically in action‐angle variables, so it is possible to obtain its entire orbit history very rapidly without numerical integration. Furthermore by evolving backwards in time, we can obtain arbitrarily high spatial resolution for the current velocity distribution at a fixed point. Although this model cannot provide a detailed quantitative comparison with the Milky Way, its properties are sufficiently generic that it offers qualitative insight into the expected structure arising from a merger at a resolution that cannot be approached with full numerical simulations. Preliminary results indicate that the velocity‐space distribution of dark matter particles remains characterized by discrete and well‐defined peaks over an extended period of time, both for single and multimerging systems, in contrast to the simple smooth velocity distributions sometimes assumed in predicting laboratory experiment detection rates. In principle, this structure contains a wealth of information about the formation history of the Milky Way's dark halo.

Dependence of direct detection signals on the WIMP velocity distribution

“…composed of a small number of streams) to those on the scales resolved by simulations. The ultra-local velocity distribution may, however, contain some fine-grained substructure [71,72].…”

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).…”

Ultrafine dark matter structure in the solar neighbourhood

“…In this paper, we present a new method of simulating the DM distribution of the Milky Way, building on the approach developed by Fantin et al (2008, hereinafter Paper I). Applied to a merger tree describing the history of a Milky Way‐like galaxy, the model attributes to each progenitor of the halo a characteristic DM distribution, using the method developed in Paper I.…”