The impact of baryons on the direct detection of dark matter

Abstract

The spatial and velocity distributions of dark matter particles in the Milky Way Halo affect the signals expected to be observed in searches for dark matter. Results from direct detection experiments are often analyzed assuming a simple isothermal distribution of dark matter, the Standard Halo Model (SHM). Yet there has been skepticism regarding the validity of this simple model due to the complicated gravitational collapse and merger history of actual galaxies. In this paper we compare the SHM to the results of cosmological hydrodynamical simulations of galaxy formation to investigate whether or not the SHM is a good representation of the true WIMP distribution in the analysis of direct detection data. We examine two Milky Way-like galaxies from the MaGICC cosmological simulations (a) with dark matter only and (b) with baryonic physics included. The inclusion of baryons drives the shape of the DM halo to become more spherical and makes the velocity distribution of dark matter particles less anisotropic especially at large heliocentric velocities, thereby making the SHM a better fit. We also note that we do not find a significant disk-like rotating dark matter component in either of the two galaxy halos with baryons that we examine, suggesting that dark disks are not a generic prediction of cosmological hydrodynamical simulations. We conclude that in the Solar neighborhood, the SHM is in fact a good approximation to the true dark matter distribution in these cosmological simulations (with baryons) which are reasonable representations of the Milky Way, and hence can also be used for the purpose of dark matter direct detection calculations.

Spherical cows in dark matter indirect detection

“…In the case of annihilating DM, the effect of including baryons is more pronounced; DM-only halos are less spherical/more elongated than those including baryons. This effect has been previously studied and our results are consistent with previous analyses [13,19,24,[59][60][61][62][63][64].…”

Astrophysical uncertainties on the local dark matter distribution and direct detection experiments

“…[66,67] found that a Maxwellian speed distribution is a good fit to the speed distribution in their hydrodynamical simulations. This may be because adiabatic contraction makes the logarithmic slope of the density profile in the Solar neighbourhood closer to that of an isothermal sphere [67]. Even though the Maxwellian speed distribution is a good fit, there is still significant halo to halo variation in the peak speed/velocity dispersion.…”

“…Borzognia et al [66] selected galaxies from the EAGLE [69] and APOSTLE [70] projects with masses in the range (10 12 − 10 13 M ⊙ ) that satisfied observational constraints on the MW rotation curve and stellar mass. Kelso et al [67] [68] studied four MW-like galaxies, with masses in the range 0.7 − 0.9 × 10 12 M ⊙ with a variety of different merger histories (i.e. quiescent or with one or more relatively recent major mergers).…”

Dark matter astrophysical uncertainties and the neutrino floor

“…Nevertheless it is well known from simulations that the SHM is not a good description of a Milky Waylike halo. There have been numerous attempts to find empirical fitting functions to better capture the phase space structure found in N-body and hydrodynamic simulations [26,[67][68][69][70][71] as well as parameterisations that decompose the speed or velocity distribution in an astrophysics independent way [72,73]. Some studies of data from hydrodynamic simulations suggest that the standard halo model is a satisfactory approximation to the Milky Way once baryons are taken into account (e.g., Ref.…”

“…Some studies of data from hydrodynamic simulations suggest that the standard halo model is a satisfactory approximation to the Milky Way once baryons are taken into account (e.g., Ref. [70]), however others such as Sloane et al [71] claim that the SHM overpredicts the amount of dark matter in the tail and hence gives overly optimistic discovery limits. To address these concerns, and because when discriminating between WIMPs and neutrinos the high speed tail of the distribution is especially important, we show discovery limits for a range of different models.…”