Velocity substructure from Gaia and direct searches for dark matter

Abstract

Data from the Gaia satellite show that the solar neighbourhood of the Milky Way's stellar halo is imprinted with substructure from several accretion events. Evidence of these events is found in "the Shards", stars clustering with high significance in both action space and metallicity. Stars in the Shards share a common origin, likely as ancient satellite galaxies of the Milky Way, so will be embedded in dark matter (DM) counterparts. These "Dark Shards" contain two substantial streams (S1 and S2), as well as several retrograde, prograde and lower energy objects. The retrograde stream S1 has a very high Earth-frame speed of ∼ 550 km s −1 while S2 moves on a prograde, but highly polar orbit and enhances peak of the speed distribution at around 300 km s −1 . The presence of the Dark Shards locally leads to modifications of many to the fundamental properties of experimental DM signals. The S2 stream in particular gives rise to an array of effects in searches for axions and in the time dependence of nuclear recoils: shifting the peak day, inducing non-sinusoidal distortions, and increasing the importance of the gravitational focusing of DM by the Sun. Dark Shards additionally bring new features for directional signals, while also enhancing the DM flux towards Cygnus.

Dark matter substructure under the electron scattering lamppost

“…In Table I, we list v mp , t c , and b for possible local DM components we consider throughout this work: the SHM, Gaia halo and Sausage [16], and possible DM streams associated with Nyx [18], S1, S2a, and S2b [22] stellar streams. Note that we use the values of the stellar streams as benchmarks for the potential dark matter distributions associated with Nyx, S1, S2a, and S2b.…”

“…The standard halo model (SHM), in which the velocities of the DM particles follow an isotropic Maxwellian distribution, has commonly been used in the computation of DM direct detection rates. Recent work, however, has shown that the Milky Way's history has been punctuated by mergers with dwarf galaxies, which resulted in a rich variety of stellar substructure beyond the traditional halo and disk, such as the debris flow of the Gaia Sausage (also called Enceladus) [16,17], the Nyx stream [18,19], or the so-called "shards," the S1, S2a, and S2b streams [20][21][22]. 1 Since dwarf galaxies also contain DM, these mergers could also result in associated dark matter substructure within our galaxy, beyond that of what pertains to the SHM.…”

“…The impact that these new astrophysical discoveries could have on DM direct detection searches has only started to be explored recently [21][22][23][24][25][26]. In this paper, we focus on the effects that DM substructure hinted by these discoveries could have on electron recoil experiments with semiconductor targets.…”

“…Lacking an accurate description of how DM associated with the stellar streams is distributed, we will restrict ourselves to taking the velocity distributions of the stellar streams as benchmarks for their dark matter counterparts. To this end, we use the distributions for the Nyx [18], S1, S2a, and S2b [22] stellar streams. Since we are interested in exploring the relationship between astrophysics and direct detection, we take the stellar streams as mere proxies or placeholders for their associated DM distributions and make no claims about their accuracy as such.…”

Implications of the Gaia sausage for dark matter nuclear interactions

“…Another promising avenue to look for interesting signatures of DM substructure are axion searches and directional detection experiments as discussed in refs. [37][38][39]. The main drawback of the aforementioned analyses is the underlying assumption of a near perfect stellar-DM velocity correlation.…”