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Dark Shards: velocity substructure from Gaia and direct searches for dark matter

  • O'Hare, Ciaran A.J.
  • Evans, N. Wyn
  • McCabe, Christopher
  • Myeong, GyuChul
  • Belokurov, Vasily
Publication Date
Sep 10, 2019
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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 $\sim 550$ km/s while S2 moves on a prograde, but highly polar orbit and enhances peak of the speed distribution at around $300$ km/s. 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.

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