Implications of triangular features in the Gaia skymap for the Caustic Ring Model of the Milky Way halo

The Gaia map of the Milky Way reveals a pair of triangular features at nearly symmetric locations on opposite sides of the Galactic Center. In this paper we explore the implications of these features assuming they are manifestations of a caustic ring in the dark matter distribution of the Milky Way halo. The existence of a series of such rings is predicted by the Caustic Ring Model. The model's phase-space distribution is that acquired by a rethermalizing Bose–Einstein condensate of axions or axion-like particles. We show that dust is gravitationally entrained by cold axion flows and propose this as an explanation for the sharpness of the triangular features. The locations of the features imply that we on Earth are much closer to the fifth caustic ring than thought on the basis of pre-Gaia observations. Most likely we are inside its tricusp cross-section. In that case the dark matter density on Earth is dominated by four cold flows, termed Big, Little, Up and Down. If we are outside the tricusp cross-section the dark matter density on Earth is dominated by two cold flows, Big and Little. We use the triangular features in the Gaia map, and a matching feature in the IRAS map, to estimate the velocity vectors and densities of the four locally dominant flows.