The shape of the Galactic halo with $Gaia$ DR2 RR Lyrae. Anatomy of an ancient major merger

Abstract

We use the $Gaia$ DR2 RR Lyrae sample to gain an uninterrupted view of the Galactic stellar halo. We dissect the available volume in slices parallel to the Milky Way's disc to show that within $\sim30$ kpc from the Galactic centre the halo is triaxial, with the longest axis misaligned by $\sim70^{\circ}$ with respect to the Galactic $x$-axis. This anatomical procedure exposes two large diffuse over-densities aligned with the semi-major axis of the halo: the Hercules-Aquila Cloud and the Virgo Over-density. We reveal the kinematics of the entire inner halo by mapping out the amplitudes and directions of the RR Lyrae proper motions. These are then compared to simple models with different anisotropies to demonstrate that the inner halo is dominated by stars on highly eccentric orbits. We interpret the shape of the density and the kinematics of the $Gaia$ DR2 RR Lyrae as evidence in favour of a scenario in which the bulk of the halo was deposited in a single massive merger event.