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