A box full of chocolates: The rich structure of the nearby stellar halo
  revealed by Gaia and RAVE
  The hierarchical structure formation model predicts that stellar halos should
form, at least partly, via mergers. If this was a predominant formation channel
for the Milky Way's halo, imprints of this merger history in the form of moving
groups or streams should exist also in the vicinity of the Sun. Here we study
the kinematics of halo stars in the Solar neighbourhood using the very recent
first data release from the Gaia mission, and in particular the TGAS dataset,
in combination with data from the RAVE survey. Our aim is to determine the
amount of substructure present in the phase-space distribution of halo stars
that could be linked to merger debris. To characterise kinematic substructure,
we measure the velocity correlation function in our sample of halo (low
metallicity) stars. We also study the distribution of these stars in the space
of energy and two components of the angular momentum, in what we call
"Integrals of Motion" space. The velocity correlation function reveals
substructure in the form of an excess of pairs of stars with similar
velocities, well above that expected for a smooth distribution. Comparison to
cosmological simulations of the formation of stellar halos indicate that the
levels found are consistent with the Galactic halo having been built fully via
accretion. Similarly, the distribution of stars in the space of "Integrals of
motion" is highly complex. A strikingly high fraction (between 58% and upto
73%) of the stars that are somewhat less bound than the Sun are on (highly)
retrograde orbits. A simple comparison to Milky Way-mass galaxies in
cosmological hydrodynamical simulations suggests that less than 1% have such
prominently retrograde outer halos. We also identify several other
statistically significant structures in "Integrals of Motion" space that could
potentially be related to merger events.
