The Milky Way halo in the Gaia era
The Milky Way halo contains only 1% of its stars, but ~90% of the total mass, making it an ideal laboratory for mapping the distribution of dark matter on small scales as well as isolating processes at play in formation of galaxies. Hydrodynamical simulations show that the outer parts of Milky Way-like galaxies are entirely accreted from disrupting satellites, but the inner halos additionally have a component formed in situ. These processes are yet to be disentangled observationally. Combining the first year of Gaia data with ground-based spectroscopic surveys, we identified a metal-rich component of the stellar halo in the Solar neighborhood. Based on their orbital properties, we argue that these stars were formed in situ, originating from the inner Galaxy and having migrated to the Solar circle, thus becoming a part of the relaxed, inner halo population. The dynamical times are longer in the outer halo, such that tidally disrupting satellites are apparent as overdensities in the configuration space. Dozens of such events have been identified in the Milky Way halo, and provide not only irrefutable evidence for its accreted origin, but also serve as excellent tracers of the underlying gravitational potential. We show how to optimally map the distribution of matter in the Galaxy by incorporating constraints from multiple tidal streams, and chart the way forward in anticipation of the future Gaia data releases.