Twinkling relics of the last major merger: Galactic archeology with Gaia RR Lyrae stars

It was always hoped that one day we would be able to use Gaia to measure the Galactic accretion history. This started to come true just after the first  Gaia Data Releases. In particular, the peculiar kinematics of halo stars in the solar vicinity and of Globular clusters has revealed the presence of relics of the last significant merger event experienced by our Galaxy. About 10 Gyr ago an LMC-like galaxy (Gaia-Sausage/Enceladus) merged in the proto Milky Way changing its evolution and appearance. Despite the increasing number of evidence, we still lack a comprehensive and uninterrupted view of the merger relics.  The RR Lyrae stars can come at our rescue. These old and metal-poor variable stars are relatively easy to detect and "twinkle" approximately at constant luminosity. They can be used as Galactic “standard candles”  and they are bright enough to be seen at large distances (>50 kpc). In this talk, I will present a detailed 3D spatial and kinematic analysis of a clean sample of Gaia DR2 RR Lyrae. My analysis shows clearly that the inner part of the stellar halo is dominated by a triaxial shaped cloud of stars that are kinematically alike to the relics of the Gaia-Sausage found in the solar vicinity. The superior spatial coverage of this sample makes possible to trace the radial variation of the properties of the cloud of debris with unprecedented details. Surprisingly a consistent number of likely metal-rich RR Lyrae stars have been found to belong to a rotating thin-disc like component. I will discuss a possible formation mechanism of such stars and how they could possibly help us in timing the last major merger of our Galaxy.