The Milky Way Revealed by Variable Stars I: Sample Selection of RR Lyrae
  stars, and Evidence for the Merger History
  In order to study the Milky Way, RR Lyrae (RRL) variable stars identified by
Gaia, ASAS-SN and ZTF sky survey projects have been analyzed as tracers in this
work. Photometric and spectroscopic information of 3417 RRLs including proper
motions, radial velocity and metallcity are obtained from observational data of
Gaia, LAMOST, GALAH, APOGEE and RAVE. Precise distances of RRLs with typical
uncertainties less than 3% are derived by using a recent comprehensive
period-luminosity-metallicity relation. Our results from kinematical and
chemical analysis provide important clues for the assembly history of the Milky
Way, especially for the Gaia-Sausage ancient merger. The kinematical and
chemical trends found in this work are consistent with that of recent
simulations which indicated that the Gaia-Sausage merger is the dual origin of
the Galactic thick disc and halo. As recent similar works have found, the halo
RRLs sample in this work contains a subset of radially biased orbits besides a
more isotropic component. This higher orbital anisotropy component amounts to
$\beta\simeq 0.8$, and it contributes between 42% and 83% of the halo RRLs at
$4 < R(\rm kpc)<20$.
