The Milky Way Revealed by Variable Stars I: Sample Selection of RR Lyrae stars, and Evidence for the Merger History

TL;DR

This study analyzes RR Lyrae stars from Gaia, ASAS-SN, and ZTF to trace the Milky Way's structure and merger history, revealing evidence of the Gaia-Sausage merger and its role in forming the Galactic thick disc and halo.

Contribution

It provides a comprehensive sample of RR Lyrae stars with precise distances and kinematic data, offering new insights into the Milky Way's assembly and merger events.

Findings

Identification of 3417 RR Lyrae stars with accurate distances

Evidence supporting the Gaia-Sausage merger as a major Galactic event

Detection of radially biased orbits in the halo RRLs, indicating dual origins

Abstract

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 $4 < R(\rm kpc)<20$.