Kinematics of RR Lyrae stars in the Galactic bulge with OGLE-IV and Gaia DR2

Hangci Du, Shude Mao, E. Athanassoula, Juntai Shen, Pawel Pietrukowicz

Published 2020-07-02Version 1

We analyze the kinematics and spatial distribution of fundamental-mode 15,599 RR Lyrae (RRL) stars in the Galactic bulge by combining OGLE-IV photometric data and Gaia DR2 proper motions. We show that the longitudinal proper motion and the line-of-sight velocity give similar results for the rotation in the Galactic central regions. The angular velocity of bulge RRLs is found to be around $35$ km s$^{-1}$ kpc$^{-1}$, significantly smaller than that for the majority of bulge stars ($50-60$ km s$^{-1}$ kpc$^{-1}$); bulge RRLs have larger velocity dispersion (120$-$140 km s$^{-1}$) than younger stars. The dependence of the kinematics of the bulge RRLs on their metallicities is shown by their rotation curves and spatial distributions. Metal-poor RRLs ([Fe/H]<$-1$) show a smaller bar angle than metal-rich ones. We also find clues suggesting that RRLs in the bulge are not dominated by halo stars. These results might explain some previous conflicting results over bulge RRLs and help understand the chemodynamical evolution of the Galactic bulge.