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

TL;DR

This study investigates the kinematics and spatial distribution of RR Lyrae stars in the Galactic bulge using OGLE-IV and Gaia DR2 data, revealing differences in rotation and dispersion related to metallicity.

Contribution

It provides new insights into the kinematic behavior of bulge RR Lyrae stars and their metallicity dependence, improving understanding of bulge evolution.

Findings

Bulge RR Lyrae stars have lower angular velocity than other bulge stars.

Metal-poor RR Lyrae stars show a smaller bar angle.

RR Lyrae stars are not predominantly halo stars in the bulge.

Abstract

We analyze the kinematics and spatial distribution of 15,599 fundamental-mode RR Lyrae (RRL) stars in the Milky Way bulge by combining OGLE-IV photometric data and Gaia DR2 proper motions. We show that the longitudinal proper motions and the line-of-sight velocities can 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.