Deciphering the 3D structure of the old Galactic bulge from the OGLE RR Lyrae stars

TL;DR

This study analyzes over 27,000 RR Lyrae stars in the Galactic bulge, revealing its 3D triaxial ellipsoid shape, metallicity gradients, and evidence for multiple old populations, informing models of bulge formation.

Contribution

It provides the first detailed 3D structure and population analysis of RR Lyrae stars in the Galactic bulge, supporting a merger-based formation scenario.

Findings

The bulge RR Lyrae stars form a triaxial ellipsoid with a 20° inclination.

No evidence of an X-shaped structure among RR Lyrae stars.

Identification of two main old populations with different metallicities.

Abstract

We have analyzed a sample of 27,258 fundamental-mode RR Lyrae variable stars (type RRab) detected recently toward the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE) survey. The data support our earlier claim that these metal-poor stars trace closely the barred structure formed of intermediate-age red clump giants. The distance to the Galactic center (GC) inferred from the bulge RR Lyrae stars is R_0=8.27+/-0.01(stat)+/-0.40(sys) kpc. We show that their spatial distribution has the shape of a triaxial ellipsoid with an major axis located in the Galactic plane and inclined at an angle of i=20+/-3 deg to the Sun-GC line of sight. The obtained scale-length ratio of the major axis to the minor axis in the Galactic plane and to the axis vertical to the plane is 1:0.49(2):0.39(2). We do not see the evidence for the bulge RR Lyrae stars forming an X-shaped structure. Based on the light curve parameters, we derive metallicities of the RRab variables and show that there is a very mild but statistically significant radial metallicity gradient. About 60% of the bulge RRab stars form two very close sequences on the period-amplitude (or Bailey) diagram, which we interpret as two major old bulge populations: A and B. Their metallicities likely differ. Population A is about four times less abundant than the slightly more metal-poor population B. Most of the remaining stars seem to represent other, even more metal-poor populations of the bulge. The presence of multiple old populations indicates that the Milky Way bulge was initially formed through mergers.