Old Galactic Bulge in 3D from OGLE RR Lyrae Stars

We have analyzed a sample of over 27 200 fundamental-mode RR Lyr (type RRab) variable stars detected recently toward the Galactic bulge by the 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 Lyr stars is R₀=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° 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 have found that the spatial density profile of bulge RR Lyr variables can be described as a single power law with an index of -2.96. They do not form an X-shaped structure as it is observed in the case of bulge red clump giants.

Another discovery is the presence of multiple old populations being likely the result of mergers in the early history of the Milky Way. 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.

PLEASE cite the following paper when refering to these results: Pietrukowicz et al., 2015, ApJ, 811, 113.

Any comments about the data and the form of their presentation are welcome. Send your messages to this address.

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