An OGLE view of the bulge and Sagittarius

Abstract

We use observations of OGLE-III ab-type RRLyrae stars towards the Galactic centre (Soszyński et al. 2011, AcA 6,1) to study the metallicity and 3D spatial distribution of the old and metal-poor component of the Galactic bulge. Metallicities and distances to the RRLyrae stars are derived photometrically from Fourier-component analyses of their light curves, allowing the determination of distances accurate to ~7% from 0 to 40 kpc. The distance distribution of the RRLyrae stars peaks at 8.8 kpc, with the data indicating the presence of a bar-like structure inclined at ~30° to the line of sight. The dataset also exhibits a secondary concentration of stars beyond the Galactic centre at ~27 kpc. This is consistent with the distance to the Sagittarius (Sgr) dwarf galaxy, and can be attributed to the serendipitous alignment of the Sgr stream with the bulge. This dataset allows the Sgr stream to be traced in this part of the sky for the first time. The underlying metallicity distributions are determined to have mean ± intrinsic width of [Fe/H]_bulge = −1.24±0.23 dex and [Fe/H]_Sgr = −1.53±0.14 dex.