Wide-field High-precision CCD Photometry of omega Centauri and Its RR Lyrae Stars

We present wide-field and high-precision BV and Ca & Stromgren by photometry of omega Centauri and its RR Lyrae stars, which represents one of the most extensive photometric surveys to date for this cluster. The member stars of omega Cen are well discriminated from foreground Galactic disk stars from the different distributions in the hk [=(Ca-b)-(b-y)] vs. b-y diagram. The color-magnitude diagrams show the presence of several distinct red-giant branches with a red, metal-rich, sequence clearly separated from other bluer metal-poor ones. Comparison with our population models suggests the most metal-rich population is few billion years younger than the most metal-poor population. From our new BV photometry and hk metallicity measurements for RR Lyrae stars in omega Cen, we also confirm that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. From the presence of several distinct populations and the internal age-metallicity relation, we conclude omega Cen was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now.