The Not So Simple Stellar System Cen. II. Evidence in Support of a Merging Scenario

We present multiband photometry covering similar to 5 degrees x 5 degrees across omega Cen collected with the Dark Energy Camera on the 4 m Blanco telescope, combined with Hubble Space Telescope and Wide Field Imager data for the central regions. The unprecedented photometric accuracy and field coverage allows us to confirm the different spatial distribution of blue and red main-sequence stars, and of red giant branch (RGB) stars with different metallicities. The ratio of the number of blue to red main-sequence stars shows that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one, with the frequency of blue main-sequence stars increasing at a distance of similar to 20 ' from omega Cen's center. Similarly, the more metal-rich RGB stars show a more extended spatial distribution compared to the more metal-poor ones in the outskirts of the cluster. Moreover, the centers of the distributions of metal-rich and metal-poor RGB stars are shifted in different directions with respect to the geometrical center of omega Cen. We constructed stellar density profiles for the blue and red main-sequence stars; they show that the blue main-sequence stellar subpopulation has a more extended spatial distribution compared to the red main-sequence one in the outskirts of omega Cen, confirming the results based on the number ratio. We also computed the ellipticity profile of omega Cen, which has a maximum value of 0.16 at a distance of similar to 8 ' from the center, and a minimum of 0.05 at similar to 30 '; the average ellipticity is similar to 0.10. The circumstantial evidence presented in this work suggests a merging scenario for the formation of the peculiar stellar system omega Cen.