Stromgren Photometry at the Main-Sequence Turn-Off

We have observed (with vby filters) a field north of the core of the most massive globular cluster in our galaxy, ω Centauri. We have found a correlation of age and metallicity in a region which avoids the dense core and the inhomogeneous foreground dust emission shown by the IRAS satellite. Our observations show that the comparatively metal-rich stars (as defined by the (b− y) and m1 colors) are younger than the metal-poor stars by at least 3 Gyr. This correlation of metallicity with age suggests that ω Cen has enriched itself over a timescale of about 3 Gyr, and possibly longer. It is remarkable that ejecta from stellar winds combined with supernovae of type II failed to disperse the cluster’s interstellar matter at an earlier epoch, but were captured by the cluster instead. Star formation would have ceased as type Ia supernovae dispersed the remaining interstellar matter. This work and other recent evidence suggests that ω Cen could have been part of a small satellite galaxy in which all the activity occurred before it was captured by the Milky Way.