  Our recent CCD photometry (Lee et al. 1999) has shown, for the first time,
that omega Cen has several distinct stellar populations, which is reminiscent
of the Sagittarius dwarf galaxy. Here we present more detailed analysis of the
data along with the population models. We confirm the presence of several
distinct red-giant-branches (RGBs) with a red metal-rich sequence well
separated from other bluer metal-poor ones. Our population models suggest the
red clump associated with the most metal-rich RGB is about 4 Gyr younger than
the dominant metal-poor component, indicating that omega Cen was enriched over
this timescale. These features, taken together with this cluster's other
unusual characteristics, provide good evidence that 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. Mergers probably were much more frequent
in the early history of the Galaxy and omega Cen appears to be a relict of this
era.
