  We present wide-field and high-precision BV and Ca & Stromgren by photometry
of omega Centauri, 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 field stars in the hk [=(Ca-b)-(b-y)]
vs. b-y diagram. The resulting "cleaned" color-magnitude diagram (CMD) has
allowed us to obtain an accurate distribution of the red horizontal branch (HB)
and the asymptotic giant branch stars. We confirm the presence of several red
giant branches (RGBs) with the most metal-rich sequence well separated from
other bluer metal-poor ones. Our population models suggest that four
populations with different metallicities can reproduce the observed nature of
the RGB. The HB distribution is also found to be consistent with the multiple
stellar populations of the RGB. From our population models, we propose that the
most metal-rich population is about 4 Gyr younger than the dominant metal-poor
population, indicating that omega Cen was enriched over this timescale. We
identify, for the first time, a continuous and slanting RGB bump in the CMD of
omega Cen, which is due to the metallicity spread amongst the RGB stars. Our
photometry also reveals a significant population of blue straggler stars. The
discovery of several populations and the internal age-metallicity relation of
omega Cen provides 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 at the present time.
