  We present metallicity measurements based on GIRAFFE@VLT spectra of 80
subgiant-branch stars of the Galactic globular cluster Omega Centauri. The VLT
spectroscopic data are complemented by ACS/HST and<EMAIL_ADDRESS>high-accuracy
color-magnitude diagrams. We have obtained the [Fe/H] abundance for each of the
80 target stars, and the abundances of C, N, Ca, Ti, and Ba for a subset of
them, by comparison with synthetic spectra. We show that stars with
[Fe/H]<-1.25 have a large magnitude spread on the flat part of the SGB. We
interpret this as empirical evidence for an age spread. We have identified four
distinct stellar groups within the SGB region: (i) an old, metal-poor group
([Fe/H]~-1.7); (ii) an old, metal-rich group ([Fe/H]~-1.1); (iii) a young (up
to 4--5 Gyr younger than the old component) metal-poor group ([Fe/H]~-1.7);
(iv) a young, intermediate-metallicity ([Fe/H]~-1.4) group, on average 1--2 Gyr
younger than the old metal-poor population, and with an age spread that we
cannot properly quantify with the present sample. In addition, a group of SGB
stars are spread between the intermediate-metallicity and metal-rich branches
of the SGB. The spread in age within each population establishes that the
progenitor of Omega Cen system must have had a composite nature.
