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 color-magnitude diagrams from high-accuracy photometry on a \sim 10 \times 10 arcmin ^ { 2 } mosaic of ACS/ HST images centered on the cluster center , and on multicolor images of a \sim 34 \times 33 arcmin ^ { 2 } field , taken with the WFI @ ESO2.2m camera .
Our main purpose was to combine photometric data with spectroscopic data , in the hope of teasing apart some of the population mysteries that neither data set can answer on its own .
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 .
A relative age has been obtained for each star , from theoretical isochrones for its metallicity , \alpha -enhancement , and presumed He content .
We have identified four distinct stellar groups within the SGB region : ( i ) an old , metal-poor group ( [ Fe/H ] \sim - 1.7 ) ; ( ii ) an old , metal-rich group ( [ Fe/H ] \sim - 1.1 ) ; ( iii ) a young ( up to 4–5 Gyr younger than the old component ) metal-poor group ( [ Fe/H ] \sim - 1.7 ) ; ( iv ) a young , intermediate-metallicity ( [ Fe/H ] \sim - 1.4 ) group , on average 1–2 Gyr younger than the old metal-poor population , and with an age spread that we can not 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 .
We tentatively propose connections between the SGB stars and both the multiple main sequence and the red giant branch .
Finally , we discuss the implications of the multiple stellar populations on the formation and evolution of \omega  Cen .
The spread in age within each population establishes that the original system must have had a composite nature .