We present new intermediate-band Strömgren photometry based on more than 300 u,v,b,y images of the Galactic globular cluster \omega Cen .
Optical data were supplemented with new multiband near-infrared ( NIR ) photometry ( 350 J,H,K _ { s } images ) .
The final optical-NIR catalog covers a region of more than 20 \times 20 arcmin squared across the cluster center .
We use different optical-NIR color-color planes together with proper motion data available in the literature to identify candidate cluster red giant ( RG ) stars .
By adopting different Strömgren metallicity indices we estimate the photometric metallicity for \approx 4 , 000 RGs , the largest sample ever collected .
The metallicity distributions show multiple peaks ( \hbox { [ Fe / H ] } _ { phot } = -1.73 \pm 0.08 , -1.29 \pm 0.03 , -1.05 \pm 0.02 , -0.80 \pm 0.04 , -0.42 \pm 0.12 and -0.07 \pm 0.08 dex ) and a sharp cut-off in the metal-poor tail ( \hbox { [ Fe / H ] } _ { phot } \lesssim - 2 dex ) that agree quite well with spectroscopic measurements .
We identify four distinct sub-populations , namely metal-poor ( MP , \hbox { [ Fe / H ] } \leq - 1.49 ) , metal-intermediate ( MI , -1.49 < \hbox { [ Fe / H ] } \leq - 0.93 ) , metal-rich ( MR , -0.95 < \hbox { [ Fe / H ] } \leq - 0.15 ) and solar metallicity ( SM , \hbox { [ Fe / H ] } \approx 0 ) .
The last group includes only a small fraction of stars ( \sim 8 \pm 5 % ) and should be confirmed spectroscopically .
Moreover , using the difference in metallicity based on different photometric indices , we find that the 19 \pm 1 % of RGs are candidate CN-strong stars .
This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars .
The Strömgren metallicity indices display a robust correlation with \alpha -elements ( [ Ca+Si/H ] ) when moving from the metal-intermediate to the metal-rich regime ( \hbox { [ Fe / H ] } \gtrsim - 1.7 dex ) .