We present new determinations of the iron abundance for 220 stars belonging to the stellar system Terzan 5 in the Galactic bulge .
The spectra have been acquired with FLAMES at the Very Large Telescope of the European Southern Observatory and DEIMOS at the Keck II Telescope .
This is by far the largest spectroscopic sample of stars ever observed in this stellar system .
From this dataset , a subsample of targets with spectra unaffected by TiO bands was extracted and statistically decontaminated from field stars .
Once combined with 34 additional stars previously published by our group , a total sample of 135 member stars covering the entire radial extent of the system has been used to determine the metallicity distribution function of Terzan 5 .
The iron distribution clearly shows three peaks : a super-solar component at [ Fe/H ] \simeq 0.25 dex , accounting for \sim 29 % of the sample , a dominant sub-solar population at [ Fe/H ] \simeq - 0.30 dex , corresponding to \sim 62 % of the total , and a minor ( 6 % ) metal-poor component at [ Fe/H ] \simeq - 0.8 dex .
Such a broad , multi-modal metallicity distribution demonstrates that Terzan 5 is not a genuine globular cluster but the remnant of a much more complex stellar system .