The metallicity distribution of bulge clump giants in Baade’s window

Aims. We seek to constrain the formation of the Galactic bulge by analysing the detailed chemical composition of a large sample of red clump stars in Baade's window. These stars were selected to minimise the contamination by other Galactic components, so they are good tracers of the bulge metallicity distribution in Baade's window, at least for stars more metal-rich than -1.5. Methods. We used an automatic procedure to measure [Fe/H] differentially with respect to the metal-rich star Leo in a sample of 219 bulge red clump stars from R = 20 000 resolution spectra obtained with FLAMES/GIRAFFE at the VLT. For a subsample of 162 stars, we also derived [Mg/H] from spectral synthesis around the Mg i triplet at 6319 . Results. The Fe and Mg metallicity distributions are both asymmetric with median values of +0.16 and +0.21, respectively. They show only a small proportion of stars at low metallicities, extending down to [Fe/H] = -1.1 or [Mg/H] = -0.7. The iron distribution is clearly bimodal, as revealed both by a deconvolution (from observational errors) and a Gaussian decomposition. The decomposition of the observed Fe and Mg metallicity distributions into Gaussian components yields two populations of equal sizes (50% each): a metal-poor component centred on [Fe/H] = -0.30 and [Mg/H] = -0.06 with a large dispersion and a narrow metal-rich component centred on [Fe/H] = +0.32 and [Mg/H] = +0.35. The metal-poor component shows high [Mg/Fe] ratios (around 0.