The GIRAFFE Inner Bulge Survey (GIBS) II. Metallicity distributions and alpha element abundances at fixed Galactic latitude

Aims. We investigate metallicity and alpha-element abundance gradients along a Galactic longitude strip, at latitude b similar to -4 degrees, with the aim of providing observational constraints for the structure and origin of the Milky Way bulge. Methods. High-resolution (R similar to 22 500) spectra for 400 K giants, in four fields within similar to 4.8 degrees less than or similar to b less than or similar to 3.4 degrees and 10 degrees less than or similar to l less than or similar to +/- 10 degrees, were obtained within the GIRAFFE Inner Bulge Survey (GIBS) project. To this sample we added another similar to 400 stars in Baade's Window at (l, b) = (1 degrees, -4 degrees), observed with the identical instrumental configuration: FLAMES GIRAFFE in Medusa mode with HR13 setup. All target stars lie within the red clump of the bulge colour magnitude diagram, thus minimising contamination from the disc or halo stars. The spectroscopic stellar surface parameters were derived with an automatic method based on the GALA code, while the [Ca/Fe] and [Mg/Fe] abundances as a function of [Fe/H] were derived through a comparison with the synthetic spectra using MOOG. We constructed the metallicity distributions for the entire sample, and for each field individually, in order to investigate the presence of gradients or field-to-field variations in the shape of the distributions. Results. The metallicity distributions in the five fields are consistent with being drawn from a single parent population, indicating the absence of a gradient along the major axis of the Galactic bar. The global metallicity distribution is nicely fitted by two Gaussians. The metal-poor component is rather broad, with a mean at <[Fe = H]> = -0.31 dex and sigma = 0.31 dex. The metal-rich component is narrower, with mean <[Fe = H]> = +0.26 and sigma = 0.2 dex. The [Mg/Fe] ratio follows a tight trend with [Fe/H], with enhancement with respect to solar in the metal-poor regime similar to the value observed for giant stars in the local thick disc. [Ca/Fe] abundances follow a similar trend, but with a considerably larger scatter than [Mg/Fe]. A decrease in [Mg/Fe] is observed at [Fe/H] = -0.44 dex. This knee is in agreement with our previous bulge study of K-giants along the minor axis, but is 0.1 dex lower in metallicity than the value reported for the microlensed dwarf and subgiant stars in the bulge. We found no variation in alpha-element abundance distributions between different fields.