In section 5. Galactic structure, stellar clusters and populations
The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations
This paper uses the fourth internal data release of the Gaia-ESO spectroscopic survey to characterize the properties of 2500 individual red clump stars in the Galactic bulge, sampling the range of longitude from -10deg to +8deg and latitude from -10deg to -4deg. This is the first time that such a large and homogeneous sample, over such a wide area of the bulge, has been analyzed looking at [Fe/H] and [Mg/Fe] with such high resolution and high S/N spectra. The authors present the spatial distribution of these measurements, and the correlation of these properties with kinematics. The metalicity distribution function of the bulge is confirmed to be bimodal across the whole sampled area, with metal-poor stars dominating at higher latitudes. The metal-poor stars have isotropic hot kinematics, not consistent with an X-shaped bulge, that could be the product of an early prompt dissipative collapse dominated by massive stars. The population shows a knee in the [Mg/Fe] measurements at higher [Fe/H] than in the thick disk, which suggests a higher star-formation rate. In contrast, the metal-rich stars exhibit bar-like kinematics and appear tightly associated with an X-shape bulge. These metal-rich stars also overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. The bar boxy-peanut bulge thus seems likely to have formed from the secular evolution of the early thin disk. The authors present a chemical evolution model for the entire bulge that requires a fast (< 1 Gyr) intense burst of stellar formation at early epochs.