Kinematics and stellar populations of the galactic bulge.

Abstract

In order to measure radial and/or rotation velocities and obtain metallicities for the stellar population of the Galactic bulge, several hundred spectra of giants have been obtained. These include three bulge fields at projected distances from the Galactic center of 1.5, 1.7 and 1.7 kpc, and 33 globular and open clusters. We measure metallicities based on a calibration from stars belonging to the field and to clusters of known abundances. There is a clear dependence of the kinematics on metallicity in all the fields studied, in the sense that the more metal poor stars have lower rotation and higher velocity dispersion than the more metal rich stars. In particular, we identify the giants having [Fe/H] ≤ 1.0 with an extension of the halo population to the innermost regions of the Galaxy rather than with the bulge itself. Near-IR photometry of 21 globulars clusters and bulge fields within 3 kpc of the Galactic center has also been obtained. We find a metallicity gradient with radial distance from the center, consistent with previous results obtained from optical photometry. We argue that the majority of the metal rich globulars within 3 kpc of the center are associated with the bulge population. We also argue that the RR Lyraes previously studied in bulge fields are associated with the inner halo, and that the bulge is younger than the halo. Other kinematic tracers are examined (M giants, RR Lyraes, Miras, OH/IR stars, planetary nebulae) to associate them with different Galactic components. We conclude that all the existing evidence shows that dissipation played an important role in the formation of the bulge.