Field Stars and Clusters of the Galactic Bulge: Implications for Galaxy Formation

Abstract

The results of a kinematic study of the Galactic bulge based on spectra of red giants in fields at projected distances of 1.4-1.8 kpc from the Galactic center are presented. There is a marked trend of kinematics with metallicity, in the sense that the more metal-poor population has higher velocity dispersion and lower rotation velocity than the metal-rich population. The K giants more metal--poor than [Fe/H] = -1 have halo-like kinematics, with no significant rotation and sigma = 120 km/s independent of Galactocentric distance. The velocity dispersion of the giants with [Fe/H] > -1 decreases with increasing Galactocentric distance, and this population is rotating with V = 0.9 km/s/degree. The present observations, together with the observed metallicity gradient imply bulge formation through dissipational collapse. In such a picture, low angular momentum gas lost from the formation of the halo was deposited in the bulge. The bulge would then be younger than the halo. Observations of the Galactic globular cluster system are consistent with this picture, if we associate the metal-rich globular clusters within 3 kpc of the Galactic center with the bulge rather than with the thick disk or halo. Data on the RR Lyr in Baade's window are also consistent with this picture if these stars are considered part of the inner halo rather than the bulge.