Composition of Stars in the Galactic Bulge and Inner Disk

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The origin and chemical evolution of the Galactic bulge/bar remains an unsolved problem. The Bulge Radial Velocity Assay (BRAVA) has found that the radial velocities of stars across the bulge are consistent with an N-body bar model. Yet over the same volume, a gradient in the chemical composition of stars is observed. If the bar was created via a purely dynamical process, no abundance gradient should be present. Maybe the bulge is a pseudo-bulge, and has an extended formation history, but evidence from both the turnoff age and abundance ratios of stars in the bulge are consistent with an old bulge that formed early and rapidly. Several studies now suggest that the bulge consists of multiple stellar subpopulations of differing age, metallicity, kinematics, and spatial extent. Studies continue to disagree on whether the abundances and kinematics of stars in the bulge are correlated. There is also the possibility that the disintegration of massive proto-clusters built the Galactic bulge. Within the last few years, high resolution spectroscopy of highly magnified, lensed bulge subdwarfs revealed what appears to be a stellar population that is metal rich, and 2-7 Gyr old- considerably younger than is inferred for the bulge field population from the analysis of color-magnitude diagrams. Two recent spectroscopic studies also suggest that the bulge is similar to and perhaps part of, the thick disk. Although there is no kinematic evidence for a 'classical' bulge at galactic latitudes below 8 degrees, infrared color-magnitude diagrams and some spectroscopic studies find evidence for a bulge-like population at high galactic latitude. Dr. Rich and his collaborators conduct a survey of the heavy elements across the inner bulge and bulge globular clusters to securely relate the bulge to the thick disk or the halo. Comparison of the composition of field and globular cluster stars helps to address whether the bulge formed from disintegrated globular clusters. Another part of this work is to undertake high dispersion spectroscopy of red giants at galactic latitudes above 20 degrees to assess whether the formation history of this population differs from that of the inner bulge. The research results are disseminated via refereed papers, colloquia, scientific meetings, and public lectures.