The Bulge Radial Velocity/Abundance Assay

Abstract

The Bulge Radial Velocity/Abundance Assay (BRAVA) has accomplished a survey of 10,000 red giants in the Southern Galactic bulge, approximately spanning -8 deg. < l < +8 deg. and -3 deg. <b < -8 deg., a region within roughly 1 kpc from the nucleus. We find that the Galactic bulge at b=-4 deg. displays a clear departure from solid body rotation, and that the rotation field along the major axis at b=-6 deg. and b=-8 deg is identical to that at lower latitude; this is "cylindrical" rotation, a hallmark observed in edge-on bars. Comparison of the BRAVA dataset with an N-body bar shows that >90% of the bulge population is in the bar, leaving little room for a "classical" bulge component. We also report on the first iron abundance and composition measurements in the outer bulge, at b=-8 deg. The iron abundance in this field falls on the trend of a suspected gradient measured from high resolution spectroscopy of bulge clump stars. Further, we find that the trends of [\alpha/Fe] vs [Fe/H] that characterize the bulge at lower latitude are present 1 kpc from the nucleus, consistent with a rapid (<1 Gyr) timescale for the formation of the bulge, even near its boundary. Although the dynamics of the bulge are consistent with those of a dynamically buckled N-body bar, the presence of an abundance gradient is not compatible with purely dynamical processes; we propose that missing baryonic physics is needed. We also report on the remarkable massive bulge globular cluster Terzan 5, which has a bimodal abundance and composition distribution, and is proposed as the remnant of a population of primordial building block stellar systems that formed the bulge. Terzan 5 is presently a unique case, and it is important to test whether the dissolution of systems similar to it populated the bulge.