The Bulge Radial Velocity/Abundance Assay

TL;DR

The BRAVA survey of 10,000 red giants reveals the Galactic bulge's cylindrical rotation, dominance of bar structures, and an abundance gradient, suggesting rapid formation and complex baryonic physics beyond pure dynamics.

Contribution

This study provides the first detailed kinematic and chemical abundance measurements of the outer bulge, confirming the dominance of bar structures and identifying an abundance gradient.

Findings

Bulge exhibits cylindrical rotation consistent with a bar structure.

Over 90% of bulge stars are part of the bar, minimal classical bulge component.

An iron abundance gradient exists in the outer bulge, indicating complex formation history.

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.