The Bulge Radial Velocity Assay (brava): Techniques and a Rotation Curve

We are undertaking a large scale radial velocity survey of the Galactic bulge which uses M giant stars selected from the 2MASS catalog as targets for the CTIO 4m Hydra multi-object spectrograph. The aim of this survey is to test dynamical models of the bulge and to quantify the importance, if any, of cold stellar streams in the bulge and its vicinity. Here we report on the kinematics of a strip of fields at −10◦ < l < +10◦ and b = −4◦. We construct a longitude-velocity plot for the bulge stars and the model data, and find that contrary to previous studies, the bulge does not rotate as a solid body; from −5◦ < l < +5◦ the rotation curve has a slope of roughly 100 km s kpc and flattens considerably at greater l and reaches a maximum rotation of 45 km s. This rotation is slower than that predicted by the dynamical model of Zhao (1996) and slower than found for planetary nebulae (Beaulieau et al. 2000). Our velocity dispersion profile is in good agreement with the Zhao model, but the slower rotation field suggests that the current model requires the inclusion of retrograde orbits. The high precision of our radial velocities (∼ 3 km s) yields an unexpected result: hints of cold kinematic features are seen in a number of the line of sight velocity distributions. Subject headings: Galaxy: bulge – Galaxy: kinematics and dynamics – Stars: late-type – stars:kinematics – techniques: radial velocities