The Bulge Radial Velocity Assay (BRAVA): I. Sample Selection and a Rotation Curve

Abstract

Results from the Bulge Radial Velocity Assay (BRAVA) are presented. BRAVA uses M giant stars, selected from the 2MASS catalog to lie within a bound of reddening corrected color and luminosity, as targets for the Cerro Tololo Inter-American Observatory 4-m Hydra multi-object spectrograph. Three years of observations investigate the kinematics of the Galactic bulge major and minor axes with ~3300 radial velocities from 32 bulge fields and one disk field. We construct a longitude-velocity plot for the bulge stars and find that, contrary to previous studies, the bulge does not rotate as a solid body; for |l|<4 degree the rotation curve has a slope of roughly 100 km/s/kpc and flattens considerably at greater l, reaching a maximum rotation of 75 km/s. We compare our rotation curve and velocity dispersion profile to both the self-consistent model of Zhao (1996) and to N-body models; neither satisfactorily fit both measured rotation and dispersion. We place the bulge on the plot of (Vmax/sigma) vs. epsilon and find that the bulge lies near the oblate rotator line, and very close to the parameters of NGC 4565, an edge-on spiral galaxy with a bulge similar to that of the Milky Way. We find that our summed velocity distribution of bulge stars appears to be sampled from a Gaussian distribution, with sigma=116+/-2 km/s for our summed bulge fields.