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

TL;DR

The BRAVA survey used M giant stars to measure the Galactic bulge's kinematics, revealing a non-solid-body rotation curve and providing new insights into the bulge's velocity profile and comparison with galaxy models.

Contribution

This study presents the first detailed rotation curve of the Galactic bulge based on extensive radial velocity measurements, challenging previous assumptions of solid-body rotation.

Findings

Bulge does not rotate as a solid body.

Rotation curve flattens at larger longitudes, reaching 75 km/s.

Velocity distribution is Gaussian with sigma=116 km/s.

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.