Kinematics at the Edge of the Galactic Bulge: Evidence for Cylindrical Rotation

TL;DR

This study uses radial velocity data from the BRAVA survey to demonstrate that the Galactic bulge exhibits cylindrical rotation, supporting models of secular evolution over merger-driven formation.

Contribution

It provides the first detailed kinematic evidence that the Galactic bulge rotates cylindrically, aligning with bar models and challenging classical bulge formation theories.

Findings

Galactic bulge shows cylindrical rotation at different latitudes.

Line of sight velocity distribution at b=-8 deg. is Gaussian.

Kinematics are consistent with an edge-on N-body bar model.

Abstract

We present new results from BRAVA, a large scale radial velocity survey of the Galactic bulge, using M giant stars selected from the Two Micron All Sky Survey catalogue as targets for the Cerro Tololo Inter-American Observatory 4m Hydra multi-object spectrograph. The purpose of this survey is to construct a new generation of self-consistent bar models that conform to these observations. We report the dynamics for fields at the edge of the Galactic bulge at latitudes b=-8 deg. and compare to the dynamics at b=-4 deg. We find that the rotation curve V(r) is the same at b=-8 deg. as at b=-4 deg. That is, the Galactic boxy bulge rotates cylindrically, as do boxy bulges of other galaxies. The summed line of sight velocity distribution at b=-8 deg. is Gaussian, and the binned longitude-velocity plot shows no evidence for either a (disk) population with cold dynamics or for a (classical bulge) population with hot dynamics. The observed kinematics are well modeled by an edge-on N-body bar, in agreement with published structural evidence. Our kinematic observations indicate that the Galactic bulge is a prototypical product of secular evolution in galaxy disks, in contrast with stellar population results that are most easily understood if major mergers were the dominant formation process.