The Pattern Speed of the Galactic Bar

Most late-type stars in the solar neighborhood have velocities similar to the local standard of rest (LSR), but there is a clearly separated secondary component corresponding to a slower rotation and a mean outward motion. Detailed simulations of the response of a stellar disk to a central bar show that such a bimodality is expected from outer Lindblad resonant scattering. When constraining the run of the rotation curve by the proper motion of Sagittarius A* and the terminal gas velocities, the value observed for the rotation velocity separating the two components results in a value of 53 ± 3 km s -1 kpc -1 for the pattern speed of the bar, only weakly dependent on the precise values for R 0 and bar angle ϕ.