The Effect of the Outer Lindblad Resonance of the Galactic Bar on the Local Stellar Velocity Distribution

Abstract

Hydrodynamic modeling of the inner Galaxy suggests that the radius of the outer Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of the Sun. How does this resonance affect the distribution function in the outer parts of a barred disk, and can we identify any effect of the resonance in the velocity distribution actually observed in the solar neighborhood? To answer these questions, detailed simulations of the velocity distribution, f ( v ), in the outer parts of an exponential stellar disk with nearly flat rotation curve and a rotating central bar have been performed. For a model resembling the old stellar disk, the OLR causes a distinct feature in f ( v ) over a significant fraction of the outer disk. For positions up to 2 kpc outside the OLR radius and at bar angles of ∼10°–70°, this feature takes the form of a bimodality between the dominant mode of low-velocity stars centered on the local standard of rest (LSR) and a secondary mode of stars predominantly moving outward and rotating more slowly than the LSR. Such a bimodality is indeed present in f ( v ) inferred from the Hipparcos data for late-type stars in the solar neighborhood. If one interprets this observed bimodality as induced by the OLR—and there are hardly any viable alternatives—then one is forced to deduce that the OLR radius is slightly smaller than R 0 . Moreover, by a quantitative comparison of the observed with the simulated distributions, one finds that the pattern speed of the bar is 1.85 ± 0.15 times the local circular frequency, where the error is dominated by the uncertainty in bar angle and local circular speed. Also, other, less prominent but still significant, features in the observed f ( v ) resemble properties of the simulated velocity distributions, in particular a ripple caused by orbits trapped in the outer 1:1 resonance.