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

Hydro-dynamical modeling of the inner Galaxy suggest that the radius of theouter Lindblad resonance (OLR) of the Galactic bar lies in the vicinity of theSun. How does this resonance affect the distribution function in the outerparts of a barred disk, and can we identify any effect of the resonance in thevelocity distribution f(v) actually observed in the solar neighborhood? Toanswer these questions, detailed simulations of f(v) in the outer parts of anexponential stellar disks with nearly flat rotation curves and a rotatingcentral 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 theouter disk. For positions <2kpc outside the OLR radius and at bar angles of\~10-70 degrees, f(v) inhibits a bi-modality between the low-velocity starsmoving like the local standard of rest (LSR) and a secondary mode of starspredominantly moving outward and rotating more slowly than the LSR. Such a bi-modality is indeed present in f(v) inferred from the Hipparcos datafor late-type stars in the solar neighborhood. If one interpretes this observedbi-modality as induced by the OLR -- and there are hardly any viablealternatives -- then one is forced to deduce that the OLR radius is slightlysmaller than Ro. Moreover, by a quantitative comparison of the observed withthe simulated distributions one finds that the pattern speed of the bar is1.85+/-0.15 times the local circular frequency, where the error is dominated bythe uncertainty in bar angle and local circular speed. Also other, less prominent but still significant, features in the observedf(v) resemble properties of the simulated velocity distributions, in particulara ripple caused by orbits trapped in the outer 1:1 resonance.Comment: 14 pages, 10 figures (Fig.2 in full resolution available upon request), accepted for publication in A