The 4:1 outer Lindblad resonance of a long-slow bar as an explanation for the Hercules stream

Abstract

There are multiple groups of comoving stars in the Solar neighbourhood, which can potentially be explained as the signatures of one of the fundamental resonances of non-axisymmetric structure such as the Galactic bar or spiral arms. One such stream, Hercules, has been proposed to result from the outer Lindblad resonance (OLR) of a short fast rotating bar as shown analytically, or the corotation resonance (CR) of a longer slower rotating bar as observed in an N-body model. We show that by including an m = 4 Fourier component in an analytical long bar model, with an amplitude that is typical for bars in N-body simulations, we can reproduce a Hercules like feature in the kinematics of the Solar neighbourhood. We then describe the expected symmetry in the velocity distribution arising from such a model, which we will soon be able to test with Gaia.