Feasibility of Using Orbital Resonances for Nearby Stars to Constrain the Triaxiality of the Galactic Halo

Based on exploratory simulations, Rojas-Niño et al. (2012) have proposed that triaxiality in the Galactic dark matter halo might cause lumpiness in the velocity distribution of halo stars in the Solar Neighbourhood through orbital resonances. Accordingly, they have asserted that detecting the presence of substructures in velocity space may be a method for diagnosing the shape of the dark matter halo of the Milky Way. However, this conclusion is based on a somewhat idealized initial configuration. In the current study, a more realistic set-up for the initial conditions was used to simulate the kinematic effects of a triaxial NFW halo (Vogelsberger et al. 2007) on halo stars in a galaxy similar to the Milky Way. We found the local velocity distributions in our simulations to be relatively smooth. Although resonances were indeed present and associated with specific regions of velocity space, they did not leave visible imprints in the form of substructures. It appears that only the shape of the velocity ellipsoid is significantly affected by triaxiality. We conclude that a method based on the identification of resonant orbital families in velocity space will probably not be sensitive enough for testing triaxiality of the dark matter halo of the Milky Way. Subject headings: dark matter halo, Milky Way, orbital resonances, stellar kinematics, triaxiality