Combined dynamical effects of the bar and spiral arms in a Galaxy model. Application to the solar neighbourhood

Abstract

Observational data indicate that the Milky Way is a barred spiral galaxy. Computation facilities and availability of data from Galactic surveys stimulate the appearance of models of the Galactic structure. More efforts to build dynamical models containing both spiral arms and the central bar/bulge are needed. We expand the study of the stellar dynamics in the Galaxy by adding the bar/bulge component to a model with spiral arms introduced in our previous paper. The model is tested by applying it to the solar neighborhood, where observational data are more precise. We model analytically the potential of the Galaxy to derive the force field in its equatorial plane. The model comprises an axisymmetric disc derived from the observable rotation curve, four arms with Gaussian-shaped groove profiles, and a classical elongated/oblate ellipsoidal bar/bulge structure. The parameters describing the bar/bulge are constrained by observations and the stellar dynamics, and their possible limits are determined. A basic model results in a bar of 2.9 kpc in length, with a mass of the order of a few 10$^9M_\odot$. The size and orientation of the bar are also restricted by the position of masers with VLBI distances. The bar's rotation speed is constrained to $\Omega_{\rm bar}<50$ km s$^{-1}$ kpc$^{-1}$ taking into account the allowed mass range. We conclude that our basic model is compatible with observations and with the dynamical constraints. The model explains simultaneously the bulk of the main moving groups, associated here with the spiral corotation resonance, and the Hercules stream, associated with several inner high-order spiral resonances; in particular, with the 8/1 resonance. From the dynamical constraints on the bar's angular speed, it is unlikely that the bar's OLR lies near the solar circle; moreover, its proximity would compromise the stability of the Local Arm structure.