From ridges in the velocity distribution to wiggles in the rotation curve

Abstract

Abstract Recently, the Gaiadata release 2 (DR2) showed us the richness in the kinematics of the Milky Way disc. Of particular interest is the presence of ridges covering the stellar velocity distribution, Vϕ–R; as shown by others, it is likely that these ridges are the signature of phase mixing, transient spirals, or the bar. Here, with a Galactic model containing both, bar and spirals, we found the same pattern of ridges extending from the inner to the outer disc. Interestingly, ridges in the Vϕ–R plane correlate extremely well with wiggles in the computed rotation curve (RC). Hence, although the DR2 reveals (for the first time) such substructures in a wide spatial coverage, we notice that we have always seen such a pattern of ridges, but projected into the form of wiggles in the RC. The separation and amplitude of the wiggles strongly depend on the extension and layout of ridges in the Vϕ–R plane. This means that within the RC are encoded the kinematic state of the disc and information about the bar and spiral arms. The amplitude of the wiggles suggests that similar features currently observable in external galaxies' RCs have similar origins, triggered by spirals and bars.