The imprint of arms and bars on rotation curves: in-plane and off-plane

TL;DR

This paper investigates how arms and bars influence galaxy rotation curves, revealing that spiral patterns induce non-circular motions and that off-plane rotation varies with height, linking observed features to orbital resonances and stellar dynamics.

Contribution

It provides a detailed analysis of the impact of spiral arms and bars on rotation curves, connecting kinematic features to orbital resonances and stellar eccentricities in galaxy models.

Findings

Non-circular motions are more prominent in late-type galaxies with larger pitch angles.

Rotation speed decreases with height above the galactic plane, especially in Sa galaxy models.

Wiggles in rotation curves are linked to diagonal ridges in the $V_{\phi}-R$ plane and orbital eccentricities.

Abstract

Within Rotation Curves (RC) is encoded the kinematical state of the stellar disc as well as information about the dynamical mechanisms driving the secular evolution of galaxies. To explain the characteristic features of RCs that arise by the influence of spiral patterns and bar, we study the kinematics of the stellar disc in a set of spiral galaxy models specifically tailored for this purpose. We find that, for our models, the induced non-circular motions are more prominent for spirals with larger pitch angle, the ones typical in late type galaxies. Moreover, inside corotation, stars rotate slower along the spiral arms than along the inter-arm, which translates into a local minima or maxima in the RC, respectively. We also see, from off-plane RC, that the rotation is faster for stars that at observed closer to the plane, and diminishes as one looks farther off plane; this trend is more noticeable in our Sa galaxy model than our Sc galaxy model. Additionally, in a previous work we found that the diagonal ridges in the $V_{\phi}-R$ plane, revealed through the Gaia DR2, have a resonant origin due to the spiral arms and bar and that these ridges project themselves as wiggles in the RC; here, we further notice that the development of these ridges, and the development of high orbital eccentricities in the stellar disc are the same. Hence, we conclude that, the following explanations of bumps and wiggles in RCs are equivalent: they are manifestations of diagonal ridges in the $V_{\phi}-R$ plane, or of the rearrangement of the orbital eccentricities in the stellar disc.