The Imprint of Spiral Arms on the Galactic Rotation Curve

Abstract

We discuss a model for the Milky Way obtained by fitting the observed terminal velocities with the radial acceleration relation. The resulting stellar surface density profile departs from a smooth exponential disk, having bumps and wiggles that correspond to massive spiral arms. These features are used to estimate the term for the logarithmic density gradient in the Jeans equation, which turn out to have exactly the right location and amplitude to reconcile the apparent discrepancy between the stellar rotation curve and that of the interstellar gas. This model also predicts a gradually declining rotation curve outside the solar circle with slope $-1.7\;\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{kpc}^{-1}$, as subsequently observed.