Deviations from circular velocity and relation to column density at the solar circle. The top panel illustrates the Galactic azimuthal profile of the difference between azimuthal velocity, vϕ, and circular velocity, v_c(R), at R₀ = 8.179 kpc. The bottom panel shows the azimuthal surface density profile. The vertical dashed lines are the potential minima of the underlying spiral arm potential. Rotation of the system is from left to right.