The Strength of the Dynamical Spiral Perturbation in the Galactic Disk
  The mean Galactocentric radial velocities $\langle v_{R}\rangle(R,\varphi)$
of luminous red giant stars within the mid-plane of the Milky Way reveal a
spiral signature, which could plausibly reflect the response to a
non-axisymmetric perturbation of the gravitational potential in the Galactic
disk. We apply a simple steady-state toy model of a logarithmic spiral to
interpret these observations, and find a good qualitative and quantitative
match. Presuming that the amplitude of the gravitational potential perturbation
is proportionate to that in the disk's surface mass density, we estimate the
surface mass density amplitude to be $\Sigma_{\rm max} (R_{\odot})\approx
5.5\,\rm M_{\odot}\,pc^{-2}$ at the solar radius when choosing a fixed pattern
speed of $\Omega_{\mathrm p}=12\,\rm km\,s^{-1}\,kpc^{-1}$. Combined with the
local disk density, this implies a surface mass density contrast between the
arm and inter-arm regions of approximately $\pm 10\%$ at the solar radius, with
an increases towards larger radii. Our model constrains the pitch angle of the
dynamical spiral arms to be approximately $12^{\circ}$.
