Weighing the Galactic disk using phase-space spirals II. Most stringent
  constraints to a thin dark disk using Gaia EDR3
  Using the method that was developed in the first paper of this series, we
measure the vertical gravitational potential of the Galactic disk from the
time-varying structure of the phase-space spiral, using data from Gaia as well
as supplementary radial velocity information from legacy spectroscopic surveys.
For eleven independent data samples, we inferred gravitational potentials that
were in good agreement, despite the data samples' varied and substantial
selection effects. Using a model for the baryonic matter densities, we inferred
a local halo dark matter density of $0.0085 \pm 0.0039$ M$_\odot$/pc$^3 = 0.32
\pm 0.15$ GeV/cm$^3$. We were also able to place the most stringent constraint
to the surface density of a thin dark disk with a scale height $\leq 50$ pc: an
upper 95 % confidence limit of roughly 5 M$_\odot$/pc$^2$ (compared to previous
limit of roughly 10 M$_\odot$/pc$^2$, given the same scale height). For the
inferred halo dark matter density and thin dark disk surface density, the
uncertainties are dominated by the baryonic model. With this level of
precision, our method is highly competitive with traditional methods that rely
on the assumption of a steady state. In a general sense, this illustrates that
time-varying dynamical structures are not solely obstacles to dynamical mass
measurements, but can also be regarded as assets containing useful information.
