The Photo-Astrometric Vertical Tracer Density of the Milky Way II:
  Results from Gaia
  We use Gaia photometry and astrometry to estimate the vertical spatial
structure of the Milky Way at the Solar radius, formally accounting for sample
incompleteness (the selection function) and parallax measurement uncertainty.
Our results show impressive precision demonstrating the power of the Gaia data.
However, systematic errors dominate the parameter value uncertainties. We
thoroughly test and quantify the impacts of all systematic uncertainties. The
vertical tracer density is modelled as a sum of two exponential profiles for
the thin and thick discs, together with a spherically symmetric power-law for
the stellar halo. We constrain the thin disc scale height as
${h_\mathrm{Tn}=260 \pm 3\, (\mathrm{stat}) \pm 26\,\mathrm{pc}\,
(\mathrm{sys})}$ and thick disc ${h_\mathrm{Tk}=693 \pm 7 \,(\mathrm{stat}) \pm
121\,\mathrm{pc}\, (\mathrm{sys})}$. For the halo, we obtain a power law
profile with $n_\mathrm{H}=3.543\pm0.023 \,(\mathrm{stat}) \pm0.259\,
(\mathrm{sys})$. We infer a local stellar mass density for non-compact object
stars of ${\rho_\mathrm{local}^* = 3.66\pm0.03\,(\mathrm{stat})\pm0.52
\times10^{-2}\,\mathrm{M}_\odot/\mathrm{pc}^3\,(\mathrm{sys})}$ and surface
density of ${\Sigma_\mathrm{local}^* =
23.17\pm0.08\,(\mathrm{stat})\pm2.43\,\mathrm{M}_\odot/\mathrm{pc}^2\,(\mathrm{sys})}$.
We find asymmetries above and below the disc with longer disc scale heights in
the north but a flatter halo in the south at the $\lesssim 10$ per cent level.
