The stellar velocity distribution function in the Milky Way galaxy
  The stellar velocity distribution function (DF) in the solar vicinity is
re-examined using data from the SDSS APOGEE survey's DR16 and \emph{Gaia} DR2.
By exploiting APOGEE's ability to chemically discriminate with great
reliability the thin disk, thick disk and (accreted) halo populations, we can,
for the first time, derive the three-dimensional velocity DFs for these
chemically-separated populations. We employ this smaller, but more data-rich
APOGEE+{\it Gaia} sample to build a \emph{data-driven model} of the local
stellar population velocity DFs, and use these as basis vectors for assessing
the relative density proportions of these populations over 5 $<$ $R$ $<$ 12
kpc, and $-1.5$ $<$ $z$ $<$ 2.5 kpc range as derived from the larger, more
complete (i.e., all-sky, magnitude-limited) {\it Gaia} database. We find that
81.9 $\pm$ 3.1$\%$ of the objects in the selected \emph{Gaia} data-set are
thin-disk stars, 16.6 $\pm$ 3.2$\%$ are thick-disk stars, and 1.5 $\pm$ 0.1$\%$
belong to the Milky Way stellar halo. We also find the local thick-to-thin-disk
density normalization to be $\rho_{T}(R_{\odot})$/$\rho_{t}(R_{\odot})$ = 2.1
$\pm$ 0.2$\%$, a result consistent with, but determined in a completely
different way than, typical starcount/density analyses. Using the same
methodology, the local halo-to-disk density normalization is found to be
$\rho_{H}(R_{\odot})$/($\rho_{T}(R_{\odot})$ + $\rho_{t}(R_{\odot})$) = 1.2
$\pm$ 0.6$\%$, a value that may be inflated due to chemical overlap of halo and
metal-weak thick disk stars.
