  We present a detailed analysis of the effect of an observationally determined
dark matter (DM) velocity distribution function (VDF) of the Milky Way (MW) on
DM direct detection rates. We go beyond local kinematic tracers and use
rotation curve data up to 200 kpc to construct a MW mass model and
self-consistently determine the local phase-space distribution of DM. This
approach mitigates any incomplete understanding of local dark matter-visible
matter degeneracies that can affect the determination of the VDF. Comparing
with the oft used Standard Halo Model (SHM), which assumes an isothermal VDF,
we look at how the tail of the empirically determined VDF alters our
interpretation of the present direct detection WIMP DM cross section exclusion
limits. While previous studies have suggested a very large difference (of more
than an order of magnitude) in the bounds at low DM masses, we show that
accounting for the detector response at low threshold energies, the difference
is still significant although less extreme. The change in the number of signal
events, when using the empirically determined DM VDF in contrast to the SHM
VDF, is most prominent for low DM masses for which the shape of the recoil
energy spectrum depends sensitively on the detector threshold energy as well as
detector response near the threshold. We demonstrate that these trends carry
over to the respective DM exclusion limits, modulo detailed understanding of
the experimental backgrounds. With the unprecedented precision of astrometric
data in the GAIA era, use of observationally determined DM phase-space will
become a critical and necessary ingredient for DM searches. We provide an
accurate fit to the current best observationally determined DM VDF (and
self-consistent local DM density) for use in analyzing current DM direct
detection data by the experimental community.
