  Context: While CDM models and MOND give explanations for flat rotation curves
of other galaxies, both present observational problems and the local gradient
of the Milky Way's rotation curve is not flat. Aims: We consider whether flat
rotation curves could be an artifact of an unmodelled component in spectral
shift. Methods: In the absence of astrometric determinations of radial
velocity, we apply a statistical test on a population of 20 440 Hipparcos stars
inside 300 pc with known radial velocities and with accurate parallaxes in the
New Hipparcos Reduction. Results: The test rejects the null hypothesis, there
is no systematic error in spectrographic determinations of heliocentric radial
velocity, with 99.95% confidence. In a separate test on metal-poor stars, we
find tension between calculations of the orbital velocity of the Sun from three
populations of halo stars inside and outside of a cone of 60{\deg} semi-angle
from the direction of rotation. Tension cannot be removed with only systematic
distance adjustments. Conclusions: We conclude that the most probable
explanation is an unmodelled element in spectrographic determinations of
heliocentric radial velocity with a probable cosmological origin, and propose
that this unmodelled component, rather than CDM or MOND, is responsible for the
apparent flatness of galaxy rotation curves.
