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

Indications of an Unmodelled Component in Spectrographic Measurements of Local Stars