New H I observations of Messier 31 (M31) obtained with the Effelsberg and Green Bank 100 m telescopes make it possible to measure the rotation curve of that galaxy out to ~35 kpc. Between 20 and 35 kpc, the rotation curve is nearly flat at a velocity of ~226 km s-1. A model of the mass distribution shows that at the last observed velocity point, the minimum dark-to-luminous mass ratio is ~0.5 for a total mass of 3.4 × 1011 M☉ at R < 35 kpc. This can be compared to the estimated Milky Way mass of 4.9 × 1011 M☉ for R < 50 kpc.
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Carignan et al. (2006) studied this question.
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