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Published online by Cambridge University Press: 12 April 2016
VLBI observations of H2O maser line emission from the central sub-parsec region of the galaxy NGC 4258 have recently revealed a remarkably well-defined, rapidly rotating molecular disk which is viewed nearly edge–on. We show that the peculiar clustering of the maser sources into several distinct clumps which are almost equally spaced apart from one another, and the striking asymmetry in the high-velocity maser spectrum can be naturally explained by spiral activity in the disk.