I was wondering, how can we measure the rotational velocity of a galaxy? In practice knowing the mass distributions and so on, we could calculate it by classical mechanics (or maybe GR). However people measured the rotational velocity of the galaxies and found that it doesn't correspond to the expected curve (DM or MOND). How did they do it? In a telescope I have the feeling that a galaxy appears as a single dot -1 dimensional object (impossible to see whether it's rotating or not).