The Andromeda Galaxy appears to us at an angle to the galactic disk, i.e. we are not in the Andromeda Galaxy disc's plane, nor are we near the direction that the galaxy's pole points. Therefore, due to the geometry and distances involved, it would seem to me that we are seeing the 'far edge' a few thousand years later than we are seeing the 'near edge'. How far can a galaxy spin during that those few thousand years, and therefore how distorted are we seeing Andromeda? Can we infer how Andromeda or other large spiral galaxies seen at an angle would look if light were to travel instantly?
Might this phenomenon account for some of the unusual spin properties measured in galaxies, which is attributed to dark matter?
Answer
Andromeda is around 70,000 light years across (depending on where you make the edge) so yes the positions of individual stars are shifted. But since it typically takes 250 million years for a galaxy to rotate they are only shifted by 70/250,000 of a circle = 0.1deg.
The rotation curve of a galaxy, which tells us the actual mass, depends on the velocity of stars relative to the center of the galaxy so their rotation position doesn't matter.
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