How distorted does the Andromeda Galaxy appear to us due to the speed of light?

Question

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 1

This is a multi-leveled question but I will address the central part: "How far can a galaxy spin during that those few thousand years, and therefore how distorted are we seeing Andromeda?"

Not very far. The milky way is smaller than Andromeda, but it still takes it about 250 MILLION years to rotate 1x. So if the speed is comparable to Andromeda, and if you account for the size of that galaxy (approx 250,000 light years in diameter), that would mean that the light at the rear of the galaxy is 250,000 years older than the light at the front. With that said, due to it taking an estimated 250 million years to spin, in the grand scheme of things, you are missing 1% accuracy. So if light were to travel instantly, the picture would be only 1% more accurate. (Which on the scale of a galaxy that large with approximately 1 Trillion stars, means about 10 billion stars are out of place, but still its only 1% of the total count).