Question: Do you know if there is current agreement about which edge of the Andromeda galaxy (M31) is closest to the Milky Way? I don’t believe present distance measurements are precise enough to distinguish the near and far sides with enough certainty (also not the intent of these measurements anyway). Radial velocity measurements are very good, but we also need the spiral structure information – are the arms trailing or leading. I know M31’s high inclination angle makes this hard to assess, but more recent observations and models appear to be converging on trailing arms for M31, thus the northwest edge is closer. Is this consensus generally true today? — Jon
Answer: In fact, we can measure very accurately which parts of M31 are moving towards or away from us. It turns out that the NE side of the galaxy is moving towards us (i.e. the emission from the gas in this part of the galaxy is blueshifted relative to the average recession velocity of the galaxy). But, this does not necessarily tell us which part of the galaxy is closest to us. After reviewing this question with some of my colleagues, I found out that this is not such an easy question to answer. Our best estimate is that the lower-left (SE) side of M31 is the part that is closest to us. We believe that this side is the one closest to us due to the fact that in the image of the galaxy shown below (which is an image that is sensitive to ultraviolet light from M31) the SE side of the galaxy appears to have brighter stars than the NW side. This is likely due to more dust obscuring the NW side due to its position on the far-side of M31 in relation to us.