The only dark matter is between their ears...
Interesting article, but I hope Beers was misquoted on his assertion that M31 is “twice as massive” as the Milky Way.
My understanding is that while the visible disk of M31 is much larger than that of the Milky Way, the two galaxies are comparable in mass, with the Milky Way possibly proving to be the heavyweight - e.g., M31 is 7.1×10^11 compared to the Milky Way’s 1-1.5×10^12 stellar masses. This study further expands the possible size of the Milky Way to 1.6-2.4×10^12, which positively dwarfs Andromeda, unless the implication is that the methodology Beers used to calculate our galaxy’s mass has been applied to M31 as well and found to similarly expand Andromeda’s waistline.
I am somewhat suspicious of these findings because I believe we have a fairly accurate diameter of our Galaxy, and a good estimate of the orbital period of halo stars. This alone will give a good mathematical solution to galactic mass (both our own and Andromeda’s.)
Oh so true. Don't get me started on dark matter.