Balancing mass and momentum in the local group

Abstract

In the rest frame of the Local Group (LG), the total momentum of the Milky Way (MW) and Andromeda (M31) should balance to zero. We use this fact to constrain new solutions for the solar motion with respect to the LG centre of mass, the total mass of the LG, and the individual masses of M31 and the MW. Using the set of remote LG galaxies at >350 kpc from the MW and M31, we find that the solar motion has amplitude V⊙ = 299 ± 15 km s-1 in a direction pointing towards galactic longitude l⊙ = 98{ring operator}. 4 ± 3{ring operator}. 6 and galactic latitude b⊙ = -5{ring operator}. 9 ± 3{ring operator} 0. The velocities of M31 and the MW in this rest frame give a direct measurement of their mass ratio, for which we find log10(MM31/MMW) = 0.36 ± 0.29. We combine these measurements with the virial theorem to estimate the total mass within the LG as MLG = (2.5 ± 0.4) × 1012M⊙. Our value for MLG is consistent with the sum of literature values for MMW and MM31. This suggests that the mass of the LG is almost entirely located within the two largest galaxies rather than being dispersed on larger scales or in a background medium. The outskirts of the LG are seemingly rather empty. Combining our measurement for MLG and the mass ratio, we estimate the individual masses of the MW and M31 to be MMW =(0.8±0.5)×1012M ⊙ and MM31 =(1.7±0.3)×1012M⊙, respectively. Our analysis favoursM31 being more massive than theMWby a factor of 2.3, and the uncertainties allow only a small probability (9.8 per cent) that the MW is more massive. This is consistent with other properties such as the maximum rotational velocities, total stellar content, and numbers of globular clusters and dwarf satellites, which all suggest that MM31/MMW > 1. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.