The last few months have seen the measurements of the radial velocities of all of the dwarf spheroidal companions to the Andromeda galaxy ( M31 ) using the spectrographs ( HIRES and LRIS ) on the Keck Telescope .
This paper summarises the data on the radial velocities and distances for all the companion galaxies and presents new dynamical modelling to estimate the mass of extended halo of M31 .
The best fit values for the total mass of M31 are \sim 7 - 10 \times 10 ^ { 11 } M _ { \odot } , depending on the details of the modelling .
The mass estimate is accompanied by considerable uncertainty caused by the smallness of the dataset ; for example , the upper bound on the total mass is \sim 24 \times 10 ^ { 11 } M _ { \odot } , while the lower bound is \sim 3 \times 10 ^ { 11 } M _ { \odot } .
These values are less than the most recent estimates of the most likely mass of the Milky Way halo .
Bearing in mind all the uncertainties , a fair conclusion is that the M31 halo is roughly as massive as that of the Milky Way halo .
There is no dynamical evidence for the widely held belief that M31 is more massive – it may even be less massive .