The M31 Velocity Vector. II. Radial Orbit Towards the Milky Way and Implied Local Group Mass

Understand

We determine the velocity vector of M31 with respect to the Milky Way and use this to constrain the mass of the Local Group, based on HST proper-motion measurements presented in Paper I.

• We construct N-body models for M31 to correct the measurements for the contributions from stellar motions internal to M31.
• We also estimate the center-of-mass motion independently, using the kinematics of satellite galaxies of M31 and the Local Group.
• All estimates are mutually consistent, and imply a weighted average M31 heliocentric transverse velocity of (v_W,v_N) = (-125.2+/-30.8, -73.8+/-28.4) km/s.