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

TL;DR

This study measures M31's velocity relative to the Milky Way, confirming a nearly head-on orbit, and estimates the Local Group's total mass to be about 3.17 trillion solar masses, with implications for galaxy interactions.

Contribution

It provides the first precise measurement of M31's velocity vector and refines the Local Group mass estimate using multiple independent methods.

Findings

M31's radial velocity is -109.3 km/s with respect to the Milky Way.

M31's tangential velocity is less than 34.3 km/s at 1-sigma confidence.

The total Local Group mass is estimated at approximately 3.17 x 10^12 solar masses.

Abstract

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. We correct for the reflex motion of the Sun using the most recent insights into the solar motion within the Milky Way. This implies a radial velocity of M31 with respect to the Milky Way of V_rad = -109.3+/-4.4 km/s, and a tangential velocity V_tan = 17.0 km/s (<34.3 km/s at 1-sigma confidence). Hence, the velocity vector of M31 is statistically consistent with a radial (head-on collision) orbit towards the Milky Way. We revise prior estimates for the Local Group timing mass, including corrections for cosmic bias and scatter. Bayesian combination with other mass estimates yields M_LG = M_MW(vir) + M_M31(vir) = (3.17 +/- 0.57) x 10^12 solar masses. The velocity and mass results imply at 95% confidence that M33 is bound to M31, consistent with expectation from observed tidal deformations. (Abridged)