The kinematical center and mass profile of the Local Group

TL;DR

This paper uses kinematic data and infall assumptions to locate the true center of the Local Group, revealing it is near M31 and challenging dark matter halo models based on velocity patterns.

Contribution

It introduces a method to locate the Local Group center using kinematics alone, without assuming light traces mass, and provides new insights into its mass distribution.

Findings

Center lies near M31 within a few degrees.

Distance to the center is about 0.5 Mpc from the Milky Way.

Radial velocities exclude extended dark matter halos.

Abstract

Abandoning the assumption that light traces mass, I seek the location of the center of the Local Group of galaxies based solely on kinematic data and the plausible assumption of infall. The available set of positions and radial velocities is shown to be a misleading indicator of Local Group motions, giving a direction to the center offset from the true one; statistical techniques of moderate sophistication do not catch the offset. Corrected calculations show the center to lie in the direction to M31 within the uncertainty of the method, a few degrees. The distance to the center is not well determined, lying about 0.5 Mpc from the Milky Way. The pattern of observed (galactocentric) radial velocities excludes both dynamically important `orphan haloes' and any extended dark matter halo for the Group as a whole, and shows the Group to have formed from a much more extended volume than it presently occupies. Kinematics alone indicates that the mass of the Group is concentrated effectively in M31 and the Milky Way.