MOND's Problem in Local Group

TL;DR

This paper tests Modified Newtonian Dynamics (MOND) using Local Group galaxy data and finds that MOND predicts less mass than observed, challenging its validity under simple radial orbit assumptions.

Contribution

The study applies MOND to the Local Group timing problem, revealing inconsistencies with observed galaxy motions and baryonic mass.

Findings

MOND predicts less mass than observed in the Local Group.

Radial orbit assumption conflicts with observed galaxy motions.

Non-radial orbits are required for MOND to match observations.

Abstract

I use the distances and motions of Local Group galaxies to test Modified Newtonian Dynamics (MOND). The old Local Group timing argument of Kahn & Woltjer, which assumed Newtonian gravity and a simple radial orbit, indicated that the total mass of M31 and the Milky Way far exceeds their known baryonic mass. Here I apply MOND to study the same problem. With the same radial orbit assumption, I find that the total mass of M31 and the Milky Way predicted by MOND is less than the known baryonic masses of these two galaxies. I find a similar result holds for all the dwarf galaxies in the Local Group, if they are assumed to move radially with respect to the center of mass of M31 and the Milky Way. If the known baryonic mass of M31 and Milky Way is used, MOND requires all galaxies in the Local Group to have non-radial motions with respect to the center of mass of M31and the Milky Way in order to be consistent with their observed motion and distance. The non-radial orbit requirement is inconsistent with the requirement of high radial anisotropy in order to reconcile MOND with the velocities of satellite galaxies around host galaxies from the Sloan Digital Sky Survey.