# A Plane of High Velocity Galaxies Across the Local Group

## TL;DR

This paper identifies a thin, high-velocity galaxy plane in the Local Group and proposes a MOND-based dynamical scenario involving a past MW-M31 flyby to explain the observed galaxy velocities and arrangements.

## Contribution

It presents the discovery of a high-velocity galaxy plane and develops a MOND-based model linking past galaxy interactions to current galaxy distributions.

## Key findings

- A thin plane of high-velocity galaxies with 101 kpc thickness.
- A MOND-based flyby scenario can produce high-velocity tidal dwarf galaxies.
- The probability of the observed galaxy arrangement occurring by chance is 0.0015.

## Abstract

We recently showed that several Local Group (LG) galaxies have much higher radial velocities (RVs) than predicted by a 3D dynamical model of the standard cosmological paradigm. Here, we show that 6 of these 7 galaxies define a thin plane with root mean square thickness of only 101 kpc despite a widest extent of nearly 3 Mpc, much larger than the conventional virial radius of the Milky Way (MW) or M31. This plane passes within ${\sim 70}$ kpc of the MW-M31 barycentre and is oriented so the MW-M31 line is inclined by $16^\circ$ to it. We develop a toy model to constrain the scenario whereby a past MW-M31 flyby in Modified Newtonian Dynamics (MOND) forms tidal dwarf galaxies that settle into the recently discovered planes of satellites around the MW and M31. The scenario is viable only for a particular MW-M31 orbital plane. This roughly coincides with the plane of LG dwarfs with anomalously high RVs. Using a restricted $N$-body simulation of the LG in MOND, we show how the once fast-moving MW and M31 gravitationally slingshot test particles outwards at high speeds. The most distant such particles preferentially lie within the MW-M31 orbital plane, probably because the particles ending up with the highest RVs are those flung out almost parallel to the motion of the perturber. This suggests a dynamical reason for our finding of a similar trend in the real LG, something not easily explained as a chance alignment of galaxies with an isotropic or mildly flattened distribution (probability $= {0.0015}$).