The Milky Way's plane of satellites: consistent with $\Lambda$CDM

TL;DR

This paper demonstrates that the Milky Way's satellite galaxy arrangement, previously seen as a challenge to the standard cosmological model, is actually consistent with it when considering transient planes and observational biases.

Contribution

The study shows that the apparent anisotropy and thin plane of satellites are transient features, reconciling observations with $ ext{Λ}$CDM predictions using Gaia data and new simulations.

Findings

The satellite plane is transient, not rotationally supported.

Orbital pole alignment is more common than previously thought.

Milky Way satellites are compatible with standard cosmology.

Abstract

The "plane of satellites problem" describes the arrangement of the Milky Way's 11 brightest satellite galaxies in a remarkably thin plane, possibly supported by rotation. This is in apparent contradiction to the standard cosmological model, wherein the Galaxy is surrounded by a dispersion-supported dark matter halo. Here, we show that the reported exceptional anisotropy of the satellite system is strongly contingent on a lopsided radial distribution, which earlier simulations have failed to reproduce, combined with the close but fleeting conjunction of the two most distant satellites, Leo I and Leo II. Using Gaia proper motions, we show that the orbital pole alignment is much more common than previously reported, and reveal the plane of satellites to be transient rather than rotationally supported. Comparing to new simulations, where such short-lived planes are common, we find the Milky Way satellites to be compatible with standard model expectations.