The Milky Way's plane of satellites is consistent with ΛCDM

The Milky Way is surrounded by 11 'classical' satellite galaxies in a remarkable configuration: a thin plane that is possibly rotationally supported. Such a structure is thought to be highly unlikely to arise in the standard (Lambda CDM) cosmological model (Lambda cold dark matter model, where Lambda is the cosmological constant). While other apparent discrepancies between predictions and observations of Milky Way satellite galaxies may be explained either through baryonic effects or by invoking alternative forms of dark matter particles, there is no known mechanism for making rotating satellite planes within the dispersion-supported dark matter haloes predicted to surround galaxies such as the Milky Way. This is the so-called 'plane of satellites problem', which challenges not only the Lambda CDM model but the entire concept of dark matter. Here we show that the reportedly exceptional anisotropy of the Milky Way satellites is explained, in large part, by their lopsided radial distribution combined with the temporary 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 with new simulations, where such short-lived planes are common, we find the Milky Way satellites to be compatible with standard model expectations.

Automated summary

The Milky Way's unique configuration of 11 satellite galaxies in a thin plane challenges the standard cosmological model and raises questions about the concept of dark matter. However, a new study suggests that the apparent rotational support and anisotropy of the satellites can be explained by their lopsided radial distribution and the temporary conjunction of the two most distant satellites. This transient plane of satellites aligns with simulations, supporting the standard model expectations.