The Planes of Satellite Galaxies Problem, Suggested Solutions, and Open Questions

TL;DR

Satellite galaxies around the Milky Way and Andromeda are aligned in flattened planes with correlated orbits, a phenomenon rare in simulations based on the standard cosmological model, posing a significant small-scale challenge.

Contribution

This paper reviews observational evidence of satellite galaxy planes and compares them with cosmological simulations, highlighting the discrepancy and discussing potential explanations.

Findings

Satellite planes are common in observations but rare in simulations.

Simulated analogs occur in less than 0.5% of cases.

Baryonic effects have limited impact on the satellite plane problem.

Abstract

Satellite galaxies of the Milky Way and of the Andromeda galaxy have been found to preferentially align in significantly flattened planes of satellite galaxies, and available velocity measurements are indicative of a preference of satellites in those structures to co-orbit. There is increasing evidence that such kinematically correlated satellite planes are also present around more distant hosts. Detailed comparisons show that similarly anisotropic phase-space distributions of sub-halos are exceedingly rare in cosmological simulations based on the $\Lambda$CDM paradigm. Analogs to the observed systems have frequencies of $\leq 0.5$ per cent in such simulations. In contrast to other small-scale problems, the satellite planes issue is not strongly affected by baryonic processes because the distribution of sub-halos on scales of hundreds of kpc is dominated by gravitational effects. This makes the satellite planes one of the most serious small-scale problem for $\Lambda$CDM. This review summarizes the observational evidence for planes of satellite galaxies in the Local Group and beyond, and provides an overview of how they compare to cosmological simulations. It also discusses scenarios which aim at explaining the coherence of satellite positions and orbits, and why they all are currently unable to satisfactorily resolve the issue.