New tools for studying planarity in galaxy satellite systems: Milky Way satellite planes are consistent with {\Lambda}CDM

TL;DR

This paper introduces a new measure of planarity in galaxy satellite systems, applies it to the Milky Way using Gaia data, and compares the results with cosmological simulations, finding consistency with the DM model.

Contribution

The paper proposes a novel concept of planarity that does not rely on plane count or thickness, and demonstrates its application to observational and simulated data.

Findings

Milky Way satellites show strong positional planarity but lack kinematic coherence.

Kinematically supported planes are common in DM simulations.

Observed satellite planarity aligns with DM predictions.

Abstract

We introduce a new concept -- termed "planarity" -- which aims to quantify planar structure in galaxy satellite systems without recourse to the number or thickness of planes. We use positions and velocities from the Gaia EDR3 to measure planarity in Milky Way (MW) satellites and the extent to which planes within the MW system are kinematically supported. We show that the position vectors of the MW satellites exhibit strong planarity but the velocity vectors do not, and that kinematic coherence cannot, therefore, be confirmed from current observational data. We then apply our methodology to NewHorizon, a high-resolution cosmological simulation, to compare satellite planarity in MW-like galaxies in a {\Lambda}CDM-based model to that in the MW satellite data. We demonstrate that kinematically supported planes are common in the simulation and that the observed planarity of MW satellites is not in tension with the standard {\Lambda}CDM paradigm.