Planes of Satellite Galaxies in the Magneticum Pathfinder Simulations

Abstract

Planes of satellites are observed around many galaxies. However, these observations are still considered a point of tension for the $Λ$CDM paradigm. We use the fully hydrodynamical cosmological $Λ$CDM state-of-the-art simulation Magneticum Pathfinder to investigate the existence of such planes over a large range of haloes, from Milky Way to galaxy cluster masses. To this end, we develop the Momentum in Thinnest Plane (MTP) method to identify planes and quantify the properties of their constituent satellites, considering both position and momentum. We find that thin planes ($20\%\cdot R_\mathrm{halo}$) containing at least $50\%$ of the total number of satellites can be found in almost all systems. In Milky Way mass-like systems, around 86\% of such planes are even aligned in momentum ($90\%$ of the total satellite momentum), where the fraction is smaller if more satellites are required to be inside the plane. We further find a mass dependency, with more massive systems exhibiting systematically thicker planes. This may point towards the change from continuous accretion of small objects along filaments and sheets for less massive haloes to the accretion of large objects (e.g., major mergers) dominating the growth of more massive haloes. There is no correlation between the existence of a plane and the main galaxy's morphology. Finally, we find a clear preference for the minor axes of the satellite planes and the host galaxy to be aligned, in agreement with recent observations.