2020ApJ...897...71S

We address the "plane of satellites problem" by studying planar configurations around two disk galaxies with no late major mergers, formed in zoom-in hydro-simulations. Due to the current lack of good-quality kinematic data for M31 satellites, we use only positional information. So far, positional analyses of simulations are unable to find planes as thin and populated as the observed ones. We follow a novel systematic and detailed plane searching technique to study the properties and quality of planes of satellites, in both simulations or real data. In particular, (i) we extend the four-galaxy-normal density plot method (Pawlowski et al. 2013) in a way designed to efficiently identify high-quality planes (i.e., thin and populated) without imposing extra constraints on their properties, and (ii), we apply it for the first time to simulations. Using zoom-in simulations allows us to mimic Milky Way/M31-like systems regarding the number of satellites involved as well as galactic disk effects. In both simulations, we find satellite planar configurations that are compatible, along given time intervals, with all of the spatial characteristics of observed planes identified using the same methodology. During most of these periods, planes are approximately perpendicular to the galactic disk. However, the fraction of co-orbiting satellites within them is, in general, low, suggesting time-varying satellite membership. We conclude that high-quality positional planes of satellites could be not infrequent in ΛCDM-formed disk galaxies with a quiet assembly history. Detecting kinematically coherent, time-persistent planes demands considering the full six-dimensional phase-space information of satellites.