Considerations on how to investigate planes of satellite galaxies

Abstract

The existence of a spatially thin, kinematically coherent Disk of Satellites (DoS) around the Milky Way (MW) is a problem that often garners vivacious debate in the literature or at scientific meetings. One of the most recent incarnations of this wrangle occurred with two papers by Maji et al., who argued that these structures "may be a misinterpretation of the data." These claims are in stark contrast to previous works. Motivated by this and other recent publications on this problem, we discuss necessary considerations to make, observational effects to consider, and pitfalls to avoid when investigating satellite galaxy planes such as the MW's DoS. In particular, we emphasize that conclusions need to have a statistical basis including a determination of the significance of satellite alignments, observational biases must not be ignored, and measurement errors (e.g., for proper motions) need to be considered. We discuss the general problems faced by attempts to determine the dynamical stability of the DoS via orbit integrations of MW satellite galaxies, and demonstrate that, to interpret simulations, it is helpful to compare them with a null case of isotropically distributed satellite positions and velocities. Based on these criteria, we find that the conclusions of Maji et al. do not hold up to scrutiny, and that their hydrodynamic cosmological simulation of a single host shows no evidence for a significant kinematic coherence among the simulated satellite galaxies, in contrast to the observed MW system.

The Milky Way’s disc of classical satellite galaxies in light of Gaia DR2

“…9. This confirms the expectation that the satellite galaxy phase space distribution constitutes a robust test of the cosmological model (Kroupa et al 2005;Pawlowski 2018), indicates that previous studies using dark-matter-only simulations to compare to the VPOS remain valid, and confirms earlier indications that the modelling of baryonic processes in cosmological simulations does not alleviate the planes of satellite galaxies problem (Ahmed et al 2017;Pawlowski et al 2015bPawlowski et al , 2017Pawlowski et al , 2019Shao et al 2018).…”

Planes of satellites around Milky Way/M31-mass galaxies in the FIRE simulations and comparisons with the Local Group

“…Recently, Riley & Strigari 2020 showed that globular clusters and stellar streams around the MW do not seem to be members of the satellite plane, suggesting that plane members may be recently accreted or in a particularly stable orbital configuration. Pawlowski et al 2017 noted that integrating present-day satellite orbits either forward or backward in time typically leads to the disintegration of the plane, especially when sampling measurement uncertainties on satellite galaxy positions and velocities. Shaya & Tully 2013 took a different approach and, by searching the dynamical parameter space of Local Volume satellites, found past trajectories that could possibly lead to the observed satellite planes.…”

Evolution of galactic planes of satellites in the eagle simulation

“…Even in the ideal case, that is when assuming a spherical MW halo and when neglecting satellite interactions, the Galactic plane of satellites is short lived and loses its thinness in ∼1 Gyr (Lipnicky & Chakrabarti 2017, the proper motion errors make the timescale somewhat uncertain -e.g. see Pawlowski et al 2017).…”