A comment on "Is There a Disk of Satellites Around the Milky Way?" and "The Nature of Disks of Satellites Around Milky Way-Like Galaxies"

Marcel S. Pawlowski, Jörg Dabringhausen, Benoit Famaey, Hector Flores, Francois Hammer, Rodrigo A. Ibata, Pavel Kroupa, Geraint F. Lewis, Noam I. Libeskind, Stacy S. McGaugh, David Merritt, Mathieu Puech, Yanbin Yang

Published 2017-02-20Version 1

In two recent arXiv postings, Maji et al. argue against the existence of a spatially thin, kinematically coherent Disk of Satellites (DoS) around the Milky Way (MW), and suggest that the DoS is "maybe a misinterpretation of the data". These claims are in stark contrast to previous works, and indeed we show that the conclusions of Maji et al. do not hold up to scrutiny. They lack a statistical basis since no attempts have been made to measure the significance of the found satellite alignments, observational biases have been ignored, and measurement errors such as for proper motions have not been considered. When interpreting their hydrodynamic cosmological simulation by comparing it with an alternative model of isotropically distributed satellite velocities, we find no evidence for a significant kinematic coherence among the simulated satellite galaxies, in contrast to the observed MW system. We furthermore discuss general problems faced by attempts to determine the dynamical stability of the DoS via orbit integrations of MW satellite galaxies.