A thousand shadows of Andromeda: rotating planes of satellites in the Millennium-II cosmological simulation

Understand

In a recent contribution, Bahl \& Baumgardt investigated the incidence of planar alignments of satellite galaxies in the Millennium-II simulation, and concluded that vast thin planes of dwarf galaxies, similar to that observed in the Andromeda galaxy (M31), occur frequently by chance in $\Lambda$-Cold Dark Matter cosmology.

• However, their analysis did not capture the essential fact that the observed alignment is simultaneously radially extended, yet thin, and kinematically unusual.
• With the caveat that the Millennium-II simulation may not have sufficient mass resolution to identify confidently simulacra of low-luminosity dwarf galaxies, we re-examine that simulation for planar structures, using the same method as employed by Ibata et al.
• (2013) on the real M31 satellites.