The Satellite Galaxy Planes Problem, suggested solutions and open questions

The existence of co-orbiting planes of satellite galaxies is now recognized as one of the most severe small-scale problems of cosmology. In contrast to other mismatches between observations and cosmological simulations, the satellite plane issue is not easily resolved by invoking baryonic physics. I will briefly summarize the evidence for satellite planes in the Local Group, how they compare to cosmological simulations, and why suggested formation scenarios are so far unable to satisfactorily resolve the issue. To understand the existence and origin of the satellite planes, we will have to move beyond a descriptive to a deductive use of the observed satellite distributions, and also consider additional, potentially related features and dwarf galaxy alignments in the Local Group.

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