Speaker
Description
The distribution of dwarf satellite galaxies around massive hosts serves as a useful probe of dark matter at small scales, especially since the effect of baryonic matter within the galaxies themselves on their spatial distribution is minimal. Here, I examine our neighbouring galaxy Andromeda, which hosts a strikingly asymmetrical distribution of satellites aligned towards the Milky Way. I characterise the Andromeda system’s asymmetry and test its agreement with expectations from concordance cosmology, which predicts that most satellite galaxy systems are near-isotropic – in line with other dwarf associations observed in the local Universe, which remain only weakly asymmetric. All but one of Andromeda’s 37 satellite galaxies are contained within 107 degrees of our Galaxy. In standard cosmological simulations, less than 0.3% (0.5% when accounting for possible observational incompleteness) of Andromeda-like systems show a comparably significant asymmetry, and none are as collectively lopsided as the observed satellite configuration. In conjunction with its well-studied satellite plane, these results paint the Andromeda system as an extreme outlier in the prevailing cosmological paradigm, further challenging our understanding of structure formation at small scales.