Speaker
Description
Dwarf satellite galaxies around Andromeda (M31) and the Milky Way form thin, coherently rotating planes argued to be in tension with expectations from the cold-dark-matter (CDM) model of cosmology. For M31, this disagreement is compounded by a prominent asymmetry in its satellite distribution; over 80% of its dwarfs lie in the hemisphere facing the Milky Way. To a degree, lopsided dwarf galaxy systems appear to be ubiquitous in the local Universe and may reflect an underlying asymmetry in their hosts' dark matter halo morphology - in turn carrying implications for halo mass estimates derived from satellite dynamics. Adopting a recently published set of homogeneous, RR Lyrae-based distances to the M31 satellites, we discovered that the existing asymmetry is strengthened. 34 out of 35 satellites are contained within a cone with an opening angle of 202 degrees (or 213 degrees facing the Milky Way), while the luminous dwarf M110 dominates the nearly hemispheric void on the other side. We further studied the rarity of similarly asymmetric dwarf galaxy distributions in several state-of-the-art cosmological simulations. Even when accounting for the look-elsewhere effect in selecting a preferred opening angle, less than 0.4% of M31 analogs host satellite systems that match or exceed the observed asymmetry. The significance of the M31 satellites' observed asymmetry towards the Milky Way in CDM simulations now rivals that of M31's plane-of-satellites, cementing the Andromeda system as a striking outlier from cosmological expectations.
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