Universe Today
Although many spiral galaxies have been discovered with the odd rising rotational velocities near their outer edges, Andromeda is both one of the most prominent and the closest. Detailed studies from Corbelli et al. (2010) and Chemin et al. (2009), mapped out the rise in HI gas, showing that the velocity increases some 50 km/s in the outer 7 kiloparsecs mapped. This makes up a significant fraction of the total radius given the studies extended to only ~38 kiloparsecs. While conventional models with Dark Matter are able to reproduce the rotational velocities of the inner portions of the galaxy, they have not explained this outer feature and instead predict that it should slowly fall off.