Barycenter kinematics in Local Group analogues

We investigate a new way to identify systems similar to our Local Group (LG) of galaxies in cosmological simulations. We use as an additional constraint the velocity of the LG barycenter, which is measured accurately from cosmic microwave background data. Using different cosmological simulations we compare pairs selected with this constraint to pairs selected with traditional methods. Our method selects simulated galaxy pairs where M31's radial approach speed to the Milky Way is 2% to 7% lower depending on the cosmology, while we find no robust change in the tangential velocity. The difference in the radial component is statistically significant in 86 of the 93 cosmological models we test, but the shift is small (a few kms-1) compared to the typical velocity dispersion of LG analogues (tens of kms-1). Matching the observed barycenter velocity therefore has a measurable but modest effect on LG analogue kinematics, and is best treated as an optional refinement, rather than a necessary selection criterion.