Velocity correlations of satellite pairs around massive galaxies

Abstract Many of the satellite galaxies of the Milky Way, Andromeda, and Centaurus A show evidence of coherent motions to a larger extent than most of the systems predicted by the standard cosmological model. These findings are for satellites of galactic-mass systems and it is an open question if correlations in satellite orbits are present in systems of different masses. Here, we report an analysis of the kinematics of satellite galaxies in massive galaxy groups. Different from the Milky Way analogues, we find an excess of diametrically opposed pairs of satellites that have line-of-sight velocity offsets from the central galaxy of the same sign. This corresponds to a 5.3 sigma (p-value=6.5d-8) detection of non-random satellite motions. Such excess is predicted by up-to-date cosmological simulations but the magnitude of the effect is considerably lower than in observations. The observational data is discrepant at the 3.8 sigma and 4.1 sigma level with the predictions of the Millennium and the Illustris TNG-300 cosmological simulations, potentially indicating that massive galaxy groups assembled later in the real Universe. The detection of velocity correlations of satellite galaxies and tension with theoretical predictions is robust against changes in sample selection. Using the largest sample to date, our findings demonstrate that the motions of satellite galaxies represent a challenge to the current cosmological model.