On the Origin of the Local Group’s Peculiar Velocity

Abstract

We aim to settle the debate regarding the fraction of the Local Group's peculiar velocity that is induced by structures beyond the Great Attractor by calculating the dipole anisotropy of the largest, all-sky, truly X-ray-selected cluster sample compiled to date. The sample is the combination of the REFLEX catalog in the southern hemisphere, the eBCS sample in the north, and the CIZA survey in the Galactic plane. The composite REFLEX+eBCS+CIZA sample overcomes many of the problems inherent to previous galaxy and cluster catalogs that limited their effectiveness in determining the origin of the Local Group's motion. From the dipole anisotropy present in the cluster distribution, we determine that 44% of the Local Group's peculiar velocity is due to infall into the Great Attractor region, while 56% is in the form of a large-scale flow induced by more distant overdensities between 130 and 180 h-1 Mpc away. In agreement with previous analyses, we find that the Shapley supercluster is the single overdensity most responsible for the increase in the dipole amplitude beyond 130 h-1 Mpc, generating 30.4% of the large-scale contribution. We find that a significant portion of the remainder is due to numerous groupings and loose associations of clusters at roughly the same distance as the Shapley region. We also note the presence of a significant underdensity of clusters in the northern hemisphere roughly 150 h-1 Mpc away and suggest that the large-scale anisotropy observed in the cluster distribution near this distance may have as much to do with the presence of large overdensities in the south as it does with the lack of superclusters in the north.