On the anisotropic distribution of clusters in the local Universe

Abstract

In his 2021 lecture to the Canadian Association of Physicists Congress, P.J.E. Peebles pointed out that the brightest extra-galactic radio sources tend to be aligned with the plane of the de Vaucouleur Local Supercluster up to redshifts of z = 0.02 ($d_{\rm MW}\approx 85~\rm {Mpc}$). He then asked whether such an alignment of clusters is anomalous in the standard ΛCDM framework. In this letter, we employ an alternative, absolute orientation agnostic, measure of the anisotropy based on the inertia tensor axis ratio of these brightest sources and use a large cosmological simulation from the Flamingo suite to measure how common such an alignment of structures is. We find that only 3.5% of randomly selected regions display an anisotropy of their clusters more extreme than the one found in the local Universe’s radio data. This sets the region around the Milky Way as a 1.85σ outlier. Varying the selection parameters of the objects in the catalogue, we find that the clusters in the local Universe are never more than 2σ away from the simulations’ prediction for the same selection. We thus conclude that the reported anisotropy, whilst note-worthy, is not in tension with the ΛCDM paradigm.