Probing cosmological isotropy with Planck Sunyaev–Zeldovich galaxy clusters

We probe the hypothesis of cosmological isotropy using the Planck Sunyaev-Zeldovich (PSZ) galaxy clusters data set. Our analyses consist on a hemispherical comparison of the clusters angular distribution, searching for a preferred direction in the large-scale structure of the Universe. We obtain a maximal dipolar signal at the direction (l; b) = (53:44 ; 41:81 ) whose antipode points toward (l; b) = (233:44 ; 41:81 ). Interestingly, this antipode is marginally consistent with the anomalous Cold Spot found in the Cosmic Microwave Background, located at (l; b) ’ (209 ; 57 ), which might be possibly aligned with a supervoid at z 0:2 with 200 Mpc=h of radius. The statistical significance of this result is assessed with ensembles of Monte Carlo realisations, finding that only a small number of runs are able to reproduce a close direction to this one, hence rejecting the null hypothesis of such direction being a random fluctuation of the data. Moreover, the PSZ catalogue presents a mild discrepancy with the isotropic realisations unless we correct some e ects, such as the non-uniform exposure function of Planck’s observational strategy, on the simulated data sets. We also perform a similar analysis to a smaller, albeit optimised sub-sample of PSZ sources, finding a better concordance with isotropic realisations, yet no correlation with the supervoid is obtained this time. Thus, we conclude that the dipole anisotropy found on galaxy clusters angular distribution can be partially attributed to an anomalous feature in the large-scale structure, though the significance of this result is su ciently reduced when corrections to systematic e ects are taken into account.