Clustering properties of the CatWISE2020 quasar catalogue and their impact on the cosmic dipole anomaly

ABSTRACT The cosmic dipole anomaly – the mismatch between the dipole anisotropy observed in the sky distribution of cosmologically distant sources and that expected due to our local motion with respect to the cosmic microwave background – poses a serious challenge to the Cosmological Principle upon which the standard model of cosmology rests. Accurate measurement of the dipole ($\ell =1$) depends crucially on having control over other large-scale power ($\ell > 1$) so as to avoid biases, in particular that potentially caused by correlations among multipoles due to incomplete sky coverage, and that due to local source clustering. Currently, the most significant evidence for the cosmic dipole anomaly comes from the sample of 1.6 million mid-infrared quasars derived from the CatWISE2020 catalogue. We analyse the clustering properties of this sample by inferring the large-scale multipoles in real space, and compute the angular power spectrum on small scales to test for agreement with lambda cold dark matter ($\Lambda$CDM). Having accounted for the known trend of the quasar number counts with ecliptic latitude, we find that all other large-scale power is in fact consistent with noise, that there is in particular no evidence for an octupole ($\ell =3$) in the data, and also that the expected clustering dipole is marginal. Our results thus reaffirm the anomalously high dipole in the distribution of quasars.