Colour dependence of dipole in CatWISE2020 data

Abstract

ABSTRACT The signal of dipole anisotropy in quasar number counts is studied using the CatWISE2020 catalogue in various colour bins. It is found that the dipole signal differs significantly in two colour bins, namely $W1-W2\lt 1.1$ and $W1-W2\gt 1.1$. While the dipole in the bin $W1-W2\lt 1.1$ points close to the direction of the cosmic microwave background dipole, the dipole in the bin $W1-W2\gt 1.1$ points in the direction $(l,b) = (194^\circ \pm 7^\circ ,19^\circ \pm 4^\circ)$, quite close to the Galactic plane. Despite the proximity to the Galactic plane, we are unable to attribute this signal to a Galactic bias. If we interpret the dipole in the bin $W1-W2\lt 1.1$ as due to our local motion, the extracted velocity turns out to be $900\pm 113$ km s$^{-1}$, which deviates from the cosmic microwave background dipole velocity with approximately 4.7σ significance. We speculate that the dipole signal in both bins is of cosmological origin and that the difference may be attributed to a redshift dependence of the dipole, representing a departure from the standard $\Lambda$CDM model.