The cosmic radio dipole: Bayesian estimators on new and old radio surveys

Abstract

The cosmic radio dipole is an anisotropy in the number counts of radio sources and is analogous to the dipole seen in the cosmic microwave background (CMB). Measurements of source counts of large radio surveys have shown that, although the radio dipole is generally consistent in direction with the CMB dipole, the amplitudes are in tension. These observations present an intriguing puzzle, namely the cause of this discrepancy, with a true anisotropy breaking with the assumptions of the cosmological principle, invalidating the most common cosmological models that are built on these assumptions. We present a novel set of Bayesian estimators to determine the cosmic radio dipole and compare the results with those of commonly used methods applied to the Rapid ASKAP Continuum Survey (RACS) and the NRAO VLA Sky Survey (NVSS) radio surveys. In addition, we adapt the Bayesian estimators to take into account systematic effects known to influence large radio surveys of this kind, folding information such as the local noise floor or array configuration directly into the parameter estimation. The enhancement of these estimators allows us to greatly increase the number of sources used in the parameter estimation, yielding tighter constraints on the cosmic radio dipole estimation than previously achieved with NVSS and RACS. We extend the estimators further to work on multiple catalogues simultaneously, leading to a combined parameter estimation using both NVSS and RACS. The result is a dipole estimate that perfectly aligns with the CMB dipole in terms of direction but with an amplitude that is three times as large, and a significance of 4.8σ. This new dipole measurement is made to an unprecedented level of precision for radio sources, which is only matched by recent results using infrared quasars.