Probing the Cosmological Principle in the counts of radio galaxies at different frequencies

TL;DR

This study measures the large-scale matter distribution dipole using radio galaxy counts from NVSS and TGSS surveys, finding larger-than-expected amplitudes that challenge current cosmological models.

Contribution

It presents the first analysis of the radio dipole in the TGSS survey and compares results with NVSS, providing new insights into large-scale anisotropies and potential systematics.

Findings

NVSS dipole is about twice the theoretical prediction.

TGSS dipole is nearly five times larger than expected.

Systematic effects likely cause the anomalously large TGSS dipole.

Abstract

According to the Cosmological Principle, the matter distribution on very large scales should have a kinematic dipole that is aligned with that of the CMB. We determine the dipole anisotropy in the number counts of two all-sky surveys of radio galaxies. For the first time, this analysis is presented for the TGSS survey, allowing us to check consistency of the radio dipole at low and high frequencies by comparing the results with the well-known NVSS survey. We match the flux thresholds of the catalogues, with flux limits chosen to minimise systematics, and adopt a strict masking scheme. We find dipole directions that are in good agreement with each other and with the CMB dipole. In order to compare the amplitude of the dipoles with theoretical predictions, we produce sets of lognormal realisations. Our realisations include the theoretical kinematic dipole, galaxy clustering, Poisson noise, simulated redshift distributions which fit the NVSS and TGSS source counts, and errors in flux calibration. The measured dipole for NVSS is $\sim\!2$ times larger than predicted by the mock data. For TGSS, the dipole is almost $\sim\! 5$ times larger than predicted, even after checking for completeness and taking account of errors in source fluxes and in flux calibration. Further work is required to understand the nature of the systematics that are the likely cause of the anomalously large TGSS dipole amplitude.