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