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Overdispersed radio source counts and excess radio dipole detection

Lukas Böhme, Dominik J. Schwarz, Prabhakar Tiwari, Morteza Pashapour-Ahmadabadi, Benedict Bahr-Kalus, Maciej Bilicki, Catherine L. Hale, Caroline S. Heneka, Thilo M. Siewert

Abstract

The source count dipole from wide-area radio continuum surveys allows us to test the cosmological standard model. Many radio sources have multiple components, which can cause an overdispersion of the source counts distribution. We account for this effect via a new Bayesian estimator, based on the negative binomial distribution. Combining the two best understood wide-area surveys, NVSS and RACS-low, and the deepest wide-area survey, LoTSS-DR2, we find that the source count dipole exceeds its expected value as the kinematic dipole amplitude from standard cosmology by a factor of $3.67 \pm 0.49$ -- a $5.4σ$ discrepancy.

Overdispersed radio source counts and excess radio dipole detection

Abstract

The source count dipole from wide-area radio continuum surveys allows us to test the cosmological standard model. Many radio sources have multiple components, which can cause an overdispersion of the source counts distribution. We account for this effect via a new Bayesian estimator, based on the negative binomial distribution. Combining the two best understood wide-area surveys, NVSS and RACS-low, and the deepest wide-area survey, LoTSS-DR2, we find that the source count dipole exceeds its expected value as the kinematic dipole amplitude from standard cosmology by a factor of -- a discrepancy.

Paper Structure

This paper contains 9 equations, 3 figures, 4 tables.

Figures (3)

  • Figure 1: Corner plot for the combined dipole estimate from LoTSS-DR2, NVSS, and RACS-low. The amplitude is presented in multiples of the expected dipole amplitude, with the green lines and dot representing the expected values based on the CMB dipole.
  • Figure 2: Maps (first and third row) and histograms (second and fourth row) of source counts from six radio continuum surveys and their best-fit Poisson and negative binomial distributions.
  • Figure 3: Declination and right ascension of source count dipole for all six radio surveys for different flux density cuts using the unconstrained estimator. The green lines and dot represent the expected value based on the CMB dipole.