Astrophysical information from the Rayleigh-Jeans Tail of the CMB

Raghunath Ghara, Garrelt Mellema, Saleem Zaroubi

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One of the explanations for the recent EDGES-LOW band 21 cm measurements of a strong absorption signal around 80 MHz is the presence of an excess radio background to the Cosmic Microwave Background (CMB). Such excess can be produced by the decay of unstable particles into small mass dark photons which have a non-zero mixing angle with electromagnetism. We use the EDGES-LOW band measurements to derive joint constraints on the properties of the early galaxies and the parameters of such a particle physics model for the excess radio background. A Bayesian analysis shows that a high star formation efficiency and X-ray emission of 4-7 × 1048 erg per solar mass in stars are required along with a suppression of star formation in halos with virial temperatures ≥ 2 × 104 K. The same analysis also suggests a 68 percent credible intervals for the mass of the decaying dark matter particles, it's lifetime, dark photon mass and the mixing angle of the dark and ordinary photon oscillation of [10-3.5, 10-2.4] eV, [101.1, 102.7] × 13.8 Gyr, [10-12.2, 10-10] eV and [10-7, 10-5.6] respectively. This implies an excess radio background which is ≈ 5.7 times stronger than the CMB around 80 MHz. This value is a factor ∼3 higher than the previous predictions which used a simplified model for the 21 cm signal.

Original languageEnglish
Article number055
JournalJournal of Cosmology and Astroparticle Physics
Issue number3
StatePublished - 1 Mar 2022

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  • axions
  • first stars
  • reionization
  • star formation


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