The Signature of Strong High-Redshift Radio Backgrounds on the Cosmic Dawn 21-cm Bispectrum

TL;DR

This paper investigates how a strong high-redshift radio background influences the 21-cm bispectrum, revealing that bispectrum measurements can serve as sensitive probes of early cosmic radio sources and their effects on the epoch of reionization.

Contribution

It introduces the use of the 21-cm bispectrum to analyze non-Gaussianity caused by high-redshift radio backgrounds, highlighting its potential to distinguish different early universe models.

Findings

Moderate radio efficiency in early galaxies significantly impacts the small-scale 21-cm bispectrum.

Delayed heating transition shifts the bispectrum sign change to lower redshifts (~11).

The 21-cm bispectrum is highly sensitive to radio background fluctuations.

Abstract

Measurements from the Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission 2 (ARCADE-2) reveal a strong radio background in the GHz frequency range. Since the cosmological 21-cm signal is measured relative to the background radiation temperature, the presence of a radio excess can significantly alter its characteristics. Previous studies have explored the impact of an inhomogeneous radio background on the global 21-cm signal and 21-cm power spectrum. This non-uniform radio background is also expected to introduce substantial non-Gaussianity. In this work, using the bispectrum, we analyze the non-Gaussianity in the 21-cm signal in the presence of an excess galactic radio background and investigate how line-of-sight radio fluctuations from early galaxies influence its nature. We find that even a moderate enhancement in radio efficiency in early galaxies significantly affects the small-scale 21-cm bispectrum. Furthermore, the delayed heating transition caused by a galactic radio background shifts the sign change in the squeezed-limit bispectrum to lower redshifts ($z\sim11$), providing a potential observational signature for distinguishing different radio background models. These results demonstrate that the 21-cm bispectrum, particularly in the squeezed limit, is highly sensitive to radio background fluctuations, making it a powerful tool for probing high-redshift radio-loud sources and the physics of the early cosmic epoch.