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

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.

Figures (10)

• Figure 1: Top panel: Global signal as a function of redshift for the astrophysical models considered in this work. We also show the absolute differences in the global signal for the external (green solid line) and moderate LoS radio fluctuation model (teal solid line) compared to the standard astrophysical model. Bottom panel: The 21-cm power spectrum at $k=1.0 \ \rm{Mpc^{-1}}$ as a function of redshift for the five different models. The relative differences of three excess radio models with respect to the standard astrophysical model are also shown.
• Figure 2: Top panel: Probability density functions (PDFs) of the 21-cm brightness temperature, measured relative to the mean temperature and divided by the standard deviation, for various models at $z = 20$: standard astrophysical model (black solid line), external radio background model (green solid line), isotropic galactic radio fluctuation model (orange solid line), and LoS galactic radio fluctuation models with $f_{\rm{Radio}}=60$ (teal solid line) or $f_{\rm{Radio}}=3000$ (blue solid line). Note that the PDFs are normalized to a total area of unity, and the standard deviation for these five cases is $\sigma= 15.47, 18.23, 18.73, 24.89$ and 611.0 mK, respectively. We also compare to a Gaussian PDF (black dotted curve), which has mean 0 and $\sigma=1$ in these normalized units. Bottom panel: We show the difference between each PDF and the Gaussian PDF, for the various models. This quantifies one kind of non-Gaussianity in the 21-cm brightness temperature. Also shown for comparison is a zero difference (black dotted line).
• Figure 3: Skewness and kurtosis as a function of redshift, for the same models as shown in Figure \ref{['fig:histogram_1D_pdf']}.
• Figure 4: The 21-cm SABS at $z=20$ for the limiting L-isosceles triangle ($n \rightarrow 1$, i.e., $k_2 \rightarrow k_1$) versus $\cos \theta$ at $k_{\rm{1}} = 0.07, 0.244$ and $0.851$ Mpc$^{-1}$. The square and triangle markers represent positive and negative values of the bispectrum, respectively. The colored lines in each panel show various models: CMB-only case (black), external uniform background (green), isotropic (orange) and LoS galactic radio fluctuations (teal).
• Figure 5: The 21-cm SABS at $z=20$ for the limiting linear triangles ($\cos\theta \rightarrow 1$) versus $n$ at $k_{\rm{1}} = 0.07, 0.244$ and $0.851$ Mpc$^{-1}$. The markers and colors are as in Figure \ref{['fig:bispectrum_isosceles_triangles']}.