Modeling the Radio Background from the First Black Holes at Cosmic Dawn: Implications for the 21 cm Absorption Amplitude (1803.01815v3)

Abstract: We estimate the 21 cm Radio Background from accretion onto the first intermediate-mass Black Holes between $z\approx 30$ and $z\approx 16$. Combining potentially optimistic, but plausible, scenarios for black hole formation and growth with empirical correlations between luminosity and radio-emission observed in low-redshift active galactic nuclei, we find that a model of black holes forming in molecular cooling halos is able to produce a 21 cm background that exceeds the Cosmic Microwave Background (CMB) at $z \approx 17$ though models involving larger halo masses are not entirely excluded. Such a background could explain the surprisingly large amplitude of the 21 cm absorption feature recently reported by the EDGES collaboration. Such black holes would also produce significant X-ray emission and contribute to the $0.5-2$ keV soft X-ray background at the level of $\approx 10^{-13}-10^{-12}$ erg sec$^{-1}$ cm$^{-2}$ deg$^{-2}$, consistent with existing constraints. In order to avoid heating the IGM over the EDGES trough, these black holes would need to be obscured by Hydrogen column depths of $ N_\text{H} \sim 5 \times 10^{23} \text{cm}^{-2}$. Such black holes would avoid violating contraints on the CMB optical depth from Planck if their UV photon escape fractions were below $f_{\text{esc}} \lesssim 0.1$, which would be a natural result of $N_\text{H} \sim 5 \times 10^{23} \text{cm}^{-2}$ imposed by an unheated IGM.

Modeling the Radio Background from the First Black Holes at Cosmic Dawn

The paper "Modeling the Radio Background from the First Black Holes at Cosmic Dawn: Implications for the 21 cm Absorption Amplitude" by Ewall-Wice et al. investigates the potential contributions of early black holes to the cosmic radio background. The authors propose that accretion processes on intermediate-mass black holes during cosmic dawn could generate a sufficiently strong radio background to exceed the Cosmic Microwave Background (CMB) by the redshift $z \approx 17$. This proposition could effectively explain the large 21 cm absorption signal detected by the EDGES collaboration.

Key Findings

1. Radio Background Predictions: The paper models the radio emissions from black holes forming in molecular cooling halos under different assumptions regarding black hole formation, growth, and radio loudness characteristics. With optimistic assumptions, the radio background from these sources is predicted to surpass the CMB, potentially explaining the EDGES observation.
2. X-ray and Soft X-ray Background: Early black holes would produce significant X-ray emissions, contributing to the soft X-ray background. The paper finds this is plausible within existing constraints, with the contribution level ranging from $10^{-13}$ to $10^{-12}$ erg sec$^{-1}$ cm$^{-2}$ deg$^{-2}$.
3. Column Density Requirements: To avoid heating the intergalactic medium (IGM) significantly before the EDGES trough, the black holes would need to be obscured by hydrogen column densities approximately $5 \times 10^{23}$ cm$^{-2}$. Such obscuring would naturally limit UV photon escape, aligning with constraints from Planck observations.
4. Modeling Black Hole Populations: The authors explore black hole formation in halos with varying masses and demonstrate that models with black holes in smaller molecular cooling halos can produce a radio background sufficient for the EDGES amplitude. Larger halo models are not entirely excluded but require different parameter tuning.

Implications and Future Directions

The implications of this research are substantial for our understanding of the early universe and the processes involved in cosmic dawn. By showing a scenario where early black hole activity can explain the anomalous 21 cm signal, the paper broadens the scope of astrophysical phenomena contributing to the radio background.

1. Practical Implications: The framework developed here provides a new lens for interpreting radio astronomy discoveries and contributes to our understanding of high-redshift radio sources.
2. Theoretical Insights: The findings potentially alter the narrative on black hole seed formation and growth in the early universe, suggesting that early black holes could play a more significant role in cosmic evolution than previously considered.
3. Future Research: There is a need for refined models incorporating radiative feedback mechanisms and observational data, which could provide more detailed insights into the role of black holes during cosmic dawn. Additionally, exploring different accretion scenarios and improving constraints on black hole populations at high redshifts will enhance this line of inquiry.

This paper offers a comprehensive analysis of black hole contributions to the cosmic radio background, posing significant questions for further exploration in the quest to understand the conditions and phenomena of cosmic dawn.