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arXiv · 2506.12827

21 cm Signal from the Thermal Evolution of Lyman-$α$ during Cosmic Dawn

Janakee Raste·Shiv K. Sethi
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Abstract

The Lyman-$α$ photons couple the spin temperature of neutral hydrogen (HI) to the kinetic temperature during the era of cosmic dawn. During this process, they also exchange energy with the medium, heating and cooling the HI. In addition, we expect X-ray photons to heat the mostly neutral gas during this era. We solve this coupled system (Lyman-$α$-HI system along with X-ray heating) for a period of 500 Myr (redshift range $8 <z < 25$). Our main results are: (a) Without X-ray heating, the temperature of the gas reaches an equilibrium which is nearly independent of photon intensity and only weakly dependent on the expansion of the universe. The main determinant of the quasi-static temperature is the ratio of injected and continuum Lyman-$α$ photons. (b) While X-ray photons provide an additional source of heating at initial times, for large enough Lyman-$α$ photon intensity, the system tends to reach the same quasi-static temperature as expected without additional heating. This limit is reached when the density of photons close to the Lyman-$α$ resonance far exceeds the HI number density. (c) We compute the global HI signal for these scenarios. In the limit of the large density of Lyman-$α$ photons, the spin temperature of the hyperfine line is fixed. This freezes the global HI signal from the era of cosmic dawn and the cross-over redshift from absorption to emission. This feature depends only on the ratio of injected to continuum Lyman-$α$ photons, and the global HI signal can help us determine this ratio.

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Janakee Raste, Shiv K. Sethi. 2025-12-24. 21 cm Signal from the Thermal Evolution of Lyman-$α$ during Cosmic Dawn. https://doi.org/10.3847/1538-4357%2Fae2029

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