Impact of Ly$\alpha$ heating on the global 21-cm signal from the Cosmic Dawn

Understand

The resonance scattering of Ly$\alpha$ photons with neutral hydrogen atoms in the intergalactic medium not only couples the spin temperature to the kinetic temperature but also leads to a heating of the gas.

• We investigate the impact of this heating on the average brightness temperature of the 21-cm signal from the Cosmic Dawn in the context of the claimed detection by the EDGES low-band experiment.
• We model the evolution of the global signal taking into account the Ly$\alpha$ coupling and heating and a cooling which can be stronger than the Hubble cooling.
• Using the claimed detection of a strong absorption signal at $z\approx 17$ as a constraint, we find that a strong Ly$\alpha$ background is ruled out.