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

## TL;DR

This paper examines how Ly$ extalpha$ heating influences the 21-cm signal during Cosmic Dawn, suggesting that strong Ly$ extalpha$ backgrounds are unlikely and that additional cooling mechanisms are needed to explain observations.

## Contribution

It models the global 21-cm signal considering Ly$ extalpha$ heating and coupling, challenging previous explanations for the EDGES detection.

## Key findings

- Strong Ly$ extalpha$ background is ruled out by the EDGES signal.
- Enhanced cooling mechanisms are favored over millicharged baryon interactions.
- Ly$ extalpha$ heating significantly impacts the interpretation of the 21-cm signal.

## Abstract

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. Instead the results favour a weak Ly$\alpha$ background combined with an excess cooling mechanism which is substantially stronger than previously considered. We also show that the cooling mechanism driven by the interaction between millicharged baryons and dark matter particles no longer provides a viable explanation for the EDGES result when Ly$\alpha$ heating is taken into account.