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The resonance scattering of Ly$\alpha$ photons with neutral hydrogen atoms inthe intergalactic medium not only couples the spin temperature to the kinetictemperature but also leads to a heating of the gas. We investigate the impactof this heating on the average brightness temperature of the 21-cm signal fromthe Cosmic Dawn in the context of the claimed detection by the EDGES low-bandexperiment. We model the evolution of the global signal taking into account theLy$\alpha$ coupling and heating and a cooling which can be stronger than theHubble 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 isruled out. Instead the results favour a weak Ly$\alpha$ background combinedwith an excess cooling mechanism which is substantially stronger thanpreviously considered. We also show that the cooling mechanism driven by theinteraction between millicharged baryons and dark matter particles no longerprovides a viable explanation for the EDGES result when Ly$\alpha$ heating istaken into account.

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