Lyman alpha heating and coupling with 21 cm signal

The global 21 cm signal from the cosmic dawn is affected by a variety of heating and cooling processes. We investigate the impact of heating due to Lyman-α (Lyα) photons on the global 21 cm signal during cosmic dawn using an analytical expression of the spectrum around the Lyman-α resonance based on the so-called ‘wing approximation’. We perform a short parameter study by varying the Lyα background intensity by four orders of magnitude and establish that a strong Lyα background is necessary, though not sufficient, in order to reproduce the recently detected stronger-than-expected 21 cm signal by the EDGES Collaboration. We show that the magnitude of this Lyα heating is smaller than previously estimated in the literature by an order of magnitude or more. As a result, even a strong Lyα background is consistent with the EDGES measurement of the global 21 cm absorption signal.