Ly α coupling and heating at cosmic dawn
Shikhar Mittal, Girish Kulkarni
Abstract
ABSTRACT 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 Ly α photons on the global 21-cm signal at cosmic dawn using an analytical expression of the spectrum around the Ly α resonance based on the so-called ‘wing approximation’. We derive a new expression for the scattering correction and for the first time give a simple close-form expression for the cooling due to injected Ly α photons. We perform a short parameter study by varying the Ly α background intensity by four orders of magnitude and establish that a strong Ly α background is necessary, although not sufficient, in order to reproduce the recently detected stronger-than-expected 21-cm signal by the Experiment to Detect the Global EoR Signal (EDGES) Collaboration. We show that the magnitude of this Ly α heating is smaller than previously estimated in the literature by two orders of magnitude or more. As a result, even a strong Ly α background is consistent with the EDGES measurement. We also provide a detailed discussion on different expressions of the Ly α heating rate used in the literature.