The spin-kinetic temperature coupling and the heating rate due to Lyα scattering before reionization:: Predictions for 21 centimeter emission and absorption

We investigate the interaction of Lyalpha photons produced by the first stars in the universe with intergalactic hydrogen prior to reionization. The background Lyalpha spectral profile is obtained by solving a Fokker-Planck equation. We present accurate values of the heating and scattering rates and the spin-kinetic temperature coupling coefficient. We show that the heating rate induced by the Lyalpha scattering is much lower than found previously and is basically negligible. The dominant heating source is most likely X-rays-from the first ionizing sources, which are able to penetrate into the atomic medium. The scattering of Lya photons couples the hydrogen spin temperature to the kinetic temperature. If the first ionizing sources in the universe did not emit significant X-rays, the spin temperature would be rapidly brought down to the very low gas kinetic temperature, and a 21 cm absorption signal against the cosmic microwave background (CMB) larger than 100 mK would be predicted. However, we argue that sufficient X-rays are likely to have been emitted by the first stellar population, implying that the gas kinetic temperature should rapidly increase, turning a reduced and brief absorption signal into emission, with a smaller amplitude of similar to20 mK. The detection of the 21 cm absorption and emission features would be a hallmark in unravelling the history of the "dark age" before reionization.