Wouthuysen-Field coupling strength and application to high-redshift 21-cm radiation

The first ultraviolet sources in the universe are expected to have coupled the H I spin temperature to the gas kinetic temperature via scattering in the Ly resonance (the 'Wouthuysen-Field effect'). By establishing an H I spin temperature different from the temperature of the cosmic microwave background, the Wouthuysen-Field effect should allow observations of H I during the reionization epoch in the redshifted 21-cm hyperfine line. This paper investigates four mechanisms that can affect the strength of the Wouthuysen-Field effect that were not previously considered. (1) Photons redshifting into the H I Lyman resonances may excite an H atom and result in a radiative cascade terminating in two-photon 2s 1/2 1s 1/2 emission, rather than always degrading to Ly as usually assumed. (2) The fine structure of the Ly resonance alters the photon frequency distribution and leads to a suppression of the scattering rate.