The Subtlety Of Ly Alpha Photons: Changing The Expected Range Of The 21-Cm Signal

We present the evolution of the 21-cm signal from cosmic dawn and the epoch of reionization in an upgraded model including three subtle effects of Ly alpha radiation: Ly alpha heating, CMB heating (mediated by Ly alpha photons), and multiple scattering of Ly alpha photons. Taking these effects into account, we explore a wide range of astrophysical models and quantify the impact of these processes on the global 21-cm signal and its power spectrum. We find that Ly alpha heating raises the gas temperature by up to degrees (CMB heating adds a few more degrees) in models with weak X-ray heating, thus suppressing the predicted 21-cm signals. Varying the astrophysical parameters broadly in an attempt to cover the full plausible range of models, we find that in the upgraded model the absorption trough of the global signal reaches a lowest possible floor of -165mK at redshifts z approximate to 15-19. This is in contrast with the predictions for a pure adiabatically cooling Universe, for which the deepest possible absorption is a monotonically decreasing function of cosmic time and is -178mK at z = 19 and -216mK at z = 15, dropping to even lower values at lower redshifts. With Ly alpha and CMB heating included we also observe a strong suppression in the low-redshift power spectra, with the maximum possible power (evaluated over the ensemble of models) attenuated by a factor of 6.6 at z = 9 and K = 0.1 Mpc(-1). Finally, we find that at high redshifts corresponding to cosmic dawn, the multiple scattering of Ly alpha photons leads to an amplification of the power spectrum by a factor of similar to 2-3 in all models.