Abstract
Abstract The presence of an abundant population of low frequency photons at high redshifts (such as a radio background) can source leading order effects on the evolution of the matter and spin temperatures through rapid free-free absorptions. This effect, known as soft photon heating, can have a dramatic impact on the differential brightness temperature, ΔTb, a central observable in 21cm cosmology. Here, we introduce a semi-analytic framework to describe the dynamics of soft photon heating, providing a simplified set of evolution equations and a useful numerical scheme which can be used to study this generic effect. We also perform quasi-instantaneous and continuous soft photon injections to elucidate the different regimes in which soft photon heating is expected to impart a significant contribution to the global 21cm signal and its fluctuations. We find that soft photon backgrounds produced after recombination with spectral index γ > 3.0 undergo significant free-free absorption, and therefore this heating effect cannot be neglected. The effect becomes stronger with steeper spectral index, and in some cases the injection of a synchrotron-like spectrum (γ = 3.6) can suppress the amplitude of ΔTb relative to the standard model prediction (where an additional radio background is absent), making the global 21cm signal even more difficult to detect in these scenarios.
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