Thermal Evolution of the Intergalactic Medium due to Lyα Photons during the Cosmic Dawn

Abstract The first star-forming objects that formed at high redshifts during the cosmic dawn (CD) also emitted photons between Ly α and Lyman-limit frequencies. These photons are instrumental in coupling the spin temperature of the neutral hydrogen (H i ) atoms with the kinetic temperature of the intergalactic medium (IGM). Along with this coupling effect, these photons also impact the kinetic temperature by exchanging energy with the H i atoms. The injected Ly α photons in general cool the medium, while the continuum photons heat the medium. While studying this effect in the literature, a quasi-static profile around the Ly α frequency is assumed. In this paper, we solve the time-dependent coupled dynamics of the photon intensity profile along with the evolution of the thermal state of the IGM and H i spin temperature. It is expected that, during the CD era, the IGM has a mix of continuum photons with 10%–20% of injected photons. For this case, we show that the system reaches thermal equilibrium in around 1 Myr, with the final temperature in the range 50–100 K. This timescale is comparable to the source lifetime of Population III stars at high redshifts. One impact of switching off short-lived sources is that it can keep the system heated above the temperature of the quasi-static state. We also show that the quasi-static equilibrium for the continuum photons is only achieved on timescales of 100 Myr at z ≃ 20, comparable to the age of the Universe. We also briefly discuss how the Ly α induced heating can impact the 21 cm signal from CD.