| Abstract: Studying the nature of the first stars and the state of intergalactic medium (IGM) during the Cosmic Dawn (CD, z=10–30) is a key goal of modern cosmology. However, to correctly predict and interpret the observed signal, we need to model various astrophysical processes accurately. Motivated by this, we revisit the interaction of Lyman-α photons with the IGM at high redshifts, uncovering several new results.
It is well established that the photons between the Lyman-α and Lyman-limit frequencies, produced in the large-scale structures during the CD, redshift into the neutral IGM and undergo multiple scatterings by the HI atoms, coupling the HI spin temperature to the gas kinetic temperature. During this process, these photons also exchange energy with the medium. The photons that redshift into the Lyman-α line (continuum photons) heat up the medium, while those injected at the line centre (injected photons) cause cooling.
The inherent assumption in the literature has been that the profile near Lyman-α line centre is in quasi-static equilibrium. However, we find that the timescales to reach this equilibrium are often underestimated. Thermal feedback due to evolving IGM temperature and the short lifetime of the first luminous sources can significantly delay or prevent the onset of the equilibrium state. Moreover, the continuum photons also reach a new equilibrium profile over a much longer expansion timescale (∼100 Myr). During the quasi-static equilibrium, the IGM attains an equilibrium temperature, and we find that this temperature only depends on the source spectrum and the redshift. We also find that these Lyman- α photons can act as a source of cooling in the presence of an external source of heating, and this effect should be detectable in the shape of the 21 cm signal. |
