Simulating the 21-cm signal during the Cosmic Dawn

The anticipated radio telescope SKA is expected to detect the 21-cm signal from the Cosmic Dawn, allowing us to probe the astrophysical processes of this previously unobserved era. The 21-cm differential brightness temperature fluctuations from the Cosmic Dawn are driven by early inhomogeneous heating of the neutral intergalactic medium and variations in Lyman-alpha photon density. Inhomogeneous heating is driven by high energy, X-ray photons which have long mean free paths and thus penetrate deep into the neutral intergalactic medium. Lyman-a fluctuations depend on the soft, UV photons from these sources redshifting into Lyman-a resonance. In this thesis I present a large-volume (349Mpc comoving) suite of fully numerical radiative transfer simulations of this epoch. The simulations include the effects of helium ionisation, secondary ionisations and multi-frequency heating in order to include different types of X-ray sources (high mass X-ray binaries sources and QSO sources) in addition to black body stellar sources and Lyman-alpha fluctuations, which are added as a post-processing step. In our simulations X-ray sources are able to contribute significantly to early heating of the neutral IGM. Different X-ray models produce varying lengths and morphologies of the transition from absorption to emission. When the results are smoothed to the expected resolution of SKA1-Low the mean, rms, skewness, kurtosis and power spectra of the 21-cm differential brightness temperature are notably different for each X-ray model. These rms fluctuations for each heating model are well above the expected noise for deep integrations which suggests direct imaging of X-ray heating during the Cosmic Dawn should be possible. The presence of QSOs greatly affects the non-Gaussianity, suggesting higher order statistics may be a good observational probe of rare X-ray sources. This e_ect is decreased if the Lyman-a background is built up late. We conclude by discussing ongoing and future work on the topic.