Tomáš Šoltinský

During the early stages of the universe, the intergalactic medium cools down, collapses and forms the first galaxies. The photons generated by these luminous sources ionize and heat up their surroundings. This period in the universe’s history is known as the Epoch of Reionization. There are various observables which can be used to explore the reionization. In this study we focus on the 21cm forest which should be detected in the spectra of very distant sources in the forthcoming years by facilities such as WEAVE-LOFAR and SKA. These will rely on theoretical predictions and mock observations to guide future observing strategies and interpret data.
Therefore, the aim of this project is to improve the modelling of this observable. We study how different phenomena affect the observability of the 21cm forest signal such as the strong spin temperature coupling, peculiar motion of the gas, X-ray background, patchiness and history of the reionization.
The 21cm signal originates from the hyperfine structure of neutral hydrogen. Its ground level is split into two energy states depending on the relative orientation of the proton and electron spin. This spin can be flipped by the absorption or emission of a photon of 21cm wavelength. Photons from distant sources loose energy along the way and are absorbed by the neutral hydrogen via this spin-flip transition. This is imprinted as a forest-like structure of absorption features in the spectrum of such sources and is known as the 21cm forest. The signal shows transmission troughs corresponding to hot ionized regions and deep absorption lines caused by cold dense neutral gas.
To achieve our goal we use Sherwood-relics simulation suite, a set of state-of-the-art cosmological simulations. The simulations used track more than 17 billion particles in 40/h comoving Mpc box. They include physics such as gravity and hydrodynamics, the main drivers of the structure formation and radiative transfer, a scheme tracking photons that can heat up and ionize the intergalactic medium.
The spin temperature is a measure of relative occupation numbers of the two hyperfine states and hence affects the amplitude of the signal. It can couple to the gas kinetic temperature via collisions between particles or interactions with Lyman-α photons. A common assumption is that during the later stages of the reionization there is enough Lyman-α photons such that the spin temperature is fully coupled to the gas temperature. We show that if we perform the full calculation of the spin temperature we either enhance or reduce the signal.
The intergalactic medium has a peculiar motion, a component of its velocity deviating from the expansion of the universe caused by gravity and pressure forces. Some studies treat peculiar velocity in a simplified manner or even neglect it. In our results we observe a considerable boost in absorption by the peculiar velocities.
The X-ray background has the most dominant effect on the observability of the 21cm forest in our modelling as it provides additional heating of the gas and hence reduces the signal. Unfortunately, its history is not well constrained.
We compared two types of reionization: Patchy, in which reionization starts at different places at different times, these inhomogeneities expand and overlap; and Homogeneous reionization. In the former one there are regions that are not heated yet. These regions experience less pressure smoothing of the structures and hence there are more strong absorbers.
Finally, we explored a late- and early-end reionization scenarios in which the reionization is completed by redshift 5.3 and 6.7, respectively. In the early-end simulation the intergalactic medium is heated up and ionized faster and we have significantly less absorption features in comparison with the late-end case.