X-Rays during the Cosmic Dawn

The first billion years of the Universe, from the recombination era to the epoch of reionization still largely remain an enigma. How did the structures form and evolve in the Universe? When did the first stars begin to appear? What were their properties? What was the nature of the objects which heated and reionized the surrounding intergalactic medium? With advances in observational astronomy, it has been established that the tiny initial perturbations of the order∼ 10−5 in the cosmic microwave background (CMB) radiation grew under gravitational instability and evolved to the structures that we see today. However, the period from the Dark Ages till the epoch of reionization (EoR) remains unobserved by direct observations. Ongoing powerful telescopes such as James Webb Space Telescope, Atacama Large Millimeter/submillimeter Array, etc. aim to observe the galaxies at high redshifts. However, besides being unable to capture the faintest galaxies, they might not probe the first sources of heating and reionization. In this Thesis, we exploit the potential of the redshifted 21-cm neutral hydrogen line as an indirect yet significant complementary probe of early sources of heating of the intergalactic medium (IGM). The signal is sensitive to the thermal state of the IGM. Studies suggest that X-rays from High Mass X-ray Binaries (HMXBs) drive temperature fluctuations in the IGM, during the so-called epoch of heating (EoH). The corresponding cosmic 21-cm signal should be detectable at high signal-to-noise by the upcoming radio-interferometers such as the Hydrogen Epoch of Reionization (HERA) and Square Kilometer Array (SKA). Its timing and amplitude of fluctuations during the EoH can give reveal valuable information on the nature of early sources of heating. We predict the 21-cm signal by performing cosmological simulations using a semi-numerical simulation code, 21cmFAST. Since the early galaxies were rare and biased, with their abundances modulated by long-wavelength modes of the underlying density field, the limited size of the cosmological simulations can underestimate the amount of structure in the signal. This effect is significant, especially during the early EoH.We quantify the minimum box size needed for simulating the power spectrum of the 21-cm signal from the Cosmic Dawn up to the EoR. Current theoretical models of the EoH are based on empirical scaling relations between the population-averaged X-ray luminosity (LX) to star-formation rates (SFRs) of local galaxies. However, both theoretical models of HMXB evolution and recent observations suggest that this LX/SFR relation should have a strong dependence on metallicity. We model the impact of an evolving metallicity dependence of HMXBs on the 21-cm signal from the EoH. Motivated by the current uncertainties, we present forecasts for various mass-metallicity relations as well as LX/SFR relations. The Thesis is organized as follows. 1- Chapter 1 covers the cosmological and astrophysical framework which underlies the thesis. The topics range from the standard cosmological model to various aspects of galaxy formation and evolution. 2- Chapter 2 provides an overview of the properties of the IGM such as ionization and thermal evolution. It provides a detailed discussion on the heating of the IGM by X-rays by various X-ray sources, especially HMXBs, and their global scaling relations. The Chapter ends with a brief section on several observational probes of the IGM. 3- Chapter 3 provides details on the physics of the 21-cm signal, its role in probing the IGM, the relevant theoretical framework, observation of the signal, and the challenges it entails. 4- In Chapter 4, we work out what is the minimum size of the simulation box to simulate the 21-cm signal, without losing the structure due to missing wave modes. We quantify the bias and cosmic scatter by running simulations of different sizes and comparing the corresponding 21-cm power spectra with respect to a large simulation box, in terms of telescope noise. 5- In Chapter 5, we explore the metallicity dependence of HMXBs and its effect on the corresponding 21-cm signal. We develop a methodology to relate LX/SFR with gas-phase metallicity. We work out how well the metallicity-independent LX/SFR models can produce similar effects on the 21-cm signal as the metallicity-dependent ones. 6- In Chapter 6, we constrain the properties of the early HMXBs during the EoH. Following up the previous work of Chapter 5, we quantify how our metallicitydependent LX/SFR and mass-metallicity relations can be recovered from upcoming 21-cm observations 7- The Thesis ends with a discussion of conclusions and future aspects in the last chapter.