Modeling the Constituents of the Early Universe

Abstract

Understanding how baryons assembled into the first stars and galaxies given the underlying dark matter distribution in the early Universe remains one of the key challenges in the field of cosmology. Given the great distances and extreme faintness of the oldest celestial objects, direct observation of the high redshift Universe remains difficult. Due to the biased, small number statistics, various other avenues have been sought out and paved to construct a coherent picture of the large-scale structure of the Universe at these early times. This thesis explores several of these avenues in a series of studies aimed at characterizing the first galaxies and the star-forming molecular gas that drives their evolution in the early Universe. We model the effects of strong gravitational lensing and find that by magnifying sources that would otherwise be too faint to detect and lifting them over the instrumental detection threshold, lensing allows us to probe the luminosity function (LF) below current survey limits and constrain the low-luminosity cut-off of the Schechter LF. Using the most recent dust estimates and z ~ 4-8 LF measurements, we then model the LF evolution at higher redshifts by employing abundance matching techniques and derive the redshift evolution of the star formation rate density along with the associated cosmic reionization history. To model the molecular interstellar medium (ISM), we employ the large velocity gradient (LVG) method, a radiative transfer technique used to quantitatively analyze CO spectral line energy distributions (SLEDs). In particular, we probe the average state of the molecular gas in HDF850.1, a z ~ 5.2 submillimetre source, as well as a series of local starburst, Seyfert, and ultraluminous infrared galaxies. We identify characteristic properties of the local and high-redshift ISM, including the kinetic temperature, gas density, and column density in each case, and derive the gas masses of the various CO-emitting sources along with the corresponding CO-to-H_2 conversion factors. Furthermore, we develop a theoretical framework to estimate the intensity mapping signal and power spectrum of any CO rotational line emitted at high redshifts, particularly during the epoch of reionization. By linking the characteristic properties of emitting molecular clouds to the global properties of the host halos, we model the spatially averaged brightness temperature of all the CO transitions and find the predicted signals to be within reach of existing instruments. We further apply our formalism to compute the cumulative CO emission from star-forming galaxies throughout cosmic time. Lastly, we explore the emergence of planetary systems around carbon-enhanced metal-poor (CEMP) stars, possible relics of the early Universe residing in the halo of our galaxy. We determine the maximum distance from the host CEMP star at which carbon-rich planetesimal formation is possible and characterize the potential planetary transits across these chemically anomalous stars. We conclude by exploring the possibility of probing the stellar black hole population using astrometric observations provided by missions such as Gaia. We predict that nearly 3x10^5 astrometric binaries hosting black holes should be discovered during Gaia's five year mission. The invisible companions of astrometrically observed metal-poor, low-mass stars may be stellar remnants from the dawn of the Universe, offering to shed light on the formation of the first stellar black holes in the early stages of galaxy assembly.