The nature and consequences of cosmological halo formation: dark matter and the dark ages

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Ahn, Kyungjin

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Dark matter particles and baryons constitute a significant fraction of the mass of the universe. Dark matter (DM) halos are the scaffolding around which galaxies and clusters are built. They form when the gravitational instability of primordial density fluctuations causes regions which are denser than average to slow their cosmic expansion, recollapse, and virialize. Baryons provide valuable information about the universe by emitting observable electromagnetic waves, while undergoing complicated hydrodynamic and radiative processes. Understanding the role of baryons and dark matter in structure formation is thus a prerequisite for probing the nature of our universe. We describe here our broad attempts to derive and give physical insight to the theory of cosmological structure formation, first by focusing on dark matter halo formation and the nature of dark matter. We show that many of the results of N-body simulations of cosmological structure formation can be easily understood by the “fluid approximation” we have developed, where the usual fluid conservation equations are used to describe collisionless halo dynamics. We then study the self-interacting dark matter hypothesis by comparing our results to observations of dark-matter dominated halos. We also find that an alternative dark matter candidate, the thermal relic, can be the origin of the “missing” γ-ray background at 1–20 MeV and 511 keV line emission from the Galactic center, if the dark matter particle mass is about 20 MeV. Turning our attention to baryonic structure formation in the high redshift universe, we then use high-resolution cosmological N-body and hydrodynamic simulations of structure formation at high redshift (z > 6) to predict the signal of the 21cm line radiation from neutral hydrogen gas in the cosmic “dark ages”, before reionization. We predict that the largest contribution to the 21cm signal is due to gas in collapsed minihalos. Finally, we focus on the radiative feedback effects of the first stars to question whether the second generation star formation is promoted by such feedback effects. We find that such star formation may be promoted as a result of radiation-induced implosion of minihalos in the vicinity of the first stars.