Title: The evolution of the intergalactic medium and the formation of galaxies
Description: Galaxies form out of the collapse of dense regions of the intergalactic medium (IGM) and continued accretion from the latter fuels star formation across cosmic time At the same time, galaxies fundamentally affect the evolution of the IGM through ionizing, thermal, mechanical, and chemical feedback. The interplay between the IGM and galaxies is therefore central to a holistic understanding of both phenomena. This thesis addresses aspects of this connection in a series of investigations combining empirical input with analytic and numerical theory. After measuring the evolution of the effective Lyα optical depth of the IGM between z =2 and z =4.2 from a sample of 86 quasar spectra in chapter 2, we synthesize in chapter 3 the implications of this measurement for the evolution of the cosmic ionizing background and its sources, quasars and galaxies. The empirical constraints thus obtained serve as the basis for a new calculation of the evolution of the spectrum of the ionizing background versus redshift in chapter 4. As the ionizing background spectrum is a fundamental ingredient to metal abundance studies and hydrodynamical simulations, this new model will allow more accurate studies of both the IGM and galaxy formation.

We also present in this chapter analytic models of the effects of HeII reionization on the spectrum of the ionizing background and on the thermal history of the IGM. In chapter 5, we focus more specifically on the assembly of galaxies and its observational signatures. We introduce a new three-dimensional Lyα radiative transfer code, αRT , and for the first time combine it with hydrodynamical simulations to predict the properties of the cooling radiation released by the cold accretion that dominates the baryonic build up of galaxies. In addition to predicting the morphologies and spectra of the cold streams, we find that the predicted Lyα cooling luminosities are critically sensitive to the thermal state of the self-shielded gas, which has not previously been adequately modeled. Using a simple approximation for the self-consistent thermal evolution of the dense gas in the simulations, we obtain the most robust cooling luminosity predictions to date.