Campus, Varian 355
One of the major goals in observational cosmology today is to understand how our Universe transitioned from the "dark ages", following recombination, into the ionized universe we can observe today. For this purpose we compiled a new data set of 34 high redshift (5.8<z<6.5) quasar spectra of moderate resolution, containing several new and unpublished objects, and reduced these spectra in a coherent and homogeneous way. The analysis of this rich data set enables new insights into the early evolutionary phase of our Universe and the early stages of AGN and galaxy formation traced by the intergalactic gas.
We analyse the proximity zones of the quasars in our data set, i.e. the regions surrounding the quasars that have been ionized by their own radiation, in order to set constraints on the onset and duration of the reionization process as well as the lifetime of these quasars. We find several objects showing exceptionally small proximity zones and argue that only a very short lifetime (t_Q < 10^5 years) can be causing these small zones by comparing our measurements to radiative transfer simulations. We analyze the implications of such short quasar lifetimes on their ionizing power, their black hole accretion rates and highlight tensions with current black hole formation theories. Additionally we discuss the imprints of the intergalactic medium (IGM) on the spectra of the quasars and constrain the evolution of the IGM opacity as well as its neutral gas fraction. We highlight their implications regarding the timing of the epoch of reionization, the morphology of the UV background radiation, and the temperature-density relation of the high-redshift IGM.