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Abstract

In this thesis we trace the cosmic history from the Epoch of Reionization to the local universe by means of several spectroscopic studies. In the first part, we analyze $34$ quasar spectra at $5.8\lesssim z_{\rm em}\lesssim 6.5$ and measure the redshift evolution of the opacity of the intergalactic medium (IGM) within the Ly$\alpha$ as well as the Ly$\beta$ forest to set new constraints on the timing and morphology of the reionization epoch. We find evidence for an extended reionization process down to $z\sim 5$, and, while the observed scatter in the Ly$\alpha$ forest optical depth can be well reproduced by current state-of-the-art simulations including spatial fluctuations in the temperature field or the ultraviolet background, we find a strong mismatch between simulations and observations in the Ly$\beta$ forest opacity, suggesting an inversion of the thermal state of the post-reionization IGM. We also measure the sizes of the quasars' proximity zones, which are regions of enhanced ionization in the vicinity of the quasars, ionized by their own radiation. We find a dependency of the proximity zone sizes to the quasars' lifetime, which presents a novel method to estimate the lifetime of individual quasars, providing unprecedented constraints on the formation and growth of supermassive black holes in the early universe. We discover three quasars with very short lifetimes, i.e. $t_{\rm Q}\sim 10^4-10^5$~yr, that pose significant challenges to all current black hole formation theories.

In the second part of this thesis we explore the structure and dynamics of the Milky Way. We develop a new data-driven model to determine precise parallaxes by combining multi-band photometry and spectroscopy to make global kinematic maps of our Galaxy from $\gtrsim 45,000$ luminous red giant stars with only $\lesssim 10\%$ parallax uncertainties. Our map extends to Galactocentric distances of $25$~kpc, well beyond the reach of parallax estimates by the \textit{Gaia} mission. Making use of these new spectrophotometric parallaxes, we determine the most precise measurement to date of the circular velocity curve of the Milky Way over a wide range of Galactocentric distances. Based on Jeans modeling in an axisymmetric gravitational potential we find that the velocity curve is gently but significantly declining.