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Abstract

Luminous quasars at redshift z >6, i.e. <1 Gyr after the Big Bang, are formidable probes of the early universe, at the edge of the Epoch of Reionization. These sources are predicted to be found in high–density peaks of the dark matter distribution at that time, surrounded by overdensities of galaxies. In this thesis, we present a search for and study of the most distant quasars, from the properties of their innermost regions, to those of their host galaxies and of their Mpc–scale environments. We search for the highest redshift quasars in the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1, PS1), discovering six new objects at z >6.5. Using optical/near–infrared spectroscopic data, we perform a homogeneous analysis of the properties of 15 quasars at z> 6.5. In short : 1) The majority of z >6.5 show large blueshifts of the broad CIV 1549 Å emission line, suggesting the presence of strong winds/outflows; 2) They already host supermassive black holes (M_BH = 0.3 - 5 x 10^9 M_sun) in their centers, which are accreting at a rate comparable to a luminosity–matched sample at z =1-3) No evolution of the Fe II/MgII abundance ratio with cosmic time is observed; 4) The sizes of their surrounding ionized bubbles weakly decrease with redshift. We present new millimeter observations of the dust continuum and of the [CII] 158 mm emission line (one of the main coolant of the intergalactic medium) in the host galaxies of four quasars, providing new accurate redshifts and [CII]/infrared luminosities. We study the Mpc–scale environment of a z =5.7 quasar, via observations with broad– and narrow–band filters. We recover no overdensities of galaxies. Among the potential explanations for these findings, are that the ionizing radiation from the quasar prevents galaxy formation, the sources in the fields are dust–obscured, or quasars do not live in the most massive dark matter halos. Finally, we report sensitive optical/near–infrared follow–up observations of gas–rich companion galaxies to four quasars at z >6, firstly detected with the Atacama Large Millimeter Array (ALMA).With the exception of one source, we detect no emission from the stellar population of these galaxies. Our limits on their stellar masses (< 10^10 M_sun) and unobscured star formation rates (<few M_sun yr^1) suggest that the companions are highly dust obscured and/or harboring a modest stellar content. In synthesis, in this thesis we show the large range of parameters of the most distant quasars, and the variety of their environments, with the aim of shading light on massive galaxy and black hole formation in the first Gyr of the universe.