Quasars at the dawn of cosmic time

Quasars beyond redshift z ~ 6 (age of the Universe < 1 Gyr) are the most luminous sources in the Universe and are believed to be the progenitors of present-day early-type massive galaxies. The immense luminosities from their active nuclei are powered by rapid accretion of matter onto a central supermassive black hole such that quasars outshine the starlight emission of their host galaxies. The fast growth of the black hole in these quasars pairs with the rapid consumption of huge gas reservoirs through vigorous episodes of star formation. Such active galactic nuclei can regulate the star formation activity in the host by expelling gas from galaxy via large-scale outflows thus constraining both the final stellar mass and dynamical properties of the host. This and other feedback mechanisms set a closely linked evolution between growing black holes and the assembling galaxies across the cosmic time, giving place to the black hole--galaxy scaling relations observed in the local Universe. The host galaxies of z > 6 quasars are therefore ideal laboratories to characterize the physical properties of the interstellar medium in extreme conditions, and they provide an insight of the interplay between star formation and black hole accretion in massive galaxies emerging from cosmic dawn. The redshift z ~ 6 range is of particular interest because it coincides with the last phase transition of the Universe -- from neutral to mostly ionized. Hence, z > 6 quasars can be used as beacons to shed new light on the properties of intergalactic medium at the end of the Epoch of Reionization. Models and numerical simulations of massive black hole formation at early epochs predict that z>6 quasars reside at the extreme peak of the large-scale density structure such that they can be used to pinpoint the first galaxy overdensities. [abridged]