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Quasars at cosmic dawn provide powerful probes of the formation and growth ofthe earliest supermassive black holes (SMBHs) in the universe, theirconnections to galaxy and structure formation, and the evolution of theintergalactic medium (IGM) at the epoch of reionization (EoR). Hundreds ofquasars have been discovered in the first billion years of cosmic history, withthe quasar redshift frontier extended to z~7.6. Observations of quasars atcosmic dawn show that: (1) The number density of luminous quasars declinesexponentially at z>5, suggesting that the earliest quasars emerge at z~10; thelack of strong evolution in their average spectral energy distributionindicates a rapid buildup of the AGN environment. (2) Billion-solar-mass BHsalready exist at z>7.5; they must form and grow in less than 700 Myr, by acombination of massive early BH seeds with highly efficient and sustainedaccretion. (3) The rapid quasar growth is accompanied by strong star formationand feedback activity in their host galaxies, which show diverse morphologicaland kinetic properties, with typical dynamical mass of lower than that impliedby the local BH/galaxy scaling relations. (4) HI absorption in quasar spectraprobes the tail end of cosmic reionization at z~5.3-6, and indicates the EoRmidpoint at 6.9 < z < 7.6 with large spatial fluctuations in IGM ionization.Observations of heavy element absorption lines suggest that the circumgalacticmedium also experiences evolution in its ionization structure and metalenrichment during the EoR.

Quasars and the Intergalactic Medium at Cosmic Dawn