The discovery and characterization of the most distant quasars

Abstract

Luminous quasars at high redshift (z>5.5) provide direct probes of the growth of supermassive black holes and their host galaxies at the epoch of cosmic reionization, which is one of the current frontiers of astrophysical research. In this thesis, z>5.5 quasars are studied from different angles. (1) The criteria to identify high-redshift quasars using the Panoramic Survey Telescope & Rapid Response System 1 (Pan-STARRS1, PS1) survey are presented. These selection criteria have resulted in 54 newly discovered quasars at z>5.5. These new discoveries almost double the number of known z>5.5 quasars and increase the number of quasars at z>6.5 from four to eight. The quasar sample spans a factor of ~20 in luminosity and shows a diverse range of properties, including a number of weak-line and radio-loud quasars. (2) Using the available data in the literature, the radio-loud fraction (RLF) of quasars at z~6 is constrained to RLF=8.1^{+5.0}_{-3.2}%. This result is consistent with a non-evolving RLF of quasars up to z=6. (3) High-redshift quasars are thought to reside in massive dark matter halos and should therefore be located in regions with an overdensity of galaxies. To find such overdensities, the first search for galaxies using narrow-band imaging in the field of a z=5.7 quasar is performed. No overdensity of galaxies is found around the quasar, with the possible implication that high-redshift quasars may not reside in the center of the most massive dark matter halos. (4) A strong [CII] emission line (an effective cooling line of the interstellar medium) in the host galaxy of a PS1-discovered quasar at z=6.54 is reported. This is the brightest quasar known at z>6.5 and given its high rest-frame UV and [CII] luminosities, it has the potential of becoming an important laboratory for the study of star formation and of the interstellar medium only ~800 Myr after the Big Bang. As a final note, the number of quasars at z>5.5 presented in this thesis marks the transition phase from studies of individual sources to statistical studies of the high-redshift quasar population, which was impossible with earlier, smaller samples.