Luminous quasars at > z 5.6 can be studied in detail with the current generation of telescopes and provide us with unique information on the first gigayear of the universe. Thus far, these studies have been statistically limited by the number of quasars known at these redshifts. Such quasars are rare, and therefore, wide-field surveys are required to identify them, and multiwavelength data are required to separate them efficiently from their main contaminants, the far more numerous cool dwarfs. In this paper, we update and extend the selection for thez 6 quasars presented in Bañados et al. (2014) using the Pan-STARRS1 (PS1) survey. We present the PS1 distant quasar sample, which currently consists of 124 quasars in the redshift range   z 5.6 6.7 that satisfy our selection criteria. Of these quasars, 77 have been discovered with PS1, and 63 of them are newly identified in this paper. We present the composite spectra of the PS1 distant quasar sample. This sample spans a factor of ∼20 in luminosity and shows a variety of emission line properties. The number of quasars at > z 5.6 presented in this work almost doubles the previously known quasars at these redshifts, marking a transition phase from studies of individual sources to statistical studies of the high-redshift quasar population, which was impossible with earlier, smaller samples.

Distant quasars are unique tracers to study the formation of the earliest supermassive black holes (SMBHs) and the history of cosmic reionization. Despite extensive efforts, only two quasars have been found at z ≥ 7.5, due to a combination of their low spatial density and the high contamination rate in quasar selection. We report the discovery of a luminous quasar at z = 7.642, J0313−1806, the most distant quasar yet known. This quasar has a bolometric luminosity of 3.6 × 1013 L ⊙. Deep spectroscopic observations reveal a SMBH with a mass of (1.6 ± 0.4) × 109 M ⊙ in this quasar. The existence of such a massive SMBH just ∼670 million years after the big bang challenges significantly theoretical models of SMBH growth. In addition, the quasar spectrum exhibits strong broad absorption line (BAL) features in C iv and Si iv, with a maximum velocity close to 20% of the speed of light. The relativistic BAL features, combined with a strongly blueshifted C iv emission line, indicate that there is a strong active galactic nucleus (AGN)-driven outflow in this system. Atacama Large Millimeter/submillimeter Array observations detect the dust continuum and [C ii] emission from the quasar host galaxy, yielding an accurate redshift of 7.6423 ± 0.0013 and suggesting that the quasar is hosted by an intensely star-forming galaxy, with a star formation rate of ∼200 M ⊙ yr−1 and a dust mass of ∼7 × 107 M ⊙. Follow-up observations of this reionization-era BAL quasar will provide a powerful probe of the effects of AGN feedback on the growth of the earliest massive galaxies.

The discovery of quasars a few hundred megayears after the Big Bang represents a major challenge to our understanding of black holes as well as galaxy formation and evolution. Quasarsʼ luminosity is produced by extreme gas accretion onto black holes, which have already reached masses of M BH >10 9 M e by z∼6. Simultaneously, their host galaxies form hundreds of stars per year, using up gas in the process. To understand which environments are able to sustain the rapid formation of these extreme sources, we started a Very Large Telescope/Multi-Unit Spectroscopic Explorer (MUSE) effort aimed at characterizing the surroundings of a sample of 5.7<z<6.6 quasars, which we have dubbed the Reionization Epoch QUasar InvEstigation with MUSE (REQUIEM) survey. We here present results of our searches for extended Lyα halos around the first 31 targets observed as part of this program. Reaching 5σ surface brightness limits of 0.1-1.1×10 −17 erg s −1 cm −2 arcsec −2 over a 1 arcsec 2 aperture, we were able to unveil the presence of 12 Lyα nebulae, eight of which are newly discovered. The detected nebulae show a variety of emission properties and morphologies with luminosities ranging from 8×10 42 to 2×10 44 erg s −1 , FWHMs between 300 and 1700 km s −1 , sizes <30 pkpc, and redshifts consistent with those of the quasar host galaxies. As the first statistical and homogeneous investigation of the circumgalactic medium of massive galaxies at the end of the reionization epoch, the REQUIEM survey enables the study of the evolution of the cool gas surrounding quasars in the first 3 Gyr of the universe. A comparison with the extended Lyα emission observed around bright (M 1450 −25 mag) quasars at intermediate redshift indicates little variations on the properties of the cool gas from z∼6 to z∼3, followed by a decline in the average surface brightness down to z∼2.

We present X-SHOOTER near-IR spectroscopy of a large sample of 38 luminous (M 1450 = −29.0 to −24.4) quasars at 5.78 < z < 7.54, which have complementary [C ii] 158μm observations from ALMA. This X-SHOOTER/ALMA sample provides us with the most comprehensive view of reionization-era quasars to date, allowing us to connect the quasar properties with those of its host galaxy. In this work we introduce the sample, discuss data reduction and spectral fitting, and present an analysis of the broad emission line properties. The measured Fe ii/Mg ii flux ratio suggests that the broad-line regions of all quasars in the sample are already enriched in iron. We also find the Mg ii line to be on average blueshifted with respect to the [C ii] redshift with a median of −391 km s−1. A significant correlation between the Mg ii−[C ii] 158μm and C iv−[C ii] 158μm velocity shifts indicates a common physical origin. Furthermore, we fRequently detect large C iv–Mg ii emission line velocity blueshifts in our sample with a median value of −1848 km s−1. While we find all other broad emission line properties not to be evolving with redshift, the median C iv–Mg ii blueshift is much larger than found in low-redshift, luminosity-matched quasars (−800 km s−1). Dividing our sample into two redshift bins, we confirm an increase of the average C iv–Mg ii blueshift with increasing redshift. Future observations of the rest-frame optical spectrum with the James Webb Space Telescope will be instrumental in further constraining the possible evolution of quasar properties in the epoch of reionization.

This is the third paper in a series aimed at finding reionization-era quasars with the combination of DESI Legacy imaging Surveys (DELS), the Pan-STARRS1 (PS1) Survey, and near-infrared imaging surveys, such as the UKIRT Hemisphere Survey (UHS), as well as the Wide-field Infrared Survey Explorer (WISE) mid-infrared survey. In this paper, we describe the updated quasar candidate selection procedure, report the discovery of 16 quasars at 6.4 z6.9 from an area of ∼13,020 deg 2 , and present the quasar luminosity function (QLF) at z∼6.7. The measured QLF follows F µ-L L 1450 1450 2.35 () in the magnitude range −27.6<M 1450 <−25.5. We determine the quasar comoving spatial density at á ñ z =6.7 and M 1450 <−26.0 to be 0.39±0.11 Gpc −3 and find the exponential density evolution parameter to be k=−0.78±0.18 from z∼6 to z∼6.7, corresponding to a rapid decline by a factor of ∼6 per unit redshift toward earlier epochs. This indicates that the rapid decline of quasar spatial density at z>5 that was found by previous works continues to z>6, at a rate significantly faster than the average decline rate between z∼3 and 5. We measured quasar comoving emissivity at z∼6.7, which indicates that high-redshift quasars are highly unlikely to make a significant contribution to hydrogen reionization. The broad absorption line quasar fraction at z6.5 is measured to be 22%. In addition, we also report the discovery of six additional quasars at z∼6 in the Appendix.