Discovery of the most distant luminous quasar to date: J0313-1806

Distant quasars are unique media for studying topics such as the formation mechanisms of first-generation seed black holes, the interaction of active galactic nuclei and their host galaxies, and the history of cosmic reionization. So far, there have been found only six quasars with z > 7, and two of them (ULAS J1342 + 0928, z = 7.54; J100758.264+211529.207, z = 7.52) have a redshift larger than 7.5. On January 20, 2021, in the Astrophysical Journal Letters, an international collaborative team of astronomers led by the University of Arizona reported a bright quasar, named J0313-1806, with a bolometric luminosity of 1.4 x 10(47) erg s(-1). The process of discovering the quasar, the technique, and the scientific significance of this discovery are explained in this paper. Their work is based on a data set obtained mainly from the Pan-STARRS1, the DESI Legacy Imaging Surveys, the UKIRT Hemisphere Survey, the VISTA Hemisphere Survey, and the Wide-field Infrared Survey Explorer Survey. To measure the redshift of this quasar more accurately, the dust continuum and the CII emission from the quasar's host galaxy were observed using the Atacama Large Millimeter/Submillimeter Array in Chile. Via spectral fitting, they calculated the quasar's redshift of 7.642, that is more than 13 billion light-years from Earth and dating back to only 670 million years after the Big Bang, which makes the quasar the most distant and earliest one. Based on the spectral fitting and the MgII virial estimator, the mass of the supermassive black hole was calculated to be 1.6x10(9)M(circle dot). The formation of such a massive supermassive black hole in a short period time imposes strict constraints on the mass and the formation model of the seed black hole. Suppose the black hole accreted at the Eddington rate, if it started its growth at redshift z = 15 - 30 (i.e., 400-570 Ma growth time), to what we observe today, it requires a 10(4)-10(5) M-circle dot seed black hole, that might be more supportive of direct-collapse black holes forming in pre-galactic dark matter halos. The quasar's spectra also exhibit evident broad absorption line features in CIV and SiIV, together with a strongly blueshifted CIV emission line, indicating the relativistic outflows with the maximum speed of up to 20% of the speed of light. In addition, the ALMA observations imply a strong star formation of 200 M-circle dot a(-1) taking place in the host galaxy of J0313-1806. The continuum observations indicate that substantial dust (7 x 10(7)M(circle dot)) was already built up in the quasar host galaxy. Those makes J0313-1806 an ideal target for probing the effects of active galactic nuclei on their host galaxies. The number of high-redshift quasars is still small, making it difficult for constraining the formation model of the seed black holes, understanding the relation of the supermassive black hole and its host galaxy and so on. We look forward to those ongoing and upcoming optical/infrared sky surveys, such as the James Webb Space Telescope (JWST) and the Large Synoptic Survey Telescope (LSST), which will bring breakthroughs in astronomy.