2018ApJ...869L...9W

Despite extensive efforts, to date only two quasars have been found at z > 7, due to a combination of low spatial density and high contamination from more ubiquitous Galactic cool dwarfs in quasar selection. This limits our current knowledge of the super-massive black hole growth mechanism and reionization history. In this Letter, we report the discovery of a luminous quasar at z = 7.021, DELS J003836.10-152723.6 (hereafter J0038-1527), selected using photometric data from Dark Energy Spectroscopic Instrument Legacy Imaging Survey, Pan-STARRS1 (PS1) imaging Survey, as well as Wide-field Infrared Survey Explore mid-infrared all-sky survey. With an absolute magnitude of M₁₄₅₀ = -27.1 and bolometric luminosity of L_Bol = 5.6 x 10¹³ L_☉, J0038-1527 is the most luminous quasar known at z > 7. Deep optical to near-infrared spectroscopic observations suggest that J0038-1527 hosts a 1.3 billion solar mass black hole accreting at the Eddington limit, with an Eddington ratio of 1.25 ± 0.19. The C IV broad emission line of J0038-1527 is blueshifted by more than 3000 km s^–1 relative to the quasar systemic redshift. More detailed investigations of the high-quality spectra reveal three extremely high-velocity C IV broad absorption lines with velocity from 0.08 to 0.14 times the speed of light and total "balnicity" index of more than 5000 km s^–1, suggesting the presence of relativistic outflows. J0038-1527 is the first quasar found at the epoch of reionization with such strong outflows, and therefore provides a unique laboratory to investigate active galactic nuclei feedback on the formation and growth of the most massive galaxies in the early universe.