The Discovery of A Luminous Broad Absorption Line Quasar at A Redshift
  of 7.02
  Despite extensive efforts, only two quasars have been found at $z>7$ to date
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 (SMBH) growth mechanism and
reionization history. In this letter, we report the discovery of a luminous
quasar at $z=7.021$, DELS J003836.10$-$152723.6 (hereafter J0038$-$1527),
selected using photometric data from DESI Legacy imaging Survey (DELS),
Pan-STARRS1 (PS1) imaging Survey, as well as Wide-field Infrared Survey Explore
($WISE$) mid-infrared all-sky survey. With an absolute magnitude of
$M_{1450}$=$-$27.1 and bolometric luminosity of $L_{\rm
Bol}$=5.6$\times$10$^{13}$ $L_\odot$, 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 BH accreting at the
Eddington limit, with an Eddington ratio of 1.25$\pm$0.19. The CIV broad
emission line of J0038$-$1527 is blue-shifted by more than 3000 km s$^{-1}$ to
the systemic redshift. More detailed investigations of the high quality spectra
reveal three extremely high velocity CIV broad absorption lines (BALs) 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
(EoR) with such strong outflows and provides a unique laboratory to investigate
AGN feedback on the formation and growth of the most massive galaxies in the
early universe.
