The web of the Giant: spectroscopic confirmation of a Large Scale
  Structure around the z=6.31 quasar SDSS J1030+0524
  We report on the spectroscopic confirmation of a large scale structure around
the luminous, z=6.31 QSO SDSS~J1030+0524, that is powered by a billion solar
mass black hole. The structure is populated by at least six members, four Lyman
Break Galaxies (LBGs) and two Lyman Alpha Emitters (LAEs). The four LBGs have
been identified among a sample of 21 i-band dropouts with z{AB}<25.5 selected
up to projected separations of 5 physical Mpc (15 arcmin) from the QSO. Their
redshifts have been determined through up to 8hr-long multi-object
spectroscopic observations at 8-10m class telescopes. The two LAEs have been
identified in a 6hr VLT/MUSE observation centered on the QSO. The redshifts of
the six galaxies cover the range 6.129-6.355. Assuming that peculiar velocities
are negligible, this range corresponds to radial separations of +/-5 physical
Mpc from the QSO, that is comparable to the projected scale of the observed LBG
distribution on the sky. We conservatively estimate that this structure is
significant at >3.5 sigma level, and that the level of the galaxy overdensity
is at least 1.5-2 within the large volume sampled (~780 physical Mpc^3). The
spectral properties of the six member galaxies (Lyalpha strength and UV
luminosity) are similar to those of field galaxies at similar redshifts. This
is the first spectroscopic identification of a galaxy overdensity around a
super-massive black hole in the first billion years of the Universe. Our
finding lends support to the idea that the most distant and massive black holes
form and grow within massive (>10^{12} Msun) dark matter halos in large scale
structures, and that the absence of earlier detections of such systems was
likely due to observational limitations.
