  Luminous quasars powered by accretion onto billion solar mass black holes
already exist at the epoch of Reionisation, when the Universe was 0.5-1 Gyr
old. These objects likely reside in over-dense regions of the Universe, and
will grow to form today's giant galaxies. How their huge black holes formed in
such short times is debated, particularly as they lie above the local black
hole mass-galaxy dynamical mass correlation, thus following the black
hole-dominance growth path. It is unknown what slowed down the black hole
growth, leading towards the symbiotic growth observed in the local Universe,
and when this process started, although black hole feedback is a likely driver.
This deadlock is due to the lack of large, homogeneous samples of high-redshift
quasars with high-quality, broad-band spectroscopic information. Here we report
results from a VLT/X-shooter survey of 30 quasars at redshift 5.8$\le$z$\le$6.6
(XQR-30). About 50% of their spectra reveal broad blue-shifted absorption line
(BAL) throughs, tracing powerful ionised winds. The BAL fraction in z$\gtrsim$6
quasars is 2-3 times higher than in quasars at z~2-4.5. XQR-30 BAL quasars
exhibit extreme outflow velocities, up to 17% of the light speed, rarely
observed at lower redshift. These outflows inject large amounts of energy into
the galaxy interstellar medium, which can contrast nuclear gas accretion,
slowing down the black-hole growth. The star-formation rate in high-z quasar
hosts is generally $>$100 M$_\odot$/yr, so these galaxies are growing at a fast
rate. The BAL phase may then mark the beginning of significant feedback, acting
first on black hole growth and possibly later on galaxy growth. The red optical
colors of BAL quasars at z$\gtrsim$6 indeed suggest that these systems are
dusty and may be caught during an initial quenching phase of obscured
accretion.
