  Most theoretical models invoke quasar driven outflows to quench star
formation in massive galaxies, this feedback mechanism is required to account
for the population of old and passive galaxies observed in the local universe.
The discovery of massive, old and passive galaxies at z=2, implies that such
quasar feedback onto the host galaxy must have been at work very early on,
close to the reionization epoch. We have observed the [CII]158um transition in
SDSSJ114816.64+525150.3 that, at z=6.4189, is one of the most distant quasars
known. We detect broad wings of the line tracing a quasar-driven massive
outflow. This is the most distant massive outflow ever detected and is likely
tracing the long sought quasar feedback, already at work in the early Universe.
The outflow is marginally resolved on scales of about 16 kpc, implying that the
outflow can really affect the whole galaxy, as required by quasar feedback
models. The inferred outflow rate, dM/dt > 3500 Msun/yr, is the highest ever
found. At this rate the outflow can clean the gas in the host galaxy, and
therefore quench star formation, in a few million years.
