  Galaxy-scale gas outflows triggered by active galactic nuclei have been
proposed as a key physical process to regulate the co-evolution of nuclear
black holes and their host galaxies. The recent detection of a massive gas
outflow in one of the most distant quasar, SDSS J1148+5251 at z = 6.4,
presented by Maiolino et al. (2012) strongly supports this idea and suggests
that strong quasar feedback is already at work at very early times. In a
previous work, Valiante et al. (2011), we have presented a hierarchical
semi-analytical model, GAMETE/ QSOdust, for the formation and evolution of
high-redshift quasars, and we have applied it to the quasar SDSS J1148+5251,
with the aim of investigating the star formation history, the nature of the
dominant stellar populations and the origin and properties of the large dust
mass observed in the host galaxy. A robust prediction of the model is that the
evolution of the nuclear black hole and of the host galaxy are tightly coupled
by quasar feedback in the form of strong galaxy-scale winds. In the present
letter, we show that the gas outflow rate predicted by GAMETE/QSOdust is in
good agreement with the lower limit of 3500 Msun/yr inferred by the
observations. According to the model, the observed outflow at z = 6.4 is
dominated by quasar feedback, as the outflow rate has already considerably
depleted the gas content of the host galaxy, leading to a down-turn in the star
formation rate at z < 7 - 8. Hence, supernova explosions give a negligible
contribution to the observed winds at z = 6.4.
