  We present the stacking analysis of a sample of 48 QSOs at 4.5<z<7.1 detected
by ALMA in the [CII] 158 micron line to investigate the presence and the
properties of massive, cold outflows traced by broad [CII] wings. We reveal
very broad [CII] wings tracing the presence of outflows with velocities in
excess of 1000 km/s. We find that the luminosity of the broad [CII] emission
increases with LAGN, while it does not significantly depend on the SFR of the
host galaxy, indicating that the central AGN is the main driving mechanism of
the [CII] outflows in these powerful, distant QSOs. From the stack of the ALMA
cubes, we derive an average outflow spatial extent of ~3.5 kpc. The average
mass outflow rate inferred from the whole sample stack is ~ 100 Msun/yr, while
for the most luminous systems it increases to ~200 Msun/yr. The associated
outflow kinetic power is about 0.1% of LAGN, while the outflow momentum rate is
about LAGN/c or lower, suggesting that these outflows are either driven by
radiation pressure onto dusty clouds or, alternatively, are driven by the
nuclear wind and energy conserving but with low coupling with the ISM. We
discuss the implications of the resulting feedback effect on galaxy evolution
in the early Universe.
