  We study the efficiency of galactic feedback in the early Universe by
stacking the [C II] 158 um emission in a large sample of normal star-forming
galaxies at 4 < z < 6 from the ALMA Large Program to INvestigate [C II] at
Early times (ALPINE) survey. Searching for typical signatures of outflows in
the high-velocity tails of the stacked [C II] profile, we observe (i)
deviations from a single-component Gaussian model in the combined residuals and
(ii) broad emission in the stacked [C II] spectrum, with velocities of |v|<~
500 km/s. The significance of these features increases when stacking the subset
of galaxies with star formation rates (SFRs) higher than the median (SFRmed =
25 Msun/yr), thus confirming their star-formation-driven nature. The estimated
mass outflow rates are comparable to the SFRs, yielding mass-loading factors of
the order of unity (similarly to local star-forming galaxies), suggesting that
star-formation-driven feedback may play a lesser role in quenching galaxies at
z > 4. From the stacking analysis of the datacubes, we find that the combined
[C II] core emission (|v|< 200 km/s) of the higher-SFR galaxies is extended on
physical sizes of ~ 30 kpc (diameter scale), well beyond the analogous [C II]
core emission of lower-SFR galaxies and the stacked far-infrared continuum. The
detection of such extended metal-enriched gas, likely tracing circumgalactic
gas enriched by past outflows, corroborates previous similar studies,
confirming that baryon cycle and gas exchanges with the circumgalactic medium
are at work in normal star-forming galaxies already at early epochs.
