  To explain the properties of the most massive low-redshift galaxies and the
shape of their mass function, recent models of galaxy evolution include strong
AGN feedback to complement starburst-driven feedback in massive galaxies. Using
the near-infrared integral-field spectrograph SPIFFI on the VLT, we searched
for direct evidence for such a feedback in the optical emission line gas around
the z=2.16 powerful radio galaxy MRC1138-262, likely a massive galaxy in
formation. The kpc-scale kinematics, with FWHMs and relative velocities <= 2400
km/s and nearly spherical spatial distribution, do not resemble large-scale
gravitational motion or starburst-driven winds. Order-of-magnitude timescale
and energy arguments favor the AGN as the only plausible candidate to
accelerate the gas, with a total energy injection of a few x 10^60 ergs or
more, necessary to power the outflow, and relatively efficient coupling between
radio jet and ISM. Observed outflow properties are in gross agreement with the
models, and suggest that AGN winds might have a similar, or perhaps larger,
cosmological significance than starburst-driven winds, if MRC1138-262 is indeed
archetypal. Moreover, the outflow has the potential to remove significant gas
fractions (<= 50%) from a >L* galaxy within a few 10 to 100 Myrs, fast enough
to preserve the observed [alpha/Fe] overabundance in massive galaxies at low
redshift. Using simple arguments, it appears that feedback like that observed
in MRC1138-262 may have sufficient energy to inhibit material from infalling
into the dark matter halo and thus regulate galaxy growth as required in some
recent models of hierarchical structure formation.
