  In the standard scenario for galaxy evolution young star-forming galaxies
transform into red bulge-dominated spheroids, where star formation has been
quenched. To explain such a transformation, a strong negative feedback
generated by accretion onto a super-massive black hole is often invoked. The
depletion of gas resulting from quasar-driven outflows should eventually stop
star-formation in the host galaxy and lead the black hole to "suicide" for
starvation. Direct observational evidence for a major quasar feedback onto the
host galaxy is still missing, since outflows previously observed in quasars are
generally associated with the ionized component of the gas, which only accounts
for a minor fraction of the total gas content, and typically occurring in the
central regions. We used the IRAM PdB Interferometer to observe CO(1-0) in Mrk
231, the closest QSO known. Thanks to the wide band we detect broad wings of
the CO line, with velocities up to 750 km/s and spatially resolved on the kpc
scale. Such broad CO wings trace a giant molecular outflow of about 700
MSun/yr, far larger than the ongoing star-formation rate (~200 MSun/yr)
observed in the host galaxy. This wind will totally expel the cold gas
reservoir in Mrk 231 in about 1e7 yrs, therefore halting the star-formation
activity on the same timescale. The inferred kinetic energy in the molecular
outflow is ~1.2e44 erg/s, corresponding to a few percent of the AGN bolometric
luminosity, very close to the fraction expected by models ascribing quasar
feedback to highly supersonic shocks generated by radiatively accelerated
nuclear winds. The direct observational evidence for quasar feedback reported
here provides solid support to the scenarios ascribing the observed properties
of local massive galaxies to quasar-induced large scale winds.
