  Large-scale outflows driven by supermassive black holes are thought to play a
fundamental role in suppressing star formation in massive galaxies. However,
direct observational evidence for this hypothesis is still lacking,
particularly in the young universe where star formation quenching is remarkably
rapid, thus requiring effective removal of gas as opposed to slow gas heating.
While outflows of ionized gas are commonly detected in massive distant
galaxies, the amount of ejected mass is too small to be able to suppress star
formation. Gas ejection is expected to be more efficient in the neutral and
molecular phases, but at high redshift these have only been observed in
starbursts and quasars. Here we report JWST spectroscopy of a massive galaxy
experiencing rapid quenching at redshift z=2.445. We detect a weak outflow of
ionized gas and a powerful outflow of neutral gas, with a mass outflow rate
that is sufficient to quench the star formation. Neither X-ray or radio
activity are detected; however, the presence of a supermassive black hole is
suggested by the properties of the ionized gas emission lines. We thus conclude
that supermassive black holes are able to rapidly suppress star formation in
massive galaxies by efficiently ejecting neutral gas.
