  Feedback is the key physical mechanism regulating galaxy formation. Stars in
galaxies form when baryons radiatively cool down and fall into gravitational
wells. Eventually, star formation quenches as gas is depleted and/or perturbed
by feedback processes, no longer being able to collapse and condense. For
massive galaxies, astronomers identify feedback from accreting supermassive
black holes (active galactic nuclei, AGN) as the main agent responsible for
quenching. We report the first spatially resolved spectroscopic observations of
a massive, completely quiescent galaxy at $z=3.714$ (Jekyll) and its
neighborhood. Jekyll is part of a galaxy pair with a compact, dusty, massive
star-forming companion (Hyde). We find large amounts of ionized and neutral gas
in the intergalactic medium around the pair, yet Jekyll has remained quiescent
for more than 500~Myr. The emitting gas is consistent with AGN photoionization,
but no AGN is observed in Jekyll. We find that, in contrast to standard
scenarios, AGN in satellite galaxies can be critical contributors for keeping
massive galaxies quiescent in the early Universe. After the accelerated
formation and quenching of the massive central galaxy, tidally induced gas
stripping additionally contributes to the star-formation regulation on
subsequent satellite galaxy generations.
