Tight relationships exist in the local universe between the central stellar properties of galaxies and the mass of their supermassive black hole^(1,2,3). These suggest galaxies and black holes co-evolve, with the main regulation mechanism being energetic feedback from accretion onto the black hole during its quasar phase^(4,5,6). A crucial question is how the relationship between black holes and galaxies evolves with time; a key epoch to probe this relationship is at the peaks of star …