Presentation #400.01 in the session “Royal Astronomical Society (RAS) Gold Medal Plenary”.
Why star formation ends in galaxies is one of the major questions in astronomy. I will describe a semi-empirical model for the linkage between black hole mass, galaxy properties, and dark halo mass. While star-forming, galaxies build their stellar mass at a rate that is set by their dark halo mass but grow BHs at a rate that is set by their central stellar density. Total emitted BH energy grows more rapidly than halo-gas binding energy in this phase, and when the total effective emitted BH energy exceeds 4× halo-gas binding energy, galaxies enter the “green valley” and begin to quench. The crossing point is clearly visible as the slanted boundary between star-forming galaxies and quenched galaxies in mass vs. radius and central stellar density. Data then suggest that BH growth accelerates in the green valley and that >90% of BH growth takes place there. When fully quenched, galaxies stop evolving and come to a halt along the classic MBH ~ σ4 relation, which is offset by about a ×10 higher in zero point than the BH scaling law for star-forming galaxies. This simple picture matches many important scaling laws for BHs and galaxies since z ~ 3. It portrays galaxy quenching as a contest between black holes and halos — halos build galaxies, which then build black holes, which then react back and quench halos.