Black holes set a gas thermostat, not an engine, in massive galaxies

Rather than acting as engines that expel gas, black holes impose an entropy ceiling on the surrounding halo; buoyant gas then floats outward to the virial radius on its own. Analysis of the FLAMINGO cosmological simulations shows this ceiling is mass-independent for isotropic thermal feedback but shifts with jet geometry. Above a critical halo mass near 10^13.5–14 solar masses, virial shocks overcome the ceiling entirely—predicting a resumption of star formation in the universe's most massive galaxies, a signal the authors find in low-redshift star-formation rates and morphologies.