Thermostats, Not Engines: A New Picture of Halo Gas Regulation

We propose that black hole feedback regulates gas in massive halos by establishing an entropy ceiling; the resulting buoyant gas migrates to the virial radius with no additional energy input required. The FLAMINGO simulations support this picture: at the virial radius, outflow entropy is mass-independent for isotropic thermal feedback but depends on the solid angle of directly heated gas for jet feedback. Above a critical halo mass $M_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}\approx {10}^{13.5\text{–}14}{M}_{\odot }$, virial shocks overwhelm the ceiling, predicting rejuvenation of star formation in the most massive galaxies, supported by new low-redshift evidence from star formation rates and morphologies.