Why black hole jets hollow out galaxy groups but fail in clusters

Using the FLAMINGO cosmological simulation, the authors dissect AGN-driven outflows into three radial zones and show that shock heating saturates at a fixed entropy ceiling (~360 keV cm²) regardless of halo mass. Whether an outflow escapes depends on how that ceiling compares to the entropy of infalling gas, which climbs with halo mass following the virial relation. Below roughly 10^13.7 solar masses the outflow wins and gas is expelled; above that threshold the inflow entropy is too high, the outflow stalls near the splashback radius, and clusters retain close to the cosmic baryon fraction. The transition mass shifts predictably with feedback strength, providing a physical handle for calibrating simulations against X-ray and Sunyaev–Zel'dovich observations.