Most of the massive elliptical galaxies in the universe stopped forming stars billions of years ago , even though plenty of hot gas remains available for star formation .
Here we present compelling evidence indicating that quenching of star formation depends on both black-hole feedback and Type Ia supernova heating .
We analyze Chandra X-ray observations of ten massive ellipticals , five with extended , potentially star-forming multiphase gas and five single-phase ellipticals with no star formation .
The ratio of cooling time to freefall time at 1–10 kpc in the multiphase galaxies is t _ { cool } / t _ { ff } \approx 10 , indicating that precipitation-driven feedback limits cooling but does not eliminate condensation .
In the same region of the single-phase galaxies , the radial profiles of gas entropy are consistent with a thermally stable ( t _ { cool } / t _ { ff } > 20 ) supernova-driven outflow that sweeps stellar ejecta out of the galaxy .
However , in one of those single-phase ellipticals ( NGC 4261 ) we find t _ { cool } / t _ { ff } \lesssim 10 at < 300 pc .
Notably , its jets are \sim 50 times more powerful than in the other nine ellipticals , in agreement with models indicating that precipitation near the black hole should switch its fueling mode from Bondi-like accretion to cold chaotic accretion .
We conclude by hypothesizing that particularly strong black-hole outbursts can shut off star formation in massive elliptical galaxies by boosting the entropy of the hot gas and flipping the system into the supernova-sweeping state .