2015ApJ...803L..21V

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 (SN) heating. We analyze Chandra X-ray observations of 10 massive ellipticals, 5 with extended, potentially star-forming multiphase gas and 5 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 ≈ 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) SN-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 ≲ 10 at < 300 pc. Notably, its jets are ∼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 SN-sweeping state.