  The inefficiency of star formation in massive elliptical galaxies is widely
believed to be caused by the interactions of an active galactic nucleus (AGN)
with the surrounding gas. Achieving a sufficiently rapid reddening of
moderately massive galaxies without expelling too many baryons has however
proven difficult for hydrodynamical simulations of galaxy formation, prompting
us to explore a new model for the accretion and feedback effects of
supermassive black holes. For high accretion rates relative to the Eddington
limit, we assume that a fraction of the accreted rest mass energy heats the
surrounding gas thermally, similar to the `quasar mode' in previous work. For
low accretion rates, we invoke a new, pure kinetic feedback model which imparts
momentum into the surrounding gas in a stochastic manner. These two modes of
feedback are motivated both by theoretical conjectures for the existence of
different types of accretion flows as well as recent observational evidence for
the importance of kinetic AGN winds in quenching galaxies. We find that a large
fraction of the injected kinetic energy in this mode thermalises via shocks in
the surrounding gas, thereby providing a distributed heating channel. In
cosmological simulations, the resulting model produces red, non star-forming
massive elliptical galaxies, and achieves realistic gas fractions, black hole
growth histories and thermodynamic profiles in large haloes.
