AGN Feedback and Multi-phase Gas in Giant Elliptical Galaxies
  Recent observations have found extended multi-phase gas in a significant
fraction of massive elliptical galaxies. We perform high-resolution
three-dimensional hydrodynamical simulations of two idealized elliptical
galaxies -- one representing a typical galaxy characterized by initial
conditions conducive to the development of thermal instability and the other
one less likely to develop thermal instability -- in order to study the
development of thermal instability and the formation of multi-phase structures.
We analyze the interplay between radiative cooling, momentum-driven AGN
feedback, star formation, and stellar feedback from both young and old stars.
We find that in one class of elliptical galaxies, the entropy of the hot halo
gas rises sharply as a function of radius, and the hot halo is thermally stable
and run-away cooling can only happen in the very center of the galaxy. In other
class of ellipticals, the hot halo gas has a cooling to free-fall time ratio
close to 10, and the non-linear perturbation driven by AGN feedback can cause
the hot gas to frequently precipitate into extended multi-phase filaments. Both
multi- and single-phase elliptical galaxies experience cooling-driven AGN
feedback cycles. Interestingly, AGN feedback maintains the multi- or
single-phase nature of the halo but does not turn multi-phase galaxies into
single-phase ones or vice versa. Some of the extended cold gas in the
multi-phase galaxy also forms young stars. The level of star formation and its
spatial distribution are in excellent agreement with {\it Hubble} observations
of nearby elliptical galaxies.
