The origin of cold gas in giant elliptical galaxies and its role in
  fueling radio-mode AGN feedback
  We present a multi-wavelength study of the interstellar medium in eight
nearby, X-ray and optically bright, giant elliptical galaxies. Using Herschel
PACS, we map the cold gas in the lines of [CII], [OI], and [OIb]. Additionally,
we present Ha+[NII] imaging of warm ionized gas with the SOAR telescope, and a
study of the hot X-ray emitting plasma with Chandra. All systems with extended
Ha emission in our sample (6/8 galaxies) display significant [CII] line
emission indicating the presence of cold gas. This emission is co-spatial with
the Ha+[NII] emitting nebulae and the lowest entropy X-ray emitting plasma. The
entropy profiles of the hot galactic atmospheres show a clear dichotomy, with
the systems displaying extended emission line nebulae having lower entropies
beyond r~1 kpc than the cold-gas-poor systems. We show that while the hot
atmospheres of the cold-gas-poor galaxies are thermally stable outside of their
innermost cores, the atmospheres of the cold-gas-rich systems are prone to
cooling instabilities. This result indicates that the cold gas is produced
chiefly by thermally unstable cooling from the hot phase. We show that cooling
instabilities may develop more easily in rotating systems and discuss an
alternative condition for thermal instability for this case. The hot
atmospheres of cold-gas-rich galaxies display disturbed morphologies indicating
that the accretion of clumpy multiphase gas in these systems may result in
variable power output of the AGN jets, potentially triggering sporadic, larger
outbursts. In the two cold-gas-poor, X-ray morphologically relaxed galaxies of
our sample, NGC 1399 and NGC 4472, powerful AGN outbursts may have destroyed or
removed most of the cold gas from the cores, allowing the jets to propagate and
deposit most of their energy further out, increasing the entropy of the hot
galactic atmospheres and leaving their cores relatively undisturbed.
