A Mechanism for Stimulating AGN Feedback by Lifting Gas in Massive
  Galaxies
  Observation shows that nebular emission, molecular gas, and young stars in
giant galaxies are associated with rising X-ray bubbles inflated by radio jets
launched from nuclear black holes. We propose a model where molecular clouds
condense from low entropy gas caught in the updraft of rising X-ray bubbles.
The low entropy gas becomes thermally unstable when it is lifted to an altitude
where its cooling time is shorter than the time required to fall to its
equilibrium location in the galaxy i.e., t_c/t_I < 1. The infall speed of a
cloud is bounded by the lesser of its free-fall and terminal speeds, so that
the infall time here can exceed the the free-fall time by a significant factor.
This mechanism is motivated by ALMA observations revealing molecular clouds
lying in the wakes of rising X-ray bubbles with velocities well below their
free-fall speeds. Our mechanism would provide cold gas needed to fuel a
feedback loop while stabilizing the atmosphere on larger scales. The observed
cooling time threshold of ~ 5x 10^8 yr --- the clear-cut signature of thermal
instability and the onset of nebular emission and star formation--- may result
from the limited ability of radio bubbles to lift low entropy gas to altitudes
where thermal instabilities can ensue. Outflowing molecular clouds are unlikely
to escape, but instead return to the central galaxy in a circulating flow. We
contrast our mechanism to precipitation models where the minimum value of
t_c/t_ff < 10 triggers thermal instability, which we find to be inconsistent
with observation.
