Abstract
Observation shows that cooling instabilities leading to nebular emission,
molecular gas, and star formation in giant galaxies are formed behind
buoyantly-rising X-ray bubbles inflated by radio jets launched from massive
nuclear black holes. We propose a model where molecular clouds condense from
hot but relatively low entropy gas lifted by X-ray bubbles to an altitude where
its cooling time is shorter than the time required for it to fall to its
equilibrium location in the galaxy i.e., t_c/t_I <~1$. Here the infall time can
exceed the free-fall time, t_ff, by factors of a few. This mechanism, which we
refer to as stimulated feedback, is motivated by recent ALMA observations of
central galaxies in clusters and groups revealing molecular clouds apparently
forming in the wakes of rising X-ray bubbles and with surprisingly low cloud
velocities. Supported by recent numerical simulations, our model would
naturally sustain a continual feedback-loop in galaxies fuelled by cooling gas
stimulated by radio-mechanical feedback itself, that otherwise stabilizes
cooling atmospheres on larger scales. The observed cooling time threshold for
the onset of nebular emission and star formation of ~ 5x10^8 yr may result from
the limited ability of radio bubbles to lift low entropy gas to an altitude
where thermal instabilities can ensue. The molecular clouds condensing from the
outflowing hot gas are unlikely to escape, but instead return to the central
galaxy in a circulating flow.
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