Precipitation models indicate that cooling instabilities form when the ratio of cooling time to free-fall in hot atmospheres surrounding massive galaxies falls below ~10, i.e., t_c/t_ff < 10. We show that this criterion is rarely met in central galaxies and is a statistically weaker indicator of cooling instabilities than cooling time alone. Hot atmospheres apparently do not experience large swings in gas density and cooling time in response to powerful AGN outbursts, which is an essential aspect of precipitation models. Based primarily on new ALMA observations, we show instead that molecular gas likely forms from low entropy gas caught in the updraft of rising X-ray bubbles. We suggest that thermal instabilities ensue when the ratio of the cooling time to the infall time of gas lifted to high altitudes falls below unity, i.e., t_c/t_I ~ 1.
Thermal Instability in Low Entropy Gas Lifted behind Buoyantly-Rising X-ray Bubbles.
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