Abstract Observations of galaxy-cluster cores reveal that AGN feedback is strongly associated with both a short central cooling time (tc ≲ 109 yr) and accumulations of cold gas (≲ 104 K). Also, the central ratio of cooling time to freefall time is rarely observed to drop below tc/tff ≈ 10, and large accumulations of cold gas are rarely observed in environments with tc/tff ≳ 30. Here we show that the critical range — 10 ≲ tc/tff ≲ 30 — plausibly results from magnetized thermal instability. We present numerical simulations of magnetized stratified atmospheres with an initially uniform magnetic field. Thermal instability in an otherwise static atmosphere with tc/tff ≈ 10 progresses to nonlinear amplitudes, causing cooler gas to accumulate, as long as the background ratio of thermal pressure to magnetic pressure is β ≲ 100. And in atmospheres with tc/tff ≈ 20, cooler gas accumulates for β ≲ 10. Magnetized atmospheres are therefore much more likely to precipitate than unmagnetized atmospheres with otherwise identical properties. We hypothesize that AGN feedback triggered by accumulations of cold gas prevents tc/tff from dropping much below 10, because cold gas inevitably precipitates out of magnetized galactic atmospheres with lower ratios, causing tc/tff to rise.
Wibking et al. (Sat,) studied this question.