2025/06/12 by Benjamin D. Wibking, Wibking, Benjamin D., G. Mark Voit +3 · 1 citation
Engineering · Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #Space Technology and Applications
paper · pdf · doi:10.48550/arxiv.2506.10277
openalex publication_date 2025/06/12 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Observations of galaxy-cluster cores reveal that AGN feedback is strongly associated with both a short central cooling time (t\rm c \lesssim 109 \rm yr) and accumulations of cold gas (\lesssim 104 \rm K). Also, the central ratio of cooling time to freefall time is rarely observed to drop below t\rm c/t\rm ff ≈ 10, and large accumulations of cold gas are rarely observed in environments with t\rm c / t\rm ff \gtrsim 30. Here we show that the critical range -- 10 \lesssim t\rm c/t\rm ff \lesssim 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 t\rm c/t\rm ff ≈ 10 progresses to nonlinear amplitudes, causing cooler gas to accumulate, as long as the background ratio of thermal pressure to magnetic pressure is β\lesssim 100. And in atmospheres with t\rm c/t\rm ff ≈ 20, cooler gas accumulates for β\lesssim 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 t\rm c/t\rm ff from dropping much below 10, because cold gas inevitably precipitates out of magnetized galactic atmospheres with lower ratios, causing t\rm c/t\rm ff to rise.