Precipitation plausible: magnetized thermal instability in the intracluster medium
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| Format: | Preprint |
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2025
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| _version_ | 1866918056368799744 |
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| author | Wibking, Benjamin D. Voit, G. Mark O'Shea, Brian W. |
| author_facet | Wibking, Benjamin D. Voit, G. Mark O'Shea, Brian W. |
| contents | Observations of galaxy-cluster cores reveal that AGN feedback is strongly associated with both a short central cooling time ($t_{\rm c} \lesssim 10^9 \, {\rm yr}$) and accumulations of cold gas ($\lesssim 10^4 \, {\rm K}$). Also, the central ratio of cooling time to freefall time is rarely observed to drop below $t_{\rm c}/t_{\rm ff} \approx 10$, 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 $t_{\rm c}/t_{\rm ff} \approx 10$ 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 $t_{\rm c}/t_{\rm ff} \approx 20$, 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2506_10277 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Precipitation plausible: magnetized thermal instability in the intracluster medium Wibking, Benjamin D. Voit, G. Mark O'Shea, Brian W. Astrophysics of Galaxies Observations of galaxy-cluster cores reveal that AGN feedback is strongly associated with both a short central cooling time ($t_{\rm c} \lesssim 10^9 \, {\rm yr}$) and accumulations of cold gas ($\lesssim 10^4 \, {\rm K}$). Also, the central ratio of cooling time to freefall time is rarely observed to drop below $t_{\rm c}/t_{\rm ff} \approx 10$, 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 $t_{\rm c}/t_{\rm ff} \approx 10$ 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 $t_{\rm c}/t_{\rm ff} \approx 20$, 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. |
| title | Precipitation plausible: magnetized thermal instability in the intracluster medium |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2506.10277 |