On saturation, TI produces a turbulent medium that helps mix phases and thermalize kinetic energy and thus completes a cycle of condensation and heating. The linear and non-linear isochoric clouds have interesting differences which potentially lead to either fragmentation of the cloud or not. In this regime, the cooling time is typically shorter than the sound-crossing time, and large-scale isochoric clouds are rendered unstable. The virial temperature in galactic haloes is lower ( ∼ 1 0 6 K) and opens the regime of isochoric TI. However, cold gas at small scales ( < λ F ) needs to be verified in observations. Thermal conduction may further trigger gyroscale instabilities and effective reduction of λ F. In addition, the transport of energy is anisotropic along magnetic fields. However, gravitational stratification can spatially constrain TI, and thermal conduction is known to stabilize all scales below the field length ( λ F). The multiphase medium (cold-dense-hot-diffuse) forms once TI saturates. The hot ( ∼ 1 0 7 K) ICM coronae allow the growth of isobaric TI. The H α filaments seen in cluster cores provide strong motivation for TI. ![]() Thermal instability (TI) potentially explains the origin of cold gas in the intracluster medium (ICM), which is heated sufficiently by AGN feedback. ![]() Institute of Astronomy, University of Cambridge, Cambridge, United Kingdom.
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