Formation of multiphase plasma in galactic haloes and an analogy to solar plasma

Abstract

Thermal instability (TI) potentially explains the origin of cold gas in the intracluster medium (ICM), which is heated sufficiently by AGN feedback. The Hαfilaments seen in cluster cores provide strong motivation for TI. The hot (∼107K) ICM coronae allow the growth of isobaric TI. The multiphase medium (cold-dense—hot-diffuse) forms once TI saturates. However, gravitational stratification can spatially constrain TI, and thermal conduction is known to stabilize all scales below the field length (λF). In addition, the transport of energy is anisotropic along magnetic fields. Thermal conduction may further trigger gyroscale instabilities and effective reduction ofλF. However, cold gas at small scales(<λF)needs to be verified in observations. The virial temperature in galactic haloes is lower (∼106K) and opens the regime of isochoric TI. In this regime, the cooling time is typically shorter than the sound-crossing time, and large-scale isochoric clouds are rendered unstable. The linear and non-linear isochoric clouds have interesting differences which potentially lead to either fragmentation of the cloud or not. On saturation, TI produces a turbulent medium that helps mix phases and thermalize kinetic energy and thus completes a cycle of condensation and heating. Various aspects of condensation, stratified turbulence, and magnetized transport are physically identical in solar coronae but scaled down to lower luminosity (similar temperatures). We will discuss the recent progress in TI, its connection to observations, and the analogy to solar prominences.