Thermal instability in cooling flows

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Abstract
The development of thermal instabilty in cooling flows is considered in detail. Gravitationally induced convection affects the structure of the density fluctuations, keeping the flow well mixed, but does not significantly suppress the overall cooling rate of thermally unstable gas. It is argued that in the bulk of most cooling flows convection is negligible and that the flow can be modelled as comoving. The equations governing inhomogeneous steady cooling flows are introduced and solved for illustrative cases of comoving flows. The form of flows with rapid large-scale convection is also calculated, although it does not fit the available data well. Inhomogeneous cooling flow solutions have many features in common with those for homogeneous cooling flows, but the mass-flow rate is generally a strong function of radius so that cooled gas is deposited widely. The density fluctuations required to cause widespread cooling of the gas are not very large, and must be present in the general intracluster medium. This suggests that widespread cooling can be occurring in all X-ray emitting clusters and groups of galaxies and that cooling flows are only present in those where the cooling gas in focused on to a well defined central galaxy.

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