The evolution of overdense regions in cluster gas

This paper considers the evolution of overdense regions in cooling flows, with particular emphasis on non-linear thermal instability and associated mass deposition. The effects of ram pressure drag, magnetic fields and large-amplitude perturbations are included. In the absence of fragmentation, almost all small overdensity perturbations evolve towards a stable size-overdensity relation. The mass spectrum of such blobs is calculated. As the gas flows inwards the blobs fragment and the least massive perturbations become unstable and are injected into the flow; thus the mass spectrum of inhomogeneities generated by the inflow is known. Magnetic fields play an important part in the evolution of overdense regions, eventually dominating the pressure and providing an important energy source in later evolution. The amount of energy present in the magnetic field is sufficient to explain the observed optical emission. (author)