Our simulations must be viewed as a rather limited toy model for the buoyant evolution of radio-lobes and the formation of ghost cavities. We have employed two major simplifications. Firstly, we have not modeled the jet-driven inflation of the bubble. Real examples of these radio-lobes will possess complex, jet-driven flows with speeds much in excess of the ICM sound speed, and will never resemble the static bubbles that we use for our initial conditions (Reynolds, Heinz & Begelman 2002). Consequently, the KH instability will be important at all stages in the life of the bubble, and does not need to wait for the RT instability to first induce bulk flows.
Secondly, we have neglected the effects of magnetic field. It is expected that fields within the radio plasma could readily achieve equipartition strengths and hence have an important effect on the dynamics of the rising bubble. In particular, magnetic fields will influence the development of both the RT and the KH instability at the contact discontinuity. The effect of magnetic fields on the buoyant evolution of ICM bubbles has recently been addressed in the context of inviscid hydrodynamics by Brüggen & Kaiser (2001), Robinson et al. (2003) and De Young (2003). Their results suggest that magnetic fields can have a strongly stabilizing effect on buoyant bubbles, effectively quenching all of the instabilities that distort and eventually shred this structures. However, both the symmetry of their calculation (2-d Cartesian) and their initial magnetic field configuration (taken from Cargill et al. 1996) makes the Robinson et al. (2003) calculation most relevant to a buoyantly rising cylindrical flux tube with purely toroidal magnetic flux, a poor approximation to an AGN blown bubble. The ability of magnetic fields to stabilize or de-stabilize the contact discontinuity is a strong function of the magnetic configuration and field strength (e.g., see Jun, Norman & Stone 1995), both of which are open issues in the case of AGN-blown bubbles.
Clearly further simulations are required to address these two issues. In fact, it would be most appropriate to treat these as coupled questions. It is believed that the magnetic field in such bubbles originates from the magnetized AGN jets. Thus, we must follow the jet-driven inflation of the bubble in order to produce a reasonable initial field configuration. This will be the subject a future publication.