A compression front associated with bulk ICM motion

The apparent displacement of the center of the cavities from the cluster center and the different position angle between the extended radio emission and the cavities suggests bulk motion of the ICM flow. Noting that the line connecting the centers of the two cavities misses the radio-galaxy core by approximately 10arcsec, corresponding to 10kpc, we can use estimates of the age of the radio source ($\sim 20$Myr; Heinz et al. 2002) to estimate that the ICM is flowing past the cD galaxy at a velocity of $500{\rm\thinspace km}\,{\rm\thinspace s}^{-1}$ projected onto the plane of the sky. The different position angle between the axis connecting the ghost cavities and the current extended radio emission demands either a change in the radio-axis itself, or some rotation in the ICM flow. Both numerical simulations (e.g., Roettiger, Loken & Burns 1997) and Chandra observations (e.g., Markevitch et al. 2003) suggest this kind of large scale ICM ``sloshing'' can readily occur after a major cluster merger.

In this picture, the bright and cool SW ridge is located at the position where we expect the radio-galaxy induced expanding ICM shell to be maximally compressed by the ICM flow. The sharp SW edge of this feature is readily interpreted at the interface between the ambient ICM (which we suppose is flowing in a NE direction) and the expanding ICM shell formed by the same period of radio-galaxy activity that formed the X-ray cavities.

One might expect that such compression would heat this material, contrary to observations. However, the fact that the cooling time of the ridge material is small demands that we consider radiative cooling effects. In the simplest case of adiabatic compression in the bremsstrahlung regime, the cooling time is proportional to $n^{-2/3}$. Hence, a weak shock will slightly reduce the cooling timescale. Radiative cooling can be further aided by the kinematics of the radio-galaxy/cluster interaction, which keeps this material in the high pressure regions of the cluster core for longer. Even given this, there appears to be a fine tuning problem; it is difficult to explain the cooling of the SW ridge unless it was on the verge of undergoing dramatic radiative cooling anyways. A detailed exploration of these hydrodynamical and radiative questions will be deferred until future publications.