5. SUMMARY

Cold fronts are important probes of plasma physics in galaxy clusters. We presented a review of recent simulation works, along with examples of relevant observations, which have provided the strongest constraints on the properties of the fronts themselves as well as the ICM physics implied by these features. Many insights have been gained from these simulation studies, but a number of open questions remain. To summarize:

• Recent simulations conclusively demonstrate that the surfaces of cold fronts should be locations of strong magnetic field layers oriented parallel to the front surface, whether on the outside of the front (as in remnant-core cold fronts) or the inside of the front (as in sloshing cold fronts). Such layers may suppress KHI, explaining the fact that KHI do not appear to be present in many observations of cold fronts. However, whether or not this is the case depends strongly on the initial magnetic field strength of the surrounding medium.
• In the absence of strong magnetic fields, viscosity may play an important role in explaining the lack of evidence of KHI in many cold fronts. Recent simulations have demonstrated that even a small viscosity may be enough to explain the observed features of some cold fronts. On the other hand, a number of cold fronts do show evidence of KHI, in the form of distortions such as the “horns” seen in the cold front of the remnant-core cold front in M89, and the “ragged” appearance of some cold fronts as seen in NGC 7618, UGC 12491, and A496. In these systems, such features imply a low viscosity for the cluster plasma. More detailed comparisons between observations and simulations for a variety of systems are necessary to determine what range of Reynolds number is permissible in the ICM.
• Where KHI suppression is observed in cold fronts, it is not clear which effect is likely to be more responsible–magnetic fields or viscosity? The most straightforward way to break this degeneracy is to use long-exposure Chandra observations of cold fronts to attempt to measure the thermal pressure deficit at the interface arising from the presence of a strongly magnetized layer. More quantitative comparisons of the observable signatures of cold fronts predicted from simulations with magnetic fields and viscosity are also needed.
• Whether or not magnetic fields suppress thermal conduction across cold fronts appears to depend on the type of cold front. Simulations of remnant-core cold fronts suggest that the highly magnetized layers effectively prevent the flow of heat to the colder side of the front. In contrast, for sloshing cold fronts, simulations predict that the regions above and below the cold fronts are connected with magnetic fields along which sufficient heat conduction can occur, erasing the sharp cold fronts. Though there are geometrical differences between the two types of cold fronts which may be responsible for this difference, the remnant-core simulations of this effect performed so far have low resolution. More MHD simulations of remnant-core cold fronts with anisotropic thermal conduction are needed to determine if these differences between these two types of cold fronts persist at higher resolution or is numeric in nature.
• The focus of future simulations should be to employ the most detailed and computationally feasible physical models available. This includes the full modeling of the Braginskii-MHD equations, with possible Larmor-radius-scale corrections as provided by future kinetic simulations of the cluster plasma. From these, more accurate hydrodynamic approximations may be derived. The role of magnetic reconnection near cold front surfaces should also be investigated.
• In future comparisons between simulations and observations of cold fronts, careful attention needs to be paid to predicting the distinct observable signatures of cold fronts under different assumptions for the underlying physics, since only if we can recognize and characterize these signatures accurately can they be used as a probe of the ICM physics. The most accurate comparisons to the observations are facilitated by synthetic X-ray observations of cold fronts, which include the effects of Poisson noise and instrumental responses.
• A more accurate determination of the plasma properties of the ICM from cold fronts will be provided by the next generation of X-ray telescopes with high-effective area and high spectral resolution, such as Athena and X-ray Surveyor, though the latter will be essential for examining the cold front interfaces with the same spatial resolution as Chandra.

The authors would like to thank the reviewers, Eugene Churazov and Chris Reynolds, for their reading of this manuscript and the helpful comments. JAZ acknowledges support from NASA though subcontract SV2-8203 to MIT from the Smithsonian Astrophysical Observatory, and through Chandra Award Number G04-15088X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060.