The currently operating X-ray imaging observatories provide us with an exquisitely detailed view of the Megaparsec-scale plasma atmospheres in nearby galaxy clusters. At z<0.05, the Chandra's 1" angular resolution corresponds to linear resolution of less than a kiloparsec, which is smaller than some interesting linear scales in the intracluster plasma. This enables us to study the previously unseen hydrodynamic phenomena in clusters: classic bow shocks driven by the infalling subclusters, and the unanticipated "cold fronts," or sharp contact discontinuities between regions of gas with different entropies. The ubiquitous cold fronts are found in mergers as well as around the central density peaks in "relaxed" clusters. They are caused by motion of cool, dense gas clouds in the ambient higher-entropy gas. These clouds are either remnants of the infalling subclusters, or the displaced gas from the cluster's own cool cores.

Both shock fronts and cold fronts provide novel tools to study the intracluster plasma on microscopic and cluster-wide scales, where the dark matter gravity, thermal pressure, magnetic fields, and ultrarelativistic particles are at play. In particular, these discontinuities provide the only way to measure the gas bulk velocities in the plane of the sky. The observed temperature jumps at cold fronts require that thermal conduction across the fronts is strongly suppressed. Furthermore, the width of the density jump in the best-studied cold front is smaller than the Coulomb mean free path for the plasma particles. These findings show that transport processes in the intracluster plasma can easily be suppressed. Cold fronts also appear less prone to hydrodynamic instabilities than expected, hinting at the formation of a parallel magnetic field layer via magnetic draping. This may make it difficult to mix different gas phases during a merger. A sharp electron temperature jump across the best-studied shock front has shown that the electron-proton equilibration timescale is much shorter than the collisional timescale; a faster mechanism has to be present. To our knowledge, this test is the first of its kind for any astrophysical plasma. We attempt a systematic review of these and other results obtained so far (experimental and numerical), and mention some avenues for further studies.

Table of contents:

• Introduction
• Cold fronts
  • Cold fronts in mergers
  • Origin and evolution of merger cold fronts
  • Cold fronts in cluster cool cores
    • Gas sloshing
    • Simulations of gas sloshing
    • Origin of density discontinuity
    • Effect of sloshing on cluster mass estimates
    • Effect of sloshing on cooling flows
    • Effect of sloshing on central abundance gradients
  • Zoology of cold fronts
• Cold fronts as experimental tool
  • Velocities of gas flows
  • Thermal conduction and diffusion across cold fronts
  • Stability of cold fronts
    • Rayleigh-Taylor instability and underlying mass
    • Kelvin-Helmholtz instability and magnetic field
    • Origin of magnetic layer
  • Possible future measurements using cold fronts
    • Plasma depletion layer and magnetic field
    • Effective viscosity of intracluster plasma
• Shock fronts as experimental tool
  • Cluster merger shocks
  • Mach number determination
  • Front width
  • Mach cone and reverse shock?
  • Test of electron-ion equilibrium
    • Comparison with other astrophysical plasmas
  • Shocks and cluster cosmic ray population
    • Shock acceleration
    • Compression of fossil electrons
    • Yet another method to measure intracluster magnetic field
• Summary