  We present the results of a very deep (500 ks) Chandra observation, along
with tailored numerical simulations, of the nearest, best resolved cluster cold
front in the sky, which lies 90 kpc (19 arcmin) to the north-west of M 87. The
northern part of the front appears the sharpest, with a width smaller than 2.5
kpc (1.5 Coulomb mean free paths; at 99 per cent confidence). Everywhere along
the front, the temperature discontinuity is narrower than 4-8 kpc and the
metallicity gradient is narrower than 6 kpc, indicating that diffusion,
conduction and mixing are suppressed across the interface. Such transport
processes can be naturally suppressed by magnetic fields aligned with the cold
front. Interestingly, comparison to magnetohydrodynamic simulations indicates
that in order to maintain the observed sharp density and temperature
discontinuities, conduction must also be suppressed along the magnetic field
lines. However, the northwestern part of the cold front is observed to have a
non-zero width. While other explanations are possible, the broadening is
consistent with the presence of Kelvin-Helmholtz instabilities (KHI) on
length-scales of a few kpc. Based on comparison with simulations, the presence
of KHI would imply that the effective viscosity of the intracluster medium is
suppressed by more than an order of magnitude with respect to the isotropic
Spitzer-like temperature dependent viscosity. Underneath the cold front, we
observe quasi-linear features that are ~10 per cent brighter than the
surrounding gas and are separated by ~15 kpc from each other in projection.
Comparison to tailored numerical simulations suggests that the observed
phenomena may be due to the amplification of magnetic fields by gas sloshing in
wide layers below the cold front, where the magnetic pressure reaches ~5-10 per
cent of the thermal pressure, reducing the gas density between the bright
features.
