  We present results from a deep Chandra X-ray observation of a merging galaxy
cluster A520. A high-resolution gas temperature map, after the subtraction of
the cluster-scale emission, reveals a long trail of dense, cool clumps ---
apparently the fragments of a cool core that has been completely stripped from
the infalling subcluster by ram pressure. In this scenario, we can assume that
the clumps are still connected by the magnetic field lines. The observed
temperature variations imply that thermal conductivity is suppressed by a
factor >100 across the presumed direction of the magnetic field (as found in
other clusters), and is also suppressed -along- the field lines by a factor of
several. Two massive clumps in the periphery of A520, visible in the weak
lensing mass map and the X-ray image, have apparently been completely stripped
of gas during the merger, but then re-accreted the surrounding high-entropy gas
upon exit from the cluster. An X-ray hydrostatic mass estimate for one of the
clumps (that has simple geometry) agrees with the lensing mass. Its current gas
mass to total mass ratio is very low, 1.5-3%, which makes it a "dark
subcluster". We also found a curious low X-ray brightness channel (likely a
low-density sheet in projection) going across the cluster along the direction
of an apparent secondary merger. The channel may be caused by plasma depletion
in a region of an amplified magnetic field (with plasma $\beta\sim 10-20$). The
shock in A520 will be studied in a separate paper.
