The bow shock, cold fronts and disintegrating cool core in the merging
  galaxy group RXJ0751.3+5012
  We present a new Chandra X-ray observation of the off-axis galaxy group
merger RXJ0751.3+5012. The hot atmospheres of the two colliding groups appear
highly distorted by the merger. The images reveal arc-like cold fronts around
each group core, produced by the motion through the ambient medium, and the
first detection of a group merger shock front. We detect a clear density and
temperature jump associated with a bow shock of Mach number M=1.9+/-0.4 ahead
of the northern group. Using galaxy redshifts and the shock velocity of
1100+/-300 km/s, we estimate that the merger axis is only 10deg from the plane
of the sky. From the projected group separation of 90 kpc, this corresponds to
a time since closest approach of 0.1 Gyr. The northern group hosts a dense,
cool core with a ram pressure stripped tail of gas extending 100 kpc. The
sheared sides of this tail appear distorted and broadened by Kelvin-Helmholtz
instabilities. We use the presence of this substructure to place an upper limit
on the magnetic field strength and, for Spitzer-like viscosity, show that the
development of these structures is consistent with the critical perturbation
length above which instabilities can grow in the intragroup medium. The
northern group core also hosts a galaxy pair, UGC4052, with a surrounding IR
and near-UV ring 40 kpc in diameter. The ring may have been produced by tidal
stripping of a smaller galaxy by UGC4052 or it may be a collisional ring
generated by a close encounter between the two large galaxies.
