Building a cluster: shocks, cavities, and cooling filaments in the
  group-group merger NGC 6338
  We present deep Chandra, XMM-Newton, Giant Metrewave Radio Telescope and
Halpha observations of the group-group merger NGC 6338. X-ray imaging and
spectral mapping show that as well as trailing tails of cool, enriched gas, the
two cool cores are embedded in an extensive region of shock heated gas with
temperatures rising to ~5 keV. The velocity distribution of the member galaxies
show that the merger is occurring primarily along the line of sight, and we
estimate that the collision has produced shocks of Mach number M=2.3 or
greater, making this one of the most violent mergers yet observed between
galaxy groups. Both cool cores host potential AGN cavities and Halpha nebulae,
indicating rapid radiative cooling. In the southern cool core around NGC 6338,
we find that the X-ray filaments associated with the Halpha nebula have low
entropies (<10 kev cm^2) and short cooling times (~200-300 Myr). In the
northern core we identify an Halpha cloud associated with a bar of dense, cool
X-ray gas offset from the dominant galaxy. We find no evidence of current jet
activity in either core. We estimate the total mass of the system and find that
the product of this group-group merger will likely be a galaxy cluster.
