A Merger Shock in Abell 2034
  We present a $250\,$ks Chandra observation of the cluster merger A2034 with
the aim of understanding the nature of a sharp edge previously characterized as
a cold front. The new data reveal that the edge is coherent over a larger
opening angle and is significantly more bow-shock-shaped than previously
thought. Within $\sim 27\,$degrees about the axis of symmetry of the edge the
density, temperature and pressure drop abruptly by factors of
$1.83^{+0.09}_{-0.08}$, $1.85^{+0.41}_{-0.41}$ and $3.4^{+0.8}_{-0.7}$,
respectively. This is inconsistent with the pressure equilibrium expected of a
cold front and we conclude that the edge is a shock front. We measure a Mach
number $M = 1.59^{+0.06}_{-0.07}$ and corresponding shock velocity $v_{\rm
shock}\simeq 2057\,$km/s. Using spectra collected at the MMT with the Hectospec
multi-object spectrograph we identify 328 spectroscopically confirmed cluster
members. Significantly, we find a local peak in the projected galaxy density
associated with a bright cluster galaxy which is located just ahead of the nose
of the shock. The data are consistent with a merger viewed within $\sim
23\,$degrees of the plane of the sky. The merging subclusters are now moving
apart along a north-south axis approximately $0.3\,$Gyr after a small impact
parameter core passage. The gas core of the secondary subcluster, which was
driving the shock, appears to have been disrupted by the merger. Without a
driving 'piston' we speculate that the shock is dying. Finally, we propose that
the diffuse radio emission near the shock is due to the revival of pre-existing
radio plasma which has been overrun by the shock.
