A Spectacular Bow Shock in the 11 keV Galaxy Cluster Around 3C 438
  We present results of deep 153 ks Chandra observations of the hot, 11 keV,
galaxy cluster associated with the radio galaxy 3C 438. By mapping the
morphology of the hot gas and analyzing its surface brightness and temperature
distributions, we demonstrate the presence of a merger bow shock. We identify
the presence of two jumps in surface brightness and in density located at
$\sim$400 kpc and $\sim$800 kpc from the cluster's core. At the position of the
inner jump, we detect a factor of $2.3\pm 0.2$ density jump, while at the
location of the outer jump, we detect a density drop of a factor of $3.5 \pm
0.7$. Combining this with the temperature distribution within the cluster, we
establish that the pressure of the hot gas is continuous at the 400 kpc jump,
while there is a factor of $6.2 \pm 2.8$ pressure discontinuity at 800 kpc
jump. From the magnitude of the outer pressure discontinuity, using the
Rankine-Hugoniot jump conditions, we determine that the sub-cluster is moving
at $M = 2.3\pm 0.5$, or approximately $2600\pm 565$ km/s through the
surrounding intracluster medium, creating the conditions for a bow shock. Based
on these findings, we conclude that the pressure discontinuity is likely the
result of an ongoing major merger between two massive clusters. Since few
observations of bow shocks in clusters have been made, this detection can
contribute to the study of the dynamics of cluster mergers, which offers
insight on how the most massive clusters may have formed.
