Impacts of a Supersonic Shock Front on Star Formation in the Bullet   Cluster
  We use the Bullet Cluster (1E0657-56) to investigate the extent to which star formation in cluster galaxies is influenced by ram pressure from supersonic gas (Mach 3) during a cluster merger. While the effects of ram pressure have been studied for individual galaxies infalling into galaxy clusters, this system provides a unique opportunity to investigate the impact of dramatic merger events on the cluster galaxy population. In this analysis we use {\it Spitzer} IRAC data to study star formation. At the redshift of the cluster the 6.2 $\mu$m PAH feature is redshifted into the 8 $\mu$m band, enabling use of the m$_{4.5}$-m$_{8}$ color as a proxy for specific star formation rate. We find that the color distribution on the two sides of the shock differ by less than 2$\sigma$, and conclude that ram pressure from the shock front has no dramatic, immediate impact on the star formation of cluster galaxies in the Bullet Cluster. 
Sun Mi Chung, Anthony H. Gonzalez, Douglas Clowe, Dennis Zaritsky,   Maxim Markevitch, Christine Jones