The cluster of galaxies A754 is undergoing a merger of several large structural units. X-ray observations show the nonequilibrium state of the central part of the cluster, in which a cloud of cold plasma ∼500 kpc in size was identified amid the hotter cluster gas. The X-ray image of A754 exhibits a brightness discontinuity, which can be interpreted as a shock wave in front of a moving cloud of dense gas. The shock parameters are determined from the jump in intergalactic gas density using the ROSAT image. The estimated Mach number is M 1 = 1.71 −0.24 +0.45 at a 68% confidence level.
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A 2003 study studied this question.