Significant substructure is evident, probably due to recent mergers.
Clusters of galaxies are the largest gravitationally bound objects in the universe.
Their formation and evolution are complex, large-scale events, involving many interacting physical processes, which span a significant fraction of the age of the universe. In addition to being intrinsically complex and interesting, their properties permit us to constrain many quantities of cosmological interest, such as the ratio of baryonic to dark matter and the rate of expansion of the universe. Click on the image to download a 300 DPI JPEG version. Clusters are not static entities. Indeed, hierarchical models of large-scale structure formation lead us to expect that clusters should have formed through mergers of smaller objects.