A Cluster of Galaxies Four Billion Light-years Away

This is a NASA Hubble Space Telescope image of a portion of a remote cluster of galaxies (CL 0939+4713) that existed when the universe was two-thirds of its present age (redshift z = 0.4). Hubble's high resolution allows astronomers to study, for the first time, the shapes of galaxies which existed long ago. The Space Telescope pictures are sharp enough to distinguish between various forms of spiral galaxies, as well as galaxies in collision, some tearing material from each other, some merging into single systems. The HST picture confirms that billions of years ago, clusters of galaxies contained not only the types of galaxies dominating their descendant clusters today, but also several times as many spiral galaxies. These spiral galaxies have since disappeared through possibly a variety of processes: merger1 disruption, and fading. The Hubble images provide the first unambiguous evidence for the influence of environment on the form of a galaxy. The image was taken with HST's Wide Field/Planetary Camera in Wide Field Camera mode, and required a six-hour exposure.