Cosmic Diagnosis; June 1993; Scientific American Magazine; by Corey S. Powell; 1 Page(s)

Like doctors, astronomers are finding that x-rays offer an invaluable means for examining otherwise hidden structures. Last year Trevor Ponman and his colleagues at the University of Birmingham in England announced that xray observations of hot gas in the Coma galaxy cluster show that the cluster¿s mass follows a surprisingly complicated, lumpy distribution. "It supports the notion that clusters have grown by the accumulation of blobs of galaxy groups and that the process is still happening now," Ponman explains. That discovery is especially significant because the Coma cluster, located 300 million light-years away in the constellation Coma Berenices, is the nearest and one of the best-studied rich clusters of galaxies.

Simon D. M. White of the Institute of Astronomy at the University of Cambridge and his collaborators have since amplified and expanded on Ponman¿s findings. Using data collected by the Roentgen Satellite (ROSAT), White¿s group has produced an x-ray image of the Coma cluster revealing unprecedented detail (below). White describes his work as "x-ray archaeology" because it enables him to reconstruct the process by which the Coma cluster came together. "It¿s fairly clear that you can see the remnants of previous subclumps," White says. The bright extensions of the cluster, most clearly seen at the bottom right, consist of hot gas surrounding giant galaxies that probably were once the dominant objects in their own, smaller clusters before being swallowed and merging into Coma.