Number 109 (Story #2), January 8, 1993 by Phillip F. Schewe and Ben Stein|
A DENSE CLUMP OF DARK MATTER apparently lurks within a small group of galaxies, the NGC 2300 group (containing only 3 galaxies), according to scientists using the German satellite Rosat. The mass of the dark matter was estimated to be 10 to 30 times that of the visible matter in the group, a ratio much higher than was expected for such a small group of galaxies. Dark matter cannot be observed directly of course; rather, Rosat measured x rays coming from a cloud of hot gas inside the group. The compact extent of the cloud (1.3 million light years across) and its high temperature (10 million K) cannot be accounted for by the constraining effects of the visible galaxies alone, but only by the additional shepherding presence of the dark matter. Richard Mushotzky of NASA/Goddard, one of the scientists to report on the Rosat results at the AAS meeting, suggested that if the density of dark matter inferred for NGC 2300 were typical of other areas in the universe---and small galaxy groups are more common than rich clusters (containing hundreds or thousands of galaxies) which, ironically, seem to have a lower-than-expected amount of dark matter---then there might be enough matter (dark plus visible) present to brake and reverse the expansion of the universe.