Tiny galaxies have hearts of darkness

 Small ghost galaxies, devoid of stars but harboring dense clumps of invisible matter, may outnumber the entire population of luminous galaxies in the universe.

 John Kormendy of the University of Hawaii in Honolulu and Kenneth C. Freeman of the Mt. Stromlo Observatory in Canberra, Australia, base that assertion on studies by several teams over the past 20 years that have traced the motion of stars and gas in a wide range of galaxies. Kormendy reported the results Jan. 6 at a meeting of the American Astronomical Society in Austin, Texas.

 Since the late 1970s, astronomers have come to accept that at least 90 percent of the matter in the universe is invisible. Studies of gas at the fringes of many galaxies show greater orbital velocities than the gravity exerted by visible matter can explain. Researchers conclude that some kind of unseen matter, dubbed dark matter, keeps the rapidly orbiting material from flying away. Other studies have shown that the tug of visible matter is too small to account for the velocity of stars in gas-poor galaxies.

 Analysis of several studies led Kormendy and Freeman to conclude that the tinier the galaxy, the higher its density of dark matter. Although dwarf galaxies are barely detectable fuzz balls of gas and dust, they contain dark matter with densities 100 times larger than those in giant galaxies, notes Kormendy.

"That's a result that has been hinted at for quite a long time," comments Rosemary F.G. Wyse of Johns Hopkins University in Baltimore. Kormendy agrees, noting that the finding only became clear-cut as more and diverse galaxies have been studied.

 Dwarf galaxies are known to be far more numerous than larger galaxies. By extrapolation, Kormendy and Freeman argue that galaxies with too few stars to be seen at all have even higher densities of dark matter and may be the most populous. It's even possible-but by no means certain-that these unseen galaxies, estimated to be only one ten-thousandth as massive as the Milky Way, could account for a significant fraction of the mass of the universe, Kormendy suggests.

 He explains how dark matter became such a prominent component of the small galaxies. When the first massive stars in these galaxies died, some exploded as supernovas. Because the dwarfs have relatively little mass, the explosions easily drove gas-the raw material of stars-out of the galaxies. The dark matter stayed put, however, because it reacts only to gravity. The galaxies, believed to be the first to form, became pristine relics of the dark-matter content of the early cosmos, Kormendy says.

 Wyse speculates that a dwarf galaxy recently discovered near the Milky Way has maintained its shape despite the tug of our galaxy because it possesses a dense core of dark material. The interaction between this seemingly gossamer galaxy and our own might explain the warp of the outer edge of the Milky Way, she adds.