Magazine article Science News

Tiny Galaxies Have Hearts of Darkness

Magazine article Science News

Tiny Galaxies Have Hearts of Darkness

Article excerpt

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. …

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