Phantoms in the Machine of Universe
Astronomers searching for mysterious missing matter in the universe are focusing on a new theory about “ghost galaxies” that contain few stars but are surrounded by dense halos of invisible dark matter.
An analysis of 43 galaxies shows that small, ghostly galaxies that appear to be little more than “fuzz balls” in powerful telescope images actually contain high densities of dark matter, with only a scattering of visible stars, John Kormendy of the University of Hawaii says.
Speaking here at the national meeting of the American Astronomical Society recently, Kormendy said there may be more of the small, dense galaxies than the bright, giant galaxies. They could thus contain “a significant portion” of the universe’s dark matter.
“There may be a large population of dark galaxies that contain too few stars to be discovered,” Kormendy said. “They may outnumber all of the luminous galaxies combined.”
The presence of invisible matter was determined years ago by astronomers who measured the motion of stars within galaxies. They determined that stars and clouds that shine and could be seen from Earth did not contain enough mass to hold the galaxies together.
Thus, there had to be other matter to provide the gravitational force that keeps the galaxies from flying apart.
In fact, some astronomers calculated that 90% of the matter in the universe cannot be seen, and thus is “missing.”
Because this matter is invisible, it must be dark and cold, the astronomers reasoned. Thus, it is called “cold, dark matter.”
Since then, astronomers have been scrambling to try to find and identify the missing matter, because it has profound implications about the motion and ultimate destiny of the universe.
“We’re still groping,” said Vera Rubin, a Carnegie Institution of Washington astronomer and one of the original theorists about cold, dark matter. “It is as if we are in a black room working on a black puzzle.”
Kormendy said he and his colleagues do not claim that all of the missing matter is in “ghostly galaxies,” but because the fuzz balls are so numerous, “they could add up to a significant portion of the dark matter.”
Evidence of Dwarf Stars
Another team of astronomers found support for the Kormendy theory in a Hubble Space Telescope study of a galaxy called NGC5907, which is 40 million light years away.
The Hubble focused on the galaxy for 10 hours to gather light from the very faintest of stars, said Michael Liu of UC Berkeley.
“We had expected to see hundreds of stars in the galaxy,” Liu said. “But in fact we saw very few.”
The galaxy did not have enough bright stars to provide the gravitational cohesion to hold it together, he said. Instead, there appears to be “a huge population of faint dwarf stars,” he said. “This is the first evidence for a substantial population of dwarf stars.”
Dwarf stars are too faint to be seen, but are detected by their gravitational influence on nearby bodies.
Liu said there is a halo around galaxy NGC5907, composed mostly of very dim dwarf stars. This dark matter halo is much denser than the bright halo surrounding many galaxies. Also, the high-density halo extends much farther from the galactic center than do other galaxies’ bright halos.
Rubin said the studies by the Kormendy and Liu teams “have provided a real service” in the search for dark matter. But the puzzle is far from solved.
“When we find a piece of the puzzle, we’re not sure where it fits,” she said. “But these may be two pieces of the puzzle.”
