Dwarfs behind dark puzzle posit scientists

Andy Poon
Staff writer

UBC scientists may be a step closer to finding the answer to one of astronomy's most perplexing questions: What is the galaxy made of?

"We may have identified a component of the unknown dark matter in the galaxy -- the 90 per cent of the mass of the galaxy which we thought was invisible," says Prof. Harvey Richer, an astronomer in the Physics and Astronomy Dept.

Research recently released by the four-member team which included Richer and fellow astronomer Asst. Prof. Douglas Scott indicates that ancient white dwarf stars -- the burned-out remains of normal stars like the Sun -- may make up more than half of the invisible dark matter in the Milky Way Galaxy.

The team of researchers, which also included Rodrigo Ibata from Germany and Roland Gilliland from the Space Telescope Science Institute in Baltimore, arrived at the conclusion after they compared images of the Hubble Deep Field -- the deepest optical image of the sky -- from the Hubble Space Telescope in 1995 and 1997. They found that five objects moved slightly in the foreground between the two images. The scientists believe that the objects may be old white dwarf stars.

"If this picture is correct, there will be an enormous rethinking of how galaxies formed and evolved," says Richer.

In the past, white dwarf stars have been suggested as a possible explanation of the missing matter in the galaxy but this may be the first time anybody has seen them, he says.

The team's results were published in the October issue of The Astrophysical Journal Letters. Meanwhile the team has plans to check their work this December when they will again use the Hubble Space Telescope to look at the same field and see if the previously detected motion in the five objects is confirmed.

A large consortium including astronomers from UBC, Victoria, Princeton University and Germany are also using the Canada-France-Hawaii Telescope in Hawaii to search for more local examples of these ancient white dwarfs.

For more information and images of the team's findings, visit the Web site www.astro.ubc.ca/people/richer/basic.html.