SCIENCE / ASTRONOMY : Comparison of Photos Provides New Evidence of ‘Dark Matter’ in Milky Way

Astronomers studying photographs of the sky said Monday that the Milky Way apparently is shrouded by enough invisible “dark matter” to make Earth’s home galaxy five to 10 times larger and heavier than once thought.

UC Santa Cruz astronomer Douglas N.C. Lin said the Milky Way is so massive that it is swallowing up the smaller, neighboring galaxy known as the Large Magellanic Cloud.

“What we are witnessing is galactic cannibalism in progress,” Lin said at a meeting of the American Astronomical Society at UC Berkeley.

Lin’s findings were among several presentations at the meeting that support the idea that the universe contains enormous quantities of dark matter, which can be detected only by its powerful gravitational pull.

David A. Buote of the Massachusetts Institute of Technology reported finding evidence of dark matter in an unusually flattened gas cloud around a galaxy in the constellation Cetus.

Meanwhile, University of Washington astronomer Liliya Rodrigues-Williams reported that dark matter surrounding a cluster of galaxies exerted enough gravity to bend light from more-distant quasars and focus them the way a glass lens bends and focuses light on Earth.

Scientists have suspected the existence of dark matter for 15 years, since astronomers first noticed how stars rotate in spiral galaxies. All the visible material in the galaxies--the stars, dust and gas--apparently account for no more than 10% of the gravity needed to keep the galaxies from flying apart.

No one knows what composes dark matter. Theories range from enormous clumps of completely dark and extremely dense matter called brown dwarfs to tiny and nearly weightless subatomic particles called neutrinos.

Although scientists cannot say what dark matter is, they are seeing increasing evidence of its extraordinary gravity throughout the universe--and now, with Lin’s discovery, in the Milky Way itself.

“With this information, the skeptics--even the most die-hard skeptics--will have a hard time denying the existence of dark matter,” Lin said.

Working with Burton Jones and Arnold Klemola, Lin sought to measure the amount of dark matter in the Milky Way by measuring its gravitational pull on the Large Magellanic Cloud.

To do this, he scoured astronomical archives for old photos of the smaller galaxy that also had identifiable reference points in the background, such as other stars and galaxies.

When he found a photo made in Chile in 1974, he returned to the same observatory in 1989 and duplicated the shot. After meticulously matching up the backgrounds in the two photos, he calculated how far the Large Magellanic Cloud had moved in the intervening 15 years. It was, he said, a rate equal to less than one arc second--1/3,600th of a degree across the sky--since Columbus landed in North America.

That tiny shift, which Lin compared to spotting an earthquake on the San Andreas Fault while standing atop the Empire State Building, was enough to plot the smaller galaxy’s looping, egg-shaped spiral toward the Milky Way.

With that in hand, Lin and his colleagues calculated that the Milky Way--its visible stars and the invisible dark matter--weighs as much as 600 billion suns and is at least 600,000 light-years in diameter. It could be twice that large, he added.

Until now, visual examination of the Milky Way put its diameter at closer to 120,000 light-years.

“This measurement is much more reliable than all previous measurements,” Lin said. “It proves the existence of dark matter beyond a doubt.”