Stars In Neighboring Galaxy Offer Clues To Mystery Of Dark Matter

The location of the binary star system suggests that exotic dark objects such as black holes do not make up the majority of mass in the universe.

"Our findings don't offer definitive proof," said Andrew Gould, associate professor of astronomy at Ohio State, "but we can begin to probe that possibility. It's very exciting."

Astronomers spotted the binary system within the Small Magellanic Cloud (SMC), a galaxy that orbits our own, when this system's gravity bent the rays of light from another star and magnified them like a lens.

Gravitational lensing is what happens when a massive dark object in space, such as a planet, dim star, or black hole, crosses in front of a luminous source star in the background. Here on Earth, we see the star get brighter as the lens crossesin front of it, and then fade as the lens gets farther away. This is what astronomers call a "lensing event."

Gravitational lensing is one of the few ways astronomers may detect the presence of dark, massive objects in our galaxy. If many such objects exist, they could account for the missing mass of the universe.

The dark matter, which may account for up to 99 percent of the mass of the universe, has so far eluded detection by the most powerful instruments such as the Hubble Space Telescope. Astronomers refer to these unseen dark objects as massive compact halo objects, or MACHOs.

The first lensing event in the SMC was discovered by a group of astronomers called the MACHO Collaboration in January 1997. On June 8, 1998, the MACHO Collaboration noticed this second lensing event and alerted another group, the Probing Lensing Anomalies NETwork (PLANET).

Eight institutions including Ohio State make up the PLANET Collaboration. Members gather data from four observatories: Perth Observatory in Bickley, Australia; Canopus Hill Observatory of the University of Tasmania in Hobart, Australia; South African Astronomical Observatory Sutherland, South Africa; and European Southern Observatory in La Silla, Chile.

During the two weeks that the light from this latest event grew to peak intensity, B. Scott Gaudi, a graduate student in astronomy at Ohio State, was coordinating telescope operations. Gaudi sent an announcement of the event to interested members of the astronomical community over the Internet, and directed the operation of the telescope in South Africa to observe the peak brightness of the event. Gaudi also analyzed the telescope data with software that he had developed.

The analysis appears in a paper which has been accepted for publication in the Astrophysical Journal. A preprint of the paper currently appears in an astronomy preprint server on the Internet. The World Wide Web address for the paper is http://xxx.lanl.gov/abs/astro-ph/9807086.

The way in which a gravitational lens bends the light rays of its background star indicates what type of object causes the lens. In this case, the data collected by the PLANET team confirmed that the lens object was a binary star system.

When Gaudi analyzed the data with his software, he was able to calculate the speed at which the binary system was moving through space. "That's what allowed us to distinguish whether this binary was in the halo -- and therefore possibly the dark matter that everybody's looking for -- or whether it was in the SMC," said Gaudi.

"A lens in the SMC would be traveling 10 times slower than a lens in the halo," said Gould. "From our calculations, if this lens was in the SMC, it was moving at roughly 75 kilometers per second. If it was in the halo, it would have been moving much slower, such as 35 kilometers per second. The likelihood that a halo object would travel that slowly is negligible."

The binary star system gave astronomers an especially good view of the event, because the interplay between the two stars' gravitational fields warped the lens effect and magnified the source star to 100 times its normal brightness. "A binary lens causes a distortion in the lens structure," said Gould. "Think of it this way: If you were wearing glasses, and you stood next to a road on a rainy day, you'd notice that raindrops on the glasses distorted round lights of the passing cars into diamond shapes. That's what a binary lens does."

Gould said that while these observations poke some holes in the MACHO hypothesis, they don't prove a competing idea, namely that dark mater is composed of weakly-interacting massive particles, or WIMPS. "Even if we totally ruled MACHOs out, that doesn't mean that dark matter is definitively made up of WIMPS. Before we could say that, someone would have to detect WIMP particles somehow. Our finding is just a step on the road to solving this puzzle."

Other PLANET member institutions include: the Kapteyn Institute, Rijksuniversiteit Groningen, The Netherlands; Space Telescope Science Institute, Baltimore, Maryland; South African Astronomical Observatory, Capetown, South Africa; University of Canterbury, Christchurch, New Zealand; Perth Observatory, Bickley, Western Australia; and Canopus Observatory, Hobart, Tasmania, Australia.