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A Dim Light Shines on Dark Matter
This view from the Chandra X-ray Observatory shows the cloud of hot gas surrounding NGC 4555. It's often darkest before the dawn, and in the case of dark matter, it's getting very hard to see. That's because recent discoveries made by astronomers working with NASA's orbiting Chandra X-ray Observatory are clouding our already murky understanding of dark matter and raising more questions than answers.

Image to right: Chandra's X-ray image of the NGC 4555 galaxy reveals that it is embedded in a cloud of 18 million degree Fahrenheit gas. This hot gas cloud has a diameter of about 400,000 light years across.
Credit: NASA/CXC/E. O'Sullivan et al

What's perplexing scientists are new observations made of a large, solitary, elliptical galaxy called NGC 4555. Elliptical galaxies are identified by their lack of spiral arms and often aging star population. NGC 4555 is surrounded by a gas cloud 400,000 light years wide and a searing 18 million degrees Fahrenheit in temperature.

According to conventional logic, the galaxy's hot gas should escape into space. But that's not what is happening. Instead, the gas cloud continues to envelope the galaxy as though held in place by some veiled force. Chandra scientists suspect the force may be coming from a halo of dark matter.

The name dark matter comes from the force's mysterious, hidden nature. No one knows exactly what dark matter is, but scientists estimate it composes nearly 90 percent of the cosmos. For the moment, astronomers have no way of directly detecting it, but they're confident it's real. Like tree boughs swaying in the invisible wind, astronomers see evidence of dark matter's influence throughout the universe.

While the identification of a dark matter halo around an elliptical galaxy is nothing new, they're commonly only found surrounding grouped or clustered galaxies. The discovery of dark matter encircling the lone NGC 4555 is changing that notion.

"The observed properties of NGC 4555 confirm that elliptical galaxies can possess dark matter halos of their own, regardless of their environment," said Ewan O'Sullivan, a Chandra scientist at the Harvard-Smithsonian Center for Astrophysics.

An artist's drawing of Chandra in space. This means an elliptical galaxy doesn't have to be part of the typical "galactic clique" and near similar galaxies in order to possess a dark matter halo.

But this finding doesn't necessarily guarantee that all singular elliptical galaxies have dark matter halos either.

Image to left: The Chandra Observatory was launched aboard a Space Shuttle in 1999 and is designed capture x-ray images too powerful for optical telescopes. Chandra is a member of "The Great Observatories" that includes the Hubble Space Telescope. Credit: NASA/CXC

In what seems to be an exception to the Chandra discovery, an international team of astronomers has recently located similar galaxies that appear to have no dark matter halos whatsoever. Led by Aaron Romanowsky of the United Kingdom's University of Nottingham, the group is studying elliptical galaxies using the ground-based William Herschel Telescope in La Palma, Spain. The researchers are looking for evidence of dark matter by measuring the speeds of objects within the galaxies. They've found the objects' speeds are low enough to need only the galaxy's gravity to hold them in and seemingly feature no dark matter halos.

The contrasting findings from the Chandra X-ray and Herschel Telescopes teams are causing scientists to ask even more questions about the nature of dark matter. Why do some elliptical galaxies have dark matter halos while others appear not to? Is it possible that some dark matter halos are too large to be observed by ground-based telescopes?

"This is clearly a question which deserves further consideration," concludes O'Sullivan.

So for now, the existence and behavior of dark matter halos remains clouded in mystery. But like a lot of brilliant moments in science, our deepest questions often border on the horizon of great discovery. There's a light growing in the distance and it could very well shine straight through the Chandra X-ray Observatory.
Charlie Plain
Chandra X-ray Observatory Center and NASA's John F. Kennedy Space Center