"Dark matter" is probably the most abundant stuff in the universe, making up more than 90 percent of everything out there. And yet scientists know almost nothing about it.
They can't see it because it doesn't shine or reflect light.
But astronomers in Baltimore said yesterday that they have used the Hubble Space Telescope to map the dark matter billowing out from the long-ago collision of two galaxy clusters.
They're calling it the strongest evidence yet of the existence of dark matter, and the first observation to separate it from its associated stars, galaxies and glowing gas.
"What we found is a very peculiar structure - a ring-like structure that surrounds the core of the cluster," said Johns Hopkins University research scientist M. James Jee, lead author on the study that will appear in the June 1 issue of The Astrophysical Journal.
It's not exactly a ring of dark matter, he said. Rather, it's a map of where the densest regions of dark matter must be, based on measurements of how that mass and gravity are bending the light streaming by from galaxies far in the background. Such bending was predicted in 1920 by Albert Einstein's General Theory of Relativity.
Jee likened the ring (in three dimensions actually a flattened spherical shell) to a jam-up of dark matter particles hurled outward from the collision, like commuters headed out of town, backing up behind slowing traffic ahead.
But not everyone is buying it.
"This result is meeting substantial skepticism among the astronomical community," said Richard Massey, an astronomer and cosmologist at the California Institute of Technology, who was not part of the Hubble team.
The ring's "signal" in the Hubble data is "very weak," Massey said. And it could be an artifact of the "peculiarities" of Hubble's Advanced Camera for Surveys, which captured the data. "I want to see some verification of this" from future studies.
On the other hand, Massey conceded, "it's real exciting if it's right." Dark matter is "the most common matter in the universe, and we know almost nothing about it. It's quite embarrassing."
Richard White, an astronomer at the Space Telescope Science Institute in Baltimore and a co-author of the study, said the skepticism is not unexpected.
"I have to say everybody involved with this project has gone through the same stage of initial skepticism and eventually been convinced this really is correct," he said.
The Hubble image released yesterday is a composite. The first component is a detailed photo of the galaxy cluster labeled CL0024+17, shot in 2004 with Hubble's Advanced Camera for Surveys - the workhorse instrument shut down in January because of electrical failures.
Previous studies have suggested that CL0024+17 - 5 billion light-years from Earth - is made up of two clusters of galaxies, seen one behind the other. We see them as they appeared 1 billion to 2 billion years after they collided and passed through each other along the line of sight from Earth.
Superimposed on the image is a foggy blue ring structure. It's a map, of sorts, showing where the light from 1,400 galaxies far behind the galaxy clusters has been most distorted by the gravitational influence of the unseen dark matter.
The phenomenon is called "gravitational lensing." The combined gravitational pull of all the galaxies and dark matter in the cluster tug on the passing light and stretch, distort and even multiply the images of the background galaxies that reach Earth.
Short of manufacturing dark matter in particle accelerators, which is also being tried, the mapping of gravitational lensing by dark matter is about the only way scientists have of studying it.
Jee's team derived the ring image by using computers - and the uniquely detailed Hubble images - to measure how much the cluster's dark matter was distorting the light from thousands of galaxies far behind CL0024+17. Jee likens it to studying the wind by measuring how it moves different objects.
It's the second time astronomers have mapped the dark matter surrounding a galaxy cluster. The first was announced in August by Maxim Markevitch of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. His team mapped dark matter surrounding a similar pair of colliding galaxy clusters called ZwCl0024+1652, or the "Bullet Cluster."
Those observations, however, showed the clusters colliding across the field of view, rather than head-on as seen from Earth, like CL0024+17. And while the map showed the dark matter being pulled away from the glowing gas in the cluster, it still aligned with the cluster's galaxies.
Jee's team claims its work is the first to show dark matter distribution that differs substantially from that of the ordinary, luminous matter in the galaxy cluster, strengthening the argument for its existence.
He said his result still doesn't tell theorists what dark matter is made of. Physicists have proposed a list of candidates, mostly exotic sub-atomic particles with names like axions, squarks and photinos.
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