"Dark matter in the high-redshift cluster CL 0152-1357. Gravitational
lensing analysis with the Advanced Camera for Surveys (ACS) reveals the complicated dark matter distribution (purple) in unprecedented detail when the Universe was at half its present age. The yellowish galaxies are the visible cluster member galaxies forming a filamentary structure, possibly in the process of merging. (Jee et al. 2005, Astrophysical Journal)"
Over 90% of the matter in our universe is dark matter. We can't see it. We can't touch it. But thanks to the recent work of Johns Hopkins University–Space Telescope Science Institute researchers, we know where we can find it.
Using the new and powerful Advanced Camera for Surveys, installed on the Hubble Space Telescope, the scientists created a detailed map of dark matter distribution in two young galaxy clusters. Their findings are published in the December issue of Astrophysical Journal.
Study coauthor Myungkook James Jee said the map strongly buttresses the prevalent cosmological theory, which predicts that dark matter and visible matter will be held together by gravity.
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"We found that the resulting dark matter image is strikingly similar to the foreground galaxy distribution, which lends credence to the current Cold Dark Matter structure formation theory," Jee said via e-mail. "There have been some attempts to explain the 'missing' matter problem by modifying the Newtonian physics."
Astronomers have observed that galaxies rotate as if their matter extended beyond what we can observe visually, in a halo around the visible galaxy. If scientists use different laws of motion—non-Newtonian physics—they can eliminate this need for an invisible halo of matter. If they can't, they're stuck with the presence of dark matter. There may be no need to rewrite theories, since the shape of the distribution that Jee obtained serves as evidence that dark matter is, in fact, an entity.
The researchers were able to infer the distribution of the dark matter by observing "gravitational lensing" effects on light emitted from distant galaxies about 6.5 billion years ago. While dark matter neither absorbs nor reflects light, it does have gravitational properties, and it bends the space along which light travels. If the light from a galaxy has to pass near dark matter on its way to an observer, the light may appear magnified and distorted. Because it manipulates the path and effective intensity of light, gravity, in effect, acts like a lens.
Color composite of CL0152-1357 centerCredit: Astrophysical Journal
Jee referred to an image from the study that shows the effects of a gravitational lens. (See image left.)
"Can you find the blue galaxies surrounding the central yellowish ones?" Jee said. "They are actually located far behind the yellowish galaxies, but they become so bright because the dark matter of the cluster magnifies them by perhaps a factor of five to 10."
From the distortion of the light emitted by the background galaxies, the researchers were able to calculate the density of dark matter at different points in space. They found that the dark matter clustered in haloes around the galaxies in the foreground of the image.
The discovery supports prevalent cosmological ideas. Cosmologists believe that dark matter is collision-less: The particles pass through each other and visible matter, without colliding. Jee said if the particles were collisional, the researchers would have seen a smoother distribution of dark matter. The distribution they observed looks like that of collision-less particles.
The clustering of dark matter around the foreground galaxies also supports the current theory of how structures are formed in the universe. Cosmologists believe that smaller objects are made first and then merge to form larger objects. Jee said the map lends credence to this theory by demonstrating that visible matter and dark matter, in fact, do cluster together.