For the first time, a team of astronomers has "observed" a filament of dark matter connecting two neighboring galaxy clusters. Dark matter is a type of matter that interacts only very weakly with light and itself. Its very nature is mysterious. Mapping the dark matter filament's gravity was the key to the breakthrough. The result is considered a crucial first step by scientists. It provides the first direct evidence that the universe is filled by a lacework of dark matter filaments, upon which the visible matter in the universe is distributed like small beads.

Jörg Dietrich of the physics department at the University of Michigan, together with his co-workers, examined gravitational lensing in the Abell 222 and 223 galaxy clusters. These clusters each have about 150 galaxies, are about 2.4 Gly (1 Gly being a gigalight-year, or 1 billion light-years) distant from Earth, and are separated by about 0.4 Gly. Earlier work by Dietrich's team using the 8.2 meter Subaru telescope on Mauna Kea, and the XMM-Newton x-ray space telescope discovered that these two clusters appear to be connected by a bridge of hot gas, as shown below.

Composite astrograph of the Abell 222 and 223 galaxy clusters as seen in visible light and by x-rays - the filament of dark matter between the two is suggested by the hot x-ray emitting gas (shown in dark red) gathered along the filament (Image: ESA/ XMM-Newton/ EPIC/ ESO (J. Dietrich)/ SRON (N. Werner)/ MPE (A. Finoguenov)

They suggested that the hot gas might be concentrated along a filament of dark matter, as is found in simulations of cosmological structures, but a strong case for that interpretation could not then be made.

Dietrich and his team decided to do a careful examination of the region of the two Abell clusters. They studied weak lensing effects and solved for the mass density function of the clusters and the region between them. Then by examining the mass density function of the region they were able to test their hypothesis.

A filament of dark matter has been directly detected between the galaxy clusters Abell 222 and Abell 223. The blue shading and yellow contour lines represent the density of matter Photo: (Jörg Dietrich, U-M Department of Physics)

"We found the dark matter filaments. For the first time, we can see them," said Dietrich. "It looks like there's a bridge that shows that there is additional mass beyond what the clusters contain. The clusters alone cannot explain this additional mass." At least 90 percent of the filament's mass is dark matter.

We are still left with the problem of demonstrating that dark matter filaments appear between most neighboring galaxy clusters, and the puzzle of what dark matter actually is. The discovery of a dark matter filament, however, is a huge step forward for cosmology.

This groundbreaking observation is consistent with modern cosmological models, but the story actually starts some 80 years ago.

In the 1930s, Jan Oort and Fritz Zwicky independently noticed that stars orbiting our galaxy and galaxies moving in galaxy clusters were moving faster than their escape velocity. This was not a small effect. Zwicky found that there must be about 400 times more matter than was visible. Their common conclusion was that there must be more mass hiding somewhere in these galactic objects.

For the next 40 years no additional evidence for dark matter was found. In time, however, progressively more sensitive observations supported the idea that dark matter was common and important in the Universe, largely through observation of gravitational lensing. Astronomers now believe that the Universe is composed of 73 percent dark energy, 23 percent dark matter, and only four percent normal matter and energy. Leaving aside dark energy for another day, how did we find the dark matter?

Bending light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source (Photo: Space Telescope Science Institute [STScI])

Briefly, although dark matter does not appear to interact with light, ordinary matter, or itself, it does have mass and that mass has a gravitational field. Well, gravity bends spacetime, and thus also bends light rays. As shown above, if a massive object is located between you and a distant galaxy, the light bends toward the object a bit because of its gravity, thus acting as a gravitational lens. The distant galaxy will now appear as a ring of distorted light surrounding the object. This is called an Einstein ring. The size of the ring depends on how far away and how massive the object is. In practice, however, gravitational lensing tends to stretch out a distant galaxy into a distorted arc of light extending over only a small part of a circle.

Image of a distant (~10 Gly]) galaxy as seen through the gravitational lens of the galaxy cluster RCS2 032727-132623. On the right appears a reconstructed image of the distant galaxy obtained by subtracting (approximately) the distorting effects of the gravitational lens (Photo: NASA, ESA, and Z. Levay (STScI))

The Hubble image above shows a striking examples of gravitational lensing, with a 90-degree arc of light and several distorted images of a single galaxy located about ten billion light years (Gly) away. These images are larger, brighter, and more detailed than a direct view would provide. After all, a gravitational lens made up of a cluster of galaxies focuses a lot of light. By reversing the distortion, astronomers have reconstructed approximately what the distant galaxy looks like - or looked like ten billion years ago (right hand portion of above figure).

Dark matter only shows through its gravity, so if there is enough of it around, we should be able to see its gravitational lensing effects. In fact, astronomers know how to take their observations and work out the distribution of mass that causes the lensing. This is most easily done when observing weak lensing: a minor distortion of a distant object rather than arcs and multiple images.

If a lot of mass is found where there are few stars and galaxies with little gas and dust, a reasonable hypothesis is that the mass is mostly composed of dark matter. Mind, this doesn't let us see the dark matter in a telescope. Rather, we can infer that dark matter is present by measuring the gravitational curvature near the focusing object. What's the deal with cosmological filaments?

The universe within a billion light years of Earth, showing local superclusters of galaxies - approximately 63 million galaxies are shown (Image: Richard Powell)

The map above shows the known universe within one Gly of our galaxy. Clearly, most galaxies are organized into clusters, but some are located along filaments that connect the clusters. Cosmologists believe that those filaments may largely be composed of dark matter, as shown in the Virgo Consortium image of the Millennium Simulation above.

The structure of the dark matter filaments is a remnant of the initial quantum fluctuations which dominated the Universe in its extreme youth. These filaments are very massive, and serve to guide galaxies toward the filaments. Once the galaxies have joined a filament, it provides a low-energy path for them to join the galaxy clusters which appear at the vertices of the network of filaments.

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