Home Science Space Dark matter and ordinary matter separate as two massive galaxies collide
Astronomers identify that dark matter is different from ordinary matter when they pointed the Hubble Space Telescope and the Chandra X-ray Observatory toward two very large merging galaxies that are forming the galactic cluster MACSJ0025.4-1222.

The abstract to the article, which is to be soon published in The Astrophysical Journal, is available online as “Revealing the properties of dark matter in the merging cluster MACSJ0025.4-1222,” within the Harvard University website.

Maruša Bradač, of the University of California, Santa Barbara, U.S.A., and Steven W. Allen, of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the Stanford Linear Accelerator Center (SLAC), USA, headed the team of researchers

Their team consists of Tommaso Treu, Harald Ebeling, Richard Massey, R. Glenn Morris, Anja von der Linden, and Douglas Applegate.

Dr. Bradač stated, "It is in our view an important step forward to understanding the properties of the mysterious dark matter." [MSBNC: “Dark matter detected in cosmic crash”]

The discovery helps to accumulate evidence that dark matter, a form of matter/energy, does indeed exist and provides a much better description for its physical nature.

Astronomers can only detect dark matter based on its gravitational effect on visible matter such as these gigantic galactic clusters. The research by Bradač and Allen appears to help decide whether dark matter interacts with itself other than through the force of gravity.

Dark matter is thought to account for about 22% of all matter/energy in the universe, at least according to research performed by astronomers using gravitational techniques.

About 4% of matter/energy in the universe consists of ordinary matter, while the remaining 74% consists of dark energy, another mysterious form of matter/energy found in the universe that is thought to be responsible for the universe to be expanding at an accelerating rate.

Please turn to page two so you can view the images of the pink ordinary matter and blue dark matter of the collision.

The dramatic images of the collision of the two galaxies and the separation of dark matter and ordinary matter (because of the collision) are found at the Hubble website of the European Space Agency (ESA)/National Aeronautics and Space Administration (NASA).

The website also includes the August 27, 2008 news release “Clash of clusters provides new dark matter.”

Bradač added, "Dark matter makes up five times more matter in the universe than ordinary matter. This study confirms that we are dealing with a very different kind of matter, unlike anything that we are made of. And we're able to study it in a very powerful collision of two clusters of galaxies." [MSNBC]

The Bradač/Allen team state within their abstract, “We constrain the physical nature of dark matter using the newly identified massive merging galaxy cluster MACSJ0025.4-1222. As was previously shown by the example of the Bullet Cluster (1E0657-56), such systems are ideal laboratories for detecting isolated dark matter, and distinguishing between cold dark matter (CDM) and other scenarios (e.g. self-interacting dark matter, alternative gravity theories).”

The researchers used visible-light images from the Hubble Space Telescope to determine the total mass distribution of the galactic cluster concerning both dark matter and ordinary matter.

Hubble also was able to map the dark matter using the scientific technique known as gravitational lensing—a technique in which light can be analyzed as it is “bent” around a massive object as seen between the source of the light and observers of the light (astronomers on Earth)

The images show ordinary matter in the color "blue."

The Chandra X-ray Observatory was able to image the hot gas of the ordinary matter as it spewed out x-ray radiation. These images are shown in the color "pink."

They continue to state, “MACSJ0025.4-1222 consists of two merging subclusters of similar richness at z=0.586. We measure the distribution of X-ray emitting gas from Chandra X-ray data and find it to be clearly displaced from the distribution of galaxies. A strong (information from highly distorted arcs) and weak (using weakly distorted background galaxies) gravitational lensing analysis based on Hubble Space Telescope observations and Keck arc spectroscopy confirms that the subclusters have near-equal mass.”

The researchers found that MACSJ0025-1222 is about one quadrillion times the mass of Earth’s Sun. The two galaxies that are forming the galactic cluster are merging together at millions of kilometers per hour. As this collision occurs, ordinary matter slows down in speed while dark matter is unaffected by the action—it moves through without any visible change in speed.

Page three concludes their abstract statement.

The news release also states, “The separation between material shown in pink and blue therefore provides observational evidence for dark matter and supports the view that dark matter particles interact with each other only very weakly or not at all, apart from the pull of gravity.”

They conclude in their abstract, “The total mass distribution in each of the subclusters is clearly offset … from the peak of the hot X-ray emitting gas (the main baryonic component), but aligned with the distribution of galaxies. We measure the fractions of mass in hot gas … and stars …, consistent with those of typical clusters, finding that dark matter is the dominant contributor to the gravitational field.”

And, “Under the assumption that the subclusters experienced a head-on collision in the plane of the sky, we obtain an order-of-magnitude estimate of the dark matter self-interaction cross-section …, re-affirming the results from the Bullet Cluster on the collisionless nature of dark matter.”


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William Atkins

William Atkins completed educational degrees in science (bachelor’s in physics and mathematics) from Illinois State University (Normal, United States) and business (master’s in entrepreneurship and bachelor’s in industrial relations) from Western Illinois University






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