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Press release
The black matter of a distant galaxy cluster mapped by the Hubble space telescope | |||
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Paris, July 17, 2003 |
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| An international team of astronomers(1) led by Jean-Paul Kneib, CNRS research associate (Laboratoire d’Astrophysique de l’Observatoire Midi-Pyrénées / Université Toulouse 3 - CNRS and Caltech), has mapped the distribution of black matter in a galaxy cluster using the Hubble space telescope. These results will be presented to a general assembly of the International Astronomic Union in Sydney, Australia. It is the longest observation of a galaxy cluster ever made with Hubble. This is the first time that research will provide a complete view of a galaxy cluster at a distance of over 15 million light years from the center of the cluster. By comparing the black matter map with the distribution of galaxies and the luminous matter of the cluster, researchers will be able to better understand how these clusters are formed and the role of black matter during the course of cosmic evolution. Over 80% of the mass of the universe emits no light: this is known as black matter whose characteristics and distribution within the universe are still poorly understood. Galaxy clusters are the largest stable structures in the universe and true "laboratories" for studying relationships between ordinary matter: galaxies, stars, intergalactic gases and this mysterious black matter. As early as 1937, the astronomer Fritz Zwicky noticed that the billions of stars in the thousands of galaxies in a cluster represented only a very small fraction of the total mass of these systems. Approximately 80 to 85% of the matter is actually invisible. Although the existence of this matter has been known for several decades, researchers have only recently found a technique to determine its distribution in galaxy clusters. Thousands of very faint galaxies located behind the target cluster must be observed in order to determine this. The shapes of these galaxies make it possible to reconstruct the distribution of luminous and dark matter in the cluster. The light rays that reach us from these very distant galaxies are actually bent by the enormous mass of the cluster due to a general relativity effect known as "gravitational lensing". The images of background galaxies are therefore deformed, making it possible to map the cluster mass distribution by taking exact measurements of these deformations. This international team of astronomers, working with Jean-Paul Kneib, has just mapped the black matter in a galaxy cluster known as Cl0024+1654, a difficult task since this matter by definition emits no light. To obtain the images of the cluster(2) and background galaxies, over 120 hours of observations with the Hubble space telescope were necessary. Although it is approximately 4.5 billion light years away (about a third of the age of the universe), this massive cluster takes up an area comparable to that of the full moon in the night sky. This research shows that the density of matter on a cosmic scale drops sharply as the distance from the center of the cluster increases. This result is in agreement with the most recent digital simulations. To be more specific, the team detected sub-structures on the black matter map. They were able to reveal a concentration of black matter associated with a group of galaxies that is in the process of merging with the cluster. The researchers concluded that the black matter precisely follows the galaxy distribution and on a very large scale. "At the time that the cluster is being formed, the black matter spreads out between the galaxies and acts as a bond between them. The relationship between the galaxies and black matter observed in the Cl0024+1654 cluster demonstrates that such structures are formed by the aggregation of smaller groups that already have their share of black matter themselves." explains Jean-Paul Kneib. The WFPC2 (Wide Field and Planetary Camera 2) was used to make these observations. The ACS (Advanced Camera for Survey), ten times more powerful, was just installed on the Hubble, making it possible to continue these types of observations of other clusters. It will therefore be possible to study smaller mass structures and to develop a better understanding of how galaxy clusters are formed. Thus, by tracing the distribution of black matter in more massive structures in the universe, astronomers will increase their knowledge of how these giant structures were formed and the important role that black matter certainly plays in their evolution and that of the universe in general. Reference: These results will soon be published in an article in the Astrophysical Journal. (1)
The team is made up of the following scientists: Jean-Paul Kneib (Observatoire
Midi-Pyrénées, France/Caltech, USA), Patrick Hudelot (Observatoire
Midi-Pyrénées, France), Richard S. Ellis (Caltech, USA),
Tommaso Treu (Caltech, USA), Graham P. Smith (Caltech, USA), Phil Marshall
(MRAO, UK), Oliver Czoske (Institut für Astrophysik und Extraterrestrische
Forschung, Germany), Ian Smail (University of Durham, UK) & Priya
Natarajan (Yale University, USA). Researcher
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