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A Detailed Map of Dark Matter in a Galactic Cluster Reveals How Giant Cosmic Structures Formed
Caltech News Release Embargoed for Release at 12 a.m. PDT, Thursday, July 17, 2003 A Detailed Map of Dark Matter in a Galactic Cluster Reveals How Giant Cosmic Structures Formed PASADENA, Calif.--Astrophysicists have had an exceedingly difficult time charting the mysterious stuff called dark matter that permeates the universe because it's--well--dark. Now, a unique "mass map" of a cluster of galaxies shows in unprecedented detail how dark matter is distributed with respect to the shining galaxies. The new comparison gives a convincing indication of how dark matter figures into the grand scheme of the cosmos. Using a technique based on Einstein's theory of general relativity, an international group of astronomers led by Jean-Paul Kneib, Richard Ellis, and Tommaso Treu of the California Institute of Technology mapped the mass distribution of a gigantic cluster of galaxies about 4.5 billion light-years from Earth. They did this by studying the way the cluster bends the light from other galaxies behind it. This technique, known as gravitational lensing, allowed the researchers to infer the mass contribution of the dark matter, even though it is otherwise invisible. Clusters of galaxies are the largest stable systems in the universe and ideal "laboratories" for studying the relationship between the distributions of dark and visible matter. Caltech's Fritz Zwicky realized in 1937 from studies of the motions of galaxies in the nearby Coma cluster that the visible component of a cluster--the stars in galaxies--represents only a tiny fraction of the total mass. About 80 to 85 percent of the matter is invisible. In a campaign of over 120 hours of observations using the Hubble Space Telescope, the researchers surveyed a patch of sky almost as large as the full moon, which contained the cluster and thousands of more distant galaxies behind it. The distorted shapes of these distant systems were used to map the dark matter in the foreground cluster. The study achieved a new level of precision, not only for the center of the cluster, as has been done before for many systems, but also for the previously uncharted outlying regions. The result is the most comprehensive study to date of the distribution of dark matter and its relationship to the shining galaxies. Signals were traced as far out as 15 million light-years from the cluster center, a much larger range than in previous investigations. Many researchers have tried to perform these types of measurements with ground-based telescopes, but the technique relies heavily on measuring the exact shapes of distant galaxies behind the cluster, and for this the "surgeon's eye" of the Hubble Space Telescope is far superior. The study, to be published soon in the Astrophysical Journal, reveals that the density of dark matter falls fairly sharply with distance from the cluster center, defining a limit to its distribution and hence the total mass of the cluster. The falloff in density with radius confirms a picture that has emerged from detailed computer simulations over the past years. Team member Richard Ellis said, "Although theorists have predicted the distribution of dark matter in clusters from numerical simulations based on the effects of gravity, this is the first time we have convincing observations on large scales to back them up. "Some astronomers had speculated clusters might contain large reservoirs of dark matter in their outermost regions," Ellis added. "Assuming our cluster is representative, this is not the case." In finer detail, the team noticed that some structure emerged from their map of the dark matter. For example they found localized concentrations of dark matter associated with galaxies known to be slowly falling into the system. Overall there is a striking correspondence between features in the dark matter map and that delineated by the cluster galaxies, which is an important result in the new study. "The close association of dark matter with structure in the galaxy distribution is convincing evidence that clusters like the one studied built up from the merging of smaller groups of galaxies, which were prevented from flying away by the gravitational pull of their dark matter," says Jean-Paul Kneib, who is the lead author in the publication. Future investigations will extend this work using Hubble's new camera, the Advanced Camera for Surveys (ACS), which will be trained on a second cluster later this year. ACS is 10 times more efficient than the Wide Field and Planetary Camera 2, which was used for this investigation. With the new instrument, it will be possible to study clumps of finer mass in galaxy clusters in order to investigate how the clusters originally were assembled. By tracing the distribution of dark matter in the most massive structure in the universe using the powerful trick of gravitational lensing, astronomers are making great progress towards a better understanding of how such systems were assembled, as well as toward defining the key role of dark matter. In addition to Kneib, Ellis, and Treu, the other team members are Patrick Hudelot of the Observatoire Midi-Pyrénées in France, Graham P. Smith of Caltech, Phil Marshall of the Mullard Radio Observatory in England, Oliver Czoske of the Institut für Astrophysik und Extraterrestrische Forschung in Germany, Ian Smail of the University of Durham in England, and Priya Natarajan of Yale University. For more information, please contact: Jean-Paul Kneib Caltech/Observatoire Midi-Pyr»n»es (currently in Hawaii) Phone: (808) 881-3865 E-mail: Richard Ellis Caltech Phone: (626) 395-4970 (secretary) (Australia: Cellular: 011-44-7768-923277) E-mail: |
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Central to Our Local Cluster (was - A Detailed Map of Dark Matter...)
"P" == Painius writes:
A Detailed Map of Dark Matter in a Galactic Cluster Reveals How Giant Cosmic Structures Formed P So in addition to the dark matter believed to be a significant part P of galaxies, there appear to be profound concentrations of dark P matter associated with the center of galaxy clusters as well. P Perhaps we can train the powerful Hubble on the central area of our P Local Cluster? Maybe there are distant galaxies that will be P distorted by the gravitational lensing of dark matter at our own P galaxy cluster's center? The problem is that we are not in a "real" cluster. We are in a small group of galaxies, composed of the Milky Way, the Andromeda Galaxy, M33, and various dwarfs. The total mass of this group is perhaps 100 times smaller than the typical large cluster from which gravitational lensing is seen. Moreover, we are sitting in the middle of it. The nearest cluster of any reasonable heft is the Virgo Cluster. I'm not sure if anybody's ever looked for gravitational lensing from it, but ADS or astro-ph will likely contain a reference if so. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
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Central to Our Local Cluster (was - A Detailed Map of Dark Matter...)
"Joseph Lazio" wrote...
in message ... "P" == Painius writes: P So in addition to the dark matter believed to be a significant part P of galaxies, there appear to be profound concentrations of dark P matter associated with the center of galaxy clusters as well. P Perhaps we can train the powerful Hubble on the central area of our P Local Cluster? Maybe there are distant galaxies that will be P distorted by the gravitational lensing of dark matter at our own P galaxy cluster's center? The problem is that we are not in a "real" cluster. We are in a small group of galaxies, composed of the Milky Way, the Andromeda Galaxy, M33, and various dwarfs. The total mass of this group is perhaps 100 times smaller than the typical large cluster from which gravitational lensing is seen. Moreover, we are sitting in the middle of it. Thanks, Joseph... not sure how you come by that we're in the middle of the LG. The following, while depicting us in the center, actually shows that the grouping is quite off-center. For the most part, the center appears to be off to the right and down from our galaxy... http://www.anzwers.org/free/universe/localgr.html If we zoom out x20 from the above link... http://www.anzwers.org/free/universe/virgo.html ....we find that our Local (non)Cluster is pretty much on the outskirts of the gigantic Virgo Supercluster of galaxy groups. The nearest cluster of any reasonable heft is the Virgo Cluster. I'm not sure if anybody's ever looked for gravitational lensing from it, but ADS or astro-ph will likely contain a reference if so. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html Could not find anything in ADS, but astro-ph is working on it... http://xxx.soton.ac.uk/abs/astro-ph/0003422 So Joseph, the nature of centro-cluster dark matter leans me toward lots of rogue black holes circulating along with the galaxies. Probably lots of activity and collisions. We may be very fortunate to be in the outskirts! happy days and... starry starry nights! -- Our heads up in the sky, We're so clueless of our worth... Whose sky no longer shines As we lose our Mother-Earth? As people we must learn About the care of planet parts, To leave the world a better turn-- Empower brand new hearts! Paine Ellsworth |
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Central to Our Local Cluster (was - A Detailed Map of Dark Matter...)
"P" == Painius writes:
P "Joseph Lazio" wrote... in message P ... P Perhaps we can train the powerful Hubble on the central area of our P Local Cluster? Maybe there are distant galaxies that will be P distorted by the gravitational lensing of dark matter at our own P galaxy cluster's center? The problem is that we are not in a "real" cluster. We are in a small group of galaxies, composed of the Milky Way, the Andromeda Galaxy, M33, and various dwarfs. The total mass of this group is perhaps 100 times smaller than the typical large cluster from which gravitational lensing is seen. Moreover, we are sitting in the middle of it. P Thanks, Joseph... not sure how you come by that we're in the middle P of the LG. The following, while depicting us in the center, P actually shows that the grouping is quite off-center. For the most P part, the center appears to be off to the right and down from our P galaxy... P http://www.anzwers.org/free/universe/localgr.html Depends upon how you define "middle." The Milky Way is one of the three dominant galaxies in the Local Group. Moreover, the Milky Way obscures our view of a good chunk of the sky, and some of these dwarf galaxies are quite faint and can be difficult to find. In all likelihood, the Local Group doesn't have a well-defined "center," but three, nearly equal, large gravitational wells. The nearest cluster of any reasonable heft is the Virgo Cluster. I'm not sure if anybody's ever looked for gravitational lensing from it, but ADS or astro-ph will likely contain a reference if so. P Could not find anything in ADS, but astro-ph is working on it... P http://xxx.soton.ac.uk/abs/astro-ph/0003422 P So Joseph, the nature of centro-cluster dark matter leans me toward P lots of rogue black holes circulating along with the galaxies. P Probably lots of activity and collisions. We may be very fortunate P to be in the outskirts! I'm somewhat more optimistic than you. Remember, space is big. Even if there are lots of black holes (and there are some good reasons to think that not all dark matter is in the form of black holes), there won't be that many collisions. Also, to provide some perspective on the limit quoted in this paper (optical depth ~ 10^{-3}), the microlensing optical depth toward the center of the Milky Way is only of order 10^{-6}. -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
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