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Planetary News: Hubble Space Telescope (2007)Invisible Ring of “Dark Matter” Detected 5 Billion Light Years AwayBy Amir AlexanderMay 15, 2007
Scientists studying the structure of a distant galaxy cluster billions of light years away announced today that it is surrounded by an enormous ring of “dark matter.” It is the first time that a well-defined formation of dark matter has been detected, clearly distinct from the concentrations of ordinary matter in the vicinity. If confirmed, the discovery would be a breakthrough in the study of the elusive substance said Myungkook James Jee of Johns Hopkins University, a member of the team that made the discovery. “By seeing a dark matter structure that is not traced by galaxies and hot gas, we can study how it behaves differently from normal matter” he added. No one has ever observed “dark matter” which, as its name suggests, does not emit or reflect light or any other form of radiation. Scientists wouldn’t even know that dark matter was out there, were it not that its presence makes itself felt in the very fabric of the universe. According to current cosmological theories, large-scale structures such as galaxy clusters do not have sufficient mass to hold together all by themselves. In order to explain their existence scientists must assume that an unknown and unobservable substance is present, increasing the clusters' total mass several fold and keeping them glued together by gravitational attraction. Since scientists know almost nothing about this mysterious substance except that it is, for all practical purposes, invisible, they named it “dark matter.” Hardly anything is known of the nature of dark matter, but one thing is clear: it is not rare. All the stars, galaxies, clusters, interstellar dust particles and interstellar gas in existence taken together add up to no more than one seventh of the total mass of the universe. The remaining six sevenths (6/7), scientists estimate, is made up of dark matter. Since it is practically unobservable, scientists have resorted to roundabout ways to map out its presence in the universe. Instead of looking for the telltale signs of light and radiation as they do with ordinary matter, scientists look for the gravitational effects of dark matter's presence. In particular, astronomers are makin guse of a surprising phenomenon known as “gravitational lensing” (or "microlensing") which was predicted by Einstein in his General Theory of Relativity. According to the theory, light passing in the vicinity of a large mass will be bent from its original course in certain predictable ways. It follows that although observers on Earth cannot detect dark matter directly, they can deduce its presence from the way it “lenses” light from background objects further away. That was precisely the effect Jee and his colleagues were looking for when in November of 2004 they pointed the Hubble Space Telescope at galaxy cluster Cl0024+17, 5 billion light years away. Using Hubble’s “Advanced Camera for Surveys” (ACS) they carefully mapped the lensing effect of the cluster on light from galaxies twice as distant. From this they carefully deduced the presence and distribution of matter in the cluster. The process, explained Richard White of the Space Telescope Institute in Baltimore, who is a member of the discovery team, is like looking at the pebbles in the bottom of a pond with ripples on the surface. The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies show coherent changes in their shapes due to the presence of the cluster. Identifying the hidden structure of matter in the cluster from these distortions is analogous to deducing the presence and intensity of the ripples on the surface of the pond from the distorted view of the pebbles at the bottom. Much to their surprise, Jee and his colleagues found that the distribution of matter near the cluster did not correspond to the visible structure of the galaxies. Instead, a gigantic ring of matter, 2.6 million light years in diameter, appeared to surround the visible core of the galaxy structure. Since the ring was not directly observable in any way, it could only be composed of dark matter.
At first glance the researchers could not quite believe what they had discovered. “Although the invisible matter has been found before in other galaxy structures,” explained Jee, it has never been detected to be so largely separated from the hot gas and galaxies that make up galaxy clusters.” Faced with this unexpected result, Jee’s initial reaction was that they had probably made a mistake. “I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction” he said. “I couldn’t believe my result.” But the more Jee and his team tried to get rid of the ring, the more it remained stubbornly in place. At the end of a full year of work the team was finally convinced that the ring of dark matter was truly there. Jee and his collaborators believe that the ring was formed from the collision of two galaxy clusters about two billion years ago. Computer simulations they created show that in such a collision dark matter would first fall to the center of the combined cluster and then expand outwards, eventually slowing down under the pull of gravity. This would result in a ring very much like the one discovered in cluster Cl0024+17. The fact that the collision produced a ring that is clearly distinct from the visible matter structures of the cluster is extremely significant, because it shows that dark matter behaves differently from ordinary matter under certain circumstances. “By studying the collision we are seeing how dark matter responds to gravity” said team member Holland Ford of Johns Hopkins University. “Nature is doing an experiment for us that we can’t do in a lab.” As is often the case when groundbreaking discoveries are announced, not everyone was convinced by the claims of Jee and his colleagues. Richard Massey of Caltech, who was not a member of the discovery team, praised the results but also noted that there is “considerable skepticism” in the astronomical community regarding the results. Despite Jee’s hard work, some remain convinced that the “ring” is only an artifact of the Hubble ACS camera or of incorrect handling of the data. Whether the ring around cluster Cl0024+17 is a true dark matter structure or just a mirage only time – and additional studies – will tell. But with six sevenths of the universe still waiting to be discovered, one thing is clear: the search for dark matter has only just begun. Did you like this article? Send
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