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The galaxy cluster Abell 2744 contains so many unusual phenomena that astronomers have dubbed it "Pandora's Cluster," after the mythological box said to contain all the world's ills. Unlike the eponymous box, this object contains phenomena that aren't evil (at least we hope not), and could even reveal some of the mysteries of dark matter. But what makes this cluster so interesting is not apparent to the unaided eye.
In order to complete this image of Abell 2744, astronomers compiled visible light gathered by NASA's Hubble Space Telescope, the Very Large Telescope at the European Southern Observatory, and Japan's Subaru telescope. Scientists, however, had to use x-ray emissions taken by the orbiting NASA Chandra X-Ray Observatory to reconstruct the location of hot gases (colored red). In order to map invisible dark matter (shown in blue) scientists had to look at how the light from background galaxies was distorted by its gravity, and then reconstruct the dark matter's location mathematically.
This picture of Abell 2744 unveils its history, and its anomalies. Pandora's Cluster probably formed from the collision of four different parent clusters, a process that lasted millions of years. When two of the parent clusters smashed together, their collision created a central "bullet," where the impact of gas on gas created a shock wave in matter but left the distribution of dark matter untouched. In a separate region of Pandora's cluster there is a gas-free cluster of dark matter and galaxies, where the parent clusters' collision stripped away any hot gas. In yet another region a clump of dark matter sits alone, with neither galaxies nor gas.
The segregated arrangement of galaxies, hot gas and dark matter in Abell 2744 is unprecedented. Although certain other galaxy clusters do exhibit bullets or clumps, no other system combines these features. By studying Pandora's Cluster, astronomers hope to glean more information about the behavior of dark matter and its interaction with more familiar materials.
--Sophie Bushwick
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8 Comments
Add CommentSince the existence of dark matter is unproven, this article is useless...
Reply | Report Abuse | Link to thisI agree with your sentiments, but the existence of dark matter has for many years been widely assumed by astronomers and cosmologists: its inferred existence (which cannot be directly confirmed or quantified) allows for convenient solution to some rather difficult problems they have created for themselves.
Reply | Report Abuse | Link to thisIn the case of, especially, colliding galaxy clusters the presence of dark matter is very difficult to infer, but has been established in the past ten years or so.
As the article states, "x-ray emissions taken by the orbiting NASA Chandra X-Ray Observatory to reconstruct the location of hot gases (colored red)." These hot gases of ordinary matter form the intracluster medium of galaxy clusters and are estimated to contain about twice the mass of the estimated mass of ordinary galactic matter.
As I understand, the mass of ordinary galactic matter is generally estimated based on the galaxies' Mass/Light ratios: more luminous galaxies are presumed to contain greater mass than less luminous galaxies. Unfortunately, for especially spiral, galaxies their stellar luminosities are greatly obscured by enormous clouds of massive opaque gases. Moreover, the luminosity produced by galaxies often varies by our observational perspective: a spiral galaxy viewed edge-on would far less luminous than it would if viewed full-on, providing a full view of its disc.
The inference of dark matter in galaxy collisions is derived from exceeding slight distortions to large numbers of observed background objects attributed to weak gravitational lensing by a total mass determined from a statistical analysis of the degree of minute distortions applied to the many background objects. While astronomers are exceedingly proficient and performing such detailed analyses, "These weak lensing surveys must carefully avoid a number of important sources of systematic error: the intrinsic shape of galaxies, the tendency of a camera's point spread function to distort the shape of a galaxy and the tendency of atmospheric seeing to distort images must be understood and carefully accounted for." ref.:
http://en.wikipedia.org/wiki/Gravitational_lensing
As I understand (very generally), once a total mass has been derived for an area and the total mass of observed galaxies has been estimated from M/L ratios, the difference between the two extremely difficult mass estimates is presumed to be provided by the inferred existence of dark matter.
Astronomers are confident in their very complex analyses of galaxy clusters, but there is a large potential for error...
While I expect that astronomers have been very thorough in their statistical analyses of weak gravitational lensing effects, as I understand the correspondence between effects and the mass necessary to produce them is based entirely on the gravitational effects of mass.
Reply | Report Abuse | Link to thisThe fundamental feature of colliding galactic clusters that distinguishes them from a single cluster is the rapid acceleration of cluster scale masses to high velocities. I suggest that these high velocities may significantly contribute to the lensing effects produced by cluster scale masses and commensurately reduce the gravitational mass necessary to produce the observed effects.
In the most analyzed examples of two colliding clusters, the identified region of weak gravitational lensing coincides with the location of the clusters' galaxies. As a result, any overestimation of lensing effects or underestimation of galactic masses may explain the observed effects without invoking the presence of any dark matter.
In this example of multiple galaxies of varying scales colliding (Abell 2744), the principle location of identified weak gravitational lensing effects coincides with the location of the largest clusters' galaxies. However, a smaller region that does not contain any cluster's galaxies has been identified as exhibiting gravitational lensing effects. This condition might be produced by the rapid acceleration of a smaller cluster's galaxies into the larger clusters, leaving a small region of evacuated spacetime exhibiting high velocity turbulent flow effects causing lensing effects.
Again, I'm not questioning the proficiency of astronomers in performing the established analyses of cluster collisions, but that current procedures developed for application to single galaxy clusters are being applied to high velocity galactic cluster collisions. Consideration of additional factors may be required.
Rely Wayne?! Is a hypothesis useless? I'll tell you what is useless, the opinion of an ignoramus.
Reply | Report Abuse | Link to thisThe images in this photoset is not what they think it is... It is much MORE. It is a device and not at all the result of naturally occurring phenomena. A device is made to DO something. They need to consider this as something synthetic and backward engineering will tell them the rest of the story.
Reply | Report Abuse | Link to thisI cannot help but think of the scientists who tried so hard to keep the Earth at the center of the solar system with their complicated theory of orbital ellipses for the planets. The theories of dark matter just sound so much like the same thing. Theorists are trying so hard to make the scales balance while never considering that maybe things do not add up for a reason. What if the matter that they can see, weigh and measure is being acted on by forces outside this universe? Are we like the inhabitants of Flatland trying to understand a sphere? I think the reality is that every time we look farther, we see how much more there is that we do not know.
Reply | Report Abuse | Link to thisSo there was no gravity until the theory was proven?
Reply | Report Abuse | Link to thisThe article states:
Reply | Report Abuse | Link to this"In a separate region of Pandora's cluster there is a gas-free cluster of dark matter and galaxies, where the parent clusters' collision stripped away any hot gas. In yet another region a clump of dark matter sits alone, with neither galaxies nor gas."
"The segregated arrangement of galaxies, hot gas and dark matter in Abell 2744 is unprecedented. Although certain other galaxy clusters do exhibit bullets or clumps, no other system combines these features."
In other collisions of two comparably massive galaxy clusters only the x-ray emitting intracluster mediums are segregated from the two clusters' spearate galaxies and inferred dark matter, both of which are presumed to continue beyond the point of 'collision' because neither are thought to interact: the galaxies because they are so disperse and the dark matter because of its hypothesized weakly interacting material characteristic.
Since in all other cases the inferred dark matter is coincident with the clusters' galaxies, what process is supposed to have separated them in Pandora's cluster? It is only its proposed collision of multiple galaxy clusters of varying masses and the this identification of weak lensing effects in a region segregated from any galaxies that distinguishes Abell 2744 from any other cluster collisions.
If there is no other proposed explanation for this distinguishing characteristic, I have suggested in a preceding comment how the region evacuated by a smaller cluster could be residually 'curved' by the acceleration of galaxies, producing weak lensing effects even without the presence of any dark matter...