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Dark matter core defies explanation in NASA Hubbleimage 哈勃图像呈现暗物质核心谜团 New results from NASA's Hubble Space Telescope confirm that dark matterand galaxies separated in Abell 520 instead of staying anchored together, aspredicted. 哈勃太空望远镜最新研究结果表明星系团Abell 520中暗物质和星系是分离的，并非之前预测的共存关系。 By NASA Headquarters,Washington, D.C. — Published: March 2, 2012 撰文：美国宇航局总部，华盛顿特区——发表于：2012年3月2日 This composite image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520, formed from a violent collision of massive galaxy clusters. 以上合成图片显示了正在融合中的星系团Abell 520核心暗物质、星系和炽热气体的分布情况，它是由大质量星系团剧烈碰撞后形成的。 Credit: NASA/ESA/CFHT/CXO, M.J. Jee (Univ. of California, Davis)/A. Mahdavi (San Francisco State Univ.) 供图：NASA/ESA/CFHT/CXO, M.J. Jee (加州大学戴维斯分校)/A. Mahdavi (旧金山州立大学) Astronomers using data from NASA's Hubble Space Telescope have observed what appears to be a clump of dark matter left behind from a wreck between massive clusters of galaxies. The result could challenge current theories about dark matter that predict galaxies should be anchored to the invisible substance even during the shock of a collision. 天文学家们在哈勃太空望远镜观测的数据中发现，大质量星系团碰撞后遗留下来一大团暗物质。这个观测结果有可能颠覆现有关于暗物质在星系碰撞中仍然与星系本身紧密相连的预测理论。
Abell 520 is a gigantic merger of galaxy clusters located 2.4 billion light-years away. Dark matter is not visible, although its presence and distribution is found indirectly through its effects. Dark matter can act like a magnifying glass, bending and distorting light from galaxies and clusters behind it. Astronomers can use this effect, called gravitational lensing, to infer the presence of dark matter in massive galaxy clusters. Abell 520是相距地球24亿光年外正在合并中的巨大星系团。暗物质本身无法观测，但其存在和分布状况可以通过其影响效应间接被发现。暗物质可以通过引力效应，使位于其后的星系和星系团的光线发生折射和弯曲，产生如同放大镜的效果。天文学家们正是利用这一引力透镜效应，推断出大质量星系团中暗物质的存在。
This technique revealed that dark matter in Abell 520 had collected into a "dark core," containing far fewer galaxies than would be expected if the dark matter and galaxies were anchored together. Most of the galaxies apparently have sailed far away from the collision. "This result is a puzzle," said astronomer James Jee from the University of California, Davis. "Dark matter is not behaving as predicted, and it's not obviously clear what is going on. It is difficult to explain this Hubble observation with the current theories of galaxy formation and dark matter." 同时这一技术显示出Abell 520星系团中的暗物质聚集形成了“暗物质核心”，其中包含的星系数量远少于暗物质和星系共存理论的预测值。很显然大部分的星系已经从碰撞中游离出去了。“这样的结果令人困惑”，加州大学戴维斯分校的天文学家詹姆斯·杰说道，“暗物质的状态不符合理论预测，而且我们也不清楚形成的具体原因。哈勃的观测结果与现有的星系形成和暗物质理论不符，这很难解释。”
Initial detections of dark matter in the cluster, made in 2007, were so unusual that astronomers shrugged them off as unreal because of poor data. New results from Hubble confirm that dark matter and galaxies separated in Abell 520. 最初发现星系团中的暗物质是在2007年，但这一现象过于不同寻常，而且囿于当时数据的缺乏，天文学家们只是将其作为假象放在一边不予理会。而最新的哈勃观测结果确定Abell 520星系团中的暗物质和星系是分开存在的。
One way to study the overall properties of dark matter is by analyzing collisions between galaxy clusters, the largest structures in the universe. When galaxy clusters crash, astronomers expect galaxies to tag along with the dark matter, like a dog on a leash. Clouds of hot, X-ray-emitting intergalactic gas, however, plow into one another, slow down, and lag behind the impact. 作为研究暗物质总体性质的途径之一就是分析星系团之间的碰撞，这是宇宙中最大尺度的组成结构。当星系团相撞时，天文学家认为暗物质会如同被牵住的宠物狗跟随主人一样紧随后面的星系。而炽热的气体云、星系间气体X射线辐射则会在碰撞后由于相互作用而落在后面。
That theory was supported by visible-light and X-ray observations of a colossal collision between two galaxy clusters called the Bullet Cluster. The galactic grouping has become an example of how dark matter should behave.
名为“子弹星系团”所展示的两个巨型星体团的碰撞时所观测到的可见光和X射线很好地支持了这一理论。星系团中暗物质的状态此后就作为参考的标杆。
Studies of Abell 520 show that dark matter's behavior may not be so simple. Using the original observations, astronomers found the system's core was rich in dark matter and hot gas, but contained no luminous galaxies, which normally would be seen in the same location as the dark matter. NASA's Chandra X-ray Observatory was used to detect the hot gas. Astronomers used the Canada-France-Hawaii Telescope and Subaru Telescope atop Mauna Kea in Hawaii to infer the location of dark matter by measuring the gravitationally lensed light from more distant background galaxies. 对Abell 520星系团的研究显示，暗物质的状况可能并非那么简单。在起初的观测中，天文学家发现这一系统的核心虽富含暗物质和炽热气体，但却不见发光星系的踪影，通常来说它应当与暗物质处于相同的位置。美国宇航局的钱德拉X射线天文台可以追踪炽热气体。天文学家们利用位于夏威夷莫纳克亚山顶的法国—加拿大夏威夷望远镜和昴星团天文望远镜观测，通过测量位于引力透镜后方遥远的背景星系发出的光线，推断出暗物质的位置。
The astronomers then turned to Hubble's Wide Field Planetary Camera 2, which can detect subtle distortions in the images of background galaxies and use this information to map dark matter. To astronomers' surprise, the Hubble observations helped confirm the 2007 findings. 尔后，天文学家们转向使用哈勃的2号宽视场行星照相机，检测背景星系图像中隐约存在的光线扭曲来绘制暗物质图。令天文学家们惊奇的是，哈勃的观测结果却证实了2007年的发现。
"We know of maybe six examples of high-speed galaxy cluster collisions where the dark matter has been mapped," Jee said. "But the Bullet Cluster and Abell 520 are the two that show the clearest evidence of recent mergers, and they are inconsistent with each other. No single theory explains the different behavior of dark matter in those two collisions. We need more examples." “虽然我们已经获取了大约六个高速星系团碰撞所产生的暗物质图，”杰说道，“但子弹星系团和Abell 520是其中两个有着近期最清晰证据的碰撞，两种情况不尽相同。现在还没有任何理论能够解释两次碰撞中暗物质的不同表现。我们需要更多的样本。”
The team proposed numerous explanations for the findings, but each is unsettling for astronomers. In one scenario, which would have staggering implications, some dark matter may be what astronomers call "sticky." Like two snowballs smashing together, normal matter slams together during a collision and slows down. However, dark matter blobs are thought to pass through each other during an encounter without slowing down. This scenario proposes that some dark matter interacts with itself and stays behind during an encounter. 研究团队针对发现结果提交了数份解释报告，但都未能获得天文学家的认可。在某个情况下，一些暗物质可能会以极低几率出现天文学家们所说的“粘连”情况。比如两个撞在一起的雪球，普通的物质碰撞在一起时会降低速度。但是，普遍观点认为暗物质粒子在碰撞中会互相穿越，维持原来的速度。但在这个情形下，一些暗物质与其自身发生相互作用，并在碰撞后留在原地。
Another possible explanation for the discrepancy is that Abell 520 has resulted from a more complicated interaction than the Bullet Cluster encounter. Abell 520 may have formed from a collision between three galaxy clusters, instead of just two colliding systems in the case of the Bullet Cluster. 不过，另一可能的解释就是Abell 520星系团经历了一场比子弹星系团更加复杂的碰撞和相互作用。Abell 520星系团可能形成于三个星体团的碰撞，而并非像是子弹星系团的两个。
A third possibility is that the core contained many galaxies, but they were too dim to be seen, even by Hubble. Those galaxies would have to have formed dramatically fewer stars than other normal galaxies. Armed with the Hubble data, the group will try to create a computer simulation to reconstruct the collision and see if it yields some answers to dark matter's weird behavior. 第三种可能就是其核心可能包含许多个星系，只不过光线过于暗淡，难以被哈勃观测。这些星系的恒星很有可能远少于其它普通星系。研究团队将会在今后运用更多的哈勃数据，进行计算机模拟重建碰撞过程，找寻更多关于暗物质奇特状态的蛛丝马迹。
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