This Galaxy Doesn't Have Dark Matter, and That's Weird
This picture shows NGC1052 (on left), a massive elliptical galaxy that dominates the neighborhood where NGC1052-DF2 also lives. NGC1052-DF2 is an ultra-diffuse galaxy located 65 million light-years away that seems to be missing itsdark matter. Also pictured are NGC1042 (the main spiral) and NGC1048 (the two interacting galaxies on the bottomright).
这个星系不含暗物质，这很奇怪呀（that's weird）
左图是NGC1052，一个巨大椭圆形星系，主宰着她的邻居NGC1052-DF2 。NGC1052-DF2 是一个超弥散星系，距离NGC1052有6.5亿光年那么远以至于在图片中看起来就像没有暗物质一样。图中还有NGC1042（中间的漩涡）以及NGC1048（在右下角两个相互影响的星系）。
Dark matter is the bedrock that all galaxies are anchored to. You can't get one without the other. Or so we thoughtuntil astronomers found a ghostly galaxy that doesn't appear to contain any dark matter. It's as if the universe isplaying a trick on us by flipping the laws of physics on their head — dark matter should be there, but it isn't.
暗物质是所有星系固定（be anchored to）的河床，你不能顾此失彼。我们一直这样认为，一直到天文学家发现了一个幽灵般的星系，这个星系看起来没有任何暗物质。这就像宇宙对我们开了一个玩笑，他们翻转了物理定律（flip ...on their head）这些暗物质本应在那里，然而他们并不在。
It's a "game changer" galaxy, astronomers are saying, and it's like nothing we've ever seen before.
天文学家说这是一个“改变游戏规则”的星系，我们此前从未见过类似情况（it's like nothing we've ever seen before. ）。
We may not be able to spot dark matter, but astronomers can measure its gravitational effects acting on normalmatter. For example, they can look at how fast stars cruise around a galaxy. When dark matter is present, thatgalaxy's gravity will be bulked up, causing its stars to move faster than if just normal matter were present.
我们也许看不到暗物质，但天文学家可以测量他对普通物质的引力。比如，他们可以观察星系周围星球漫游的速度。当暗物质存在时，星系的重力会增加，这会让他周围的星球移动速度比那些周围只有普通物质存在的星球快。
But in the case of NGC1052-DF2, an ultra-diffuse galaxy located 65 million light-years away, astronomers have foundthat its stars are moving in exactly the way that would be predicted if only the total mass of all the visible stuff isconsidered. In other words, dark matter doesn't seem to be exerting its gravity on normal matter in that galaxy. Andthat's weird.
但是，就NGC1052-DF2这个距离地球6.5亿光年的超漫射星球来说，天文学家发现只要能观测到周围所有观测到的物质总量，它周围星球运行的路径就可以预测。也就是，在这个星系暗物质好像对其他物体没有引力作用。这真的很奇怪。
"Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the mostdominant aspect of any galaxy," Pieter van Dokkum, of Yale University, said in a statement. "For decades, we thoughtthat galaxies start their lives as blobs of dark matter. After that everything else happens: gas falls into the dark matterhalos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way. NGC1052-DF2challenges the standard ideas of how we think galaxies form."
耶鲁大学的Pieter van Dokkum说在一次陈述中说，发现一个没有暗物质的星系出乎我们的意料，因为这种无形且神秘的物质是主宰星系的主要力量。几十年来，我们一直认为星系是以一小点暗物质为基础孕育，然后有气体落入暗物质的外晕，然后变成星球，再慢慢长大，在这样一系列过程发生后形成现在的星系。但是NGC1052-DF2的发现打破了我们对星系形成的固有观念。
Ultra-diffuse galaxies are oddities in their own right, having only been discovered in 2015 as they are very difficult todetect. However, it appears that this class of galaxy is common, but none are like NGC1052-DF2.
超弥散星系本身(in their own right）就很奇怪，因其很难被观测到直到2015年才被发现，但这组星系很寻常，并不像NGC1052-DF2.
The galaxy was discovered using the Dragonfly Telephoto Array, a telescope in New Mexico that's custom-made toseek out these elusive targets. Then, using the twin 10-meter optical and infrared telescopes at the W. M. KeckObservatory on Mauna Kea in Hawaii, the astronomers singled out 10 bright globular clusters (large compact groupsof stars orbiting the galaxy's core) and used spectral data to measure their motions. These clusters were found to beplodding along more slowly than expected, meaning there's far less mass in that galaxy than would be predicted. Infact, there's so little mass that the researchers have come to the astonishing conclusion that there's little, if any, darkmatter there.
这个星系是由Dragonfly这个天文相片矩阵发现的，这个位于新墨西哥的特制望远镜是用来寻找难发现的目标。在夏威夷州莫奈克亚山上，宇航员用双倍十米口径光学和红外望远镜筛选出（single out）10个球状聚集物（巨大的，紧密成团的众多星球环绕着星系中心运行）并用波谱数据测算出他们的运行。结果发现这些星球运行的速度比预期的要慢很多。实际上，这里的物质太少了，所以研究者得出惊人结论，这里即使有暗物质也极少。
Follow-up observations were made by the Gemini North telescope, also on Mauna Kea, so the galaxy's structure couldbe studied. With Gemini's help, the researchers ruled out interactions with other galaxies as being the cause of itsweird dark matter deficit.
后续的观察也是在莫奈克亚山上，用双子北座望远镜观测使得可以继续研究星系的结构。通过双子座望远镜，研究者可以排除其他星系的干扰，这些干扰是由奇怪的其暗物质不足导致的。
"If there is any dark matter at all, it's very little," said van Dokkum in the press release. "The stars in the galaxy canaccount for all of the mass, and there doesn't seem to be any room for dark matter."
van Dokkum在新闻稿（press release）中说，这个星系中即使有暗物质，也很少。在这个星系中的星球几乎是它所有的物质了，没有暗物质的空间了。
This finding seems to suggest that dark matter has "its own separate existence apart from other components ofgalaxies," he added. And this makes the very existence of NGC1052-DF2 a mystery. If it has no dark matter, how did iteven evolve into a galaxy?
他补充说，这个发现似乎暗示着暗物质有自己独特的存在方式，这种存在方式有别于众多星系中的其他物质。这使得NGC1052-DF2成为一个特别且神秘的存在。如果没有暗物质的话，那它是怎么演变成一个星系的呢？
In their study published in the March 29 issue of Nature, van Dokkum's team speculates that some cataclysmic eventin the galaxy may have cleared out all the dark matter and blasted away all the star-forming gases. Alternatively, thenearby massive elliptical galaxy NGC 1052 may have played a role in NGC1052-DF2's lack of dark matter billions ofyears ago when it was undergoing the early and violent stages of evolution.
5月29日发表在《自然》杂志上的研究, van Dokkum's的团队推测，在星系中发生了一些演变事件清除了所有暗物质并喷射出所有星状气体。也许在几十亿年前当它正在经历早期巨大演变阶段时，它附近的椭圆星系NGC 1052对于NGC1052-DF2缺乏暗物质也起到了重要作用。
Now, the researchers are poring over Hubble Space Telescope observations of similar galaxies to perhaps find morethat lack dark matter. If they find more, then ultra-diffuse and faint galaxies might be the norm if dark matter isn'tpresent. And that's a fascinating development in our understanding of how galaxies evolve.
如今，研究人员正在用哈勃望远镜仔细观察寻找那些也许也缺少暗物质的更小的星系。 如果可以找到更多，那么超弥散星系和微弱（faint）星系不存在暗物质可能就是标准了。那对我们在理解星系演变过程贡献将是巨大的。
"Every galaxy we knew about before has dark matter, and they all fall in familiar categories like spiral or ellipticalgalaxies," concluded van Dokkum. "But what would you get if there were no dark matter at all? Maybe this is whatyou would get."
van Dokkum说以前我们知道的所有含有暗物质的星系，他们都会被归类到熟知的椭圆星系或漩涡星系。但是如果根本没有暗物质要怎样归类？也许这就是接下来需要研究的。