天文学家发现了迄今观测到的最致密星系（矮星系）
本帖最后由 sonic5188 于 2013-9-28 21:28 编辑好久没来翻译版发帖，今天闲着没事来发个玩玩，绿色字是原文翻译，红色字是本人添加的内容。
http://www.uux.cn/attachments/2013/09/1_201309271104451NWX0.jpgThe super-dense galaxy M60-UCD1, seen in the inset, is located 54 million light-years away超致密矮星系M60-UCD1，照片中可以明显看出，坐落于5400万光年之外UCD是Ultra compact dwarf的缩写，表示超致密矮星系。
It’s a case of cosmic whodunit: What happened to all the external stars in M60-UCD1, an ultra-compact dwarf galaxy located 54 million light-years away, near the fringes of the bright M60 galaxy in the constellation Virgo?宇宙侦探小说里出现了一个奇怪的案件：超致密矮星系M60-UCD1外围的恒星都哪里去了？M60-UCD1位于室女座，距离地球5400万光年，在明亮的椭圆星系M60的边缘。
Despite its diminutive title this dwarf galaxy is packing a lot of mass — over 200 million times the mass of the sun is stuffed inside it, making M60-UCD1 one of the densest local galaxies ever discovered, if not the densest.尽管M60-UCD1身材矮小，但它却被塞入多达2亿个太阳的巨大质量，因此M60-UCD1即便不是宇宙中致密的星系，也至少是我们附近最致密的星系之一。
实际上超致密矮星系是现在星系研究领域的前沿，室女座星系团、天炉座星系团、仙后座星系团中都发现过一些超致密矮星系。
What’s more, over half of that mass is found within a region only 160 light-years across. Stars there are 25 times closer together than they are in our neighborhood of the Milky Way.更要命的是，这个矮星系超过一半的质量被发现集中在一个160光年的范围内，比银河系附近星系的恒星密度要高出25倍。
“Traveling from one star to another would be a lot easier in M60-UCD1 than it is in our galaxy,” said Jay Strader of Michigan State University in Lansing, first author of a new paper describing these results.公布这个发现的论文第一作者，密歇根州立大学（位于兰幸市）的教授Jay Strader说：“在M60-UCD1的恒星之间穿行将比在银河系容易得多。”
The question is: how did M60-UCD1 get so dense? Did it originally form as a crowded star cluster? Or is it what’s left of an even larger galaxy… one that’s had its outer stars stolen away during a run-in with another galaxy?现在问题是M60-UCD1是如何变得这么致密的？是不是从一个拥挤的球状星团演化而来？或者是一个更大的星系剩下来的核心？大部分外层恒星被另一个星系夺走？
This is where the x-ray vision of NASA’s Chandra space observatory comes in.NASA的钱德拉X射线空间天文台拍下了上述照片，发挥了极大的作用。
http://www.uux.cn/attachments/2013/09/1_201309271106001eEEW.jpgOptical HST image of M60. Dwarf galaxy M60-UCD1 is the bright object at bottom-left center.这是哈勃太空望远镜拍摄的M60星系的照片，主角是M60，照片底部中央靠左的白色亮斑就是M60-UCD1，左上角是螺旋星系NGC 4647。
After its discovery with the Hubble Space Telescope, the super-dense dwarf galaxy was observed with Chandra. Inside it, Chandra spotted a bright x-ray source which may indicate the presence of a central supermassive black hole 10 million times more massive than our sun.继哈勃太空望远镜之后，钱德拉X射线空间天文台再次观测，发现M60-UCD1里面有一个明亮的X射线源，这可能表明矮星系的中央存在一个1000万倍太阳质量的超大质量黑洞。
但椭圆星系M60核心的黑洞更加巨大，估计有45亿个太阳质量，这可是迄今发现的超大质量黑洞中排名前几的。
Since supermassive black holes aren’t found in star clusters that we know formed as star clusters, then it’s very likely that, if the x-ray source is in fact a SMBH, M60-UCD1 was once an enormous galaxy up to 200 times more massive than it is now — but its stars were lost in a galactic collision at some point.由于超大质量黑洞（SMBH）从未在星团中被发现过。如果这个X射线源来源于超大质量黑洞，可能预示着M60-UCD1曾经是一个比现在质量大200倍的星系演化而来，并在星系碰撞中失去了大部分恒星。
“We think nearly all of the stars have been pulled away from the exterior of what once was a much bigger galaxy,” said co-author Duncan Forbes of Swinburne University in Australia. “This leaves behind just the very dense nucleus of the former galaxy, and an overly massive black hole.”论文的合著者，澳大利亚斯威本科技大学的Duncan Forbes说：“我们认为，这个曾经更大的星系外围几乎所有恒星都已经被卷走，留下了非常密集的前星系核，还有一个过于巨大的黑洞。”
Estimated to be about 10 billion years old, M60-UCD1′s tragic star stripping event may have taken place fairly early in its evolution, leaving it in its diminished state for the past several billion years.M60-UCD1的年龄估计有100亿年，悲剧般的恒星剥离事件发生在其演化过程中相当早的时期，过去数十亿年中它还在持续衰弱。
(I wonder what the statute of limitations is on grand stellar larceny?)我不知道这场大规模的恒星盗窃事件的诉讼失效时间是多长？很明显M60是一个重要的嫌疑犯，不远处的NGC 4647很有可能是下一个受害者，靠近椭圆巨星系的人有危险。^_^
The results of this study are online and have been published in the September 20th issue of Astrophysical Journal Letters.这项研究结果已经发表在9月20日的《天体物理学杂志快报》，并传到网上。
啊，很难想象这个矮星系上的外星人（假设有的话）相互之间打招呼、串个门什么的，也太容易了，近得来只有零点几光年吧。
但恒星之间相互的射线太密集，如果有生命的话，都是超级强悍的。 感谢楼主分享！学习了！
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