星系碰撞创造了类星体（图）  

对远方类星体的观测显示，它们是由于大型星系碰撞而形成的。

Galaxy Collisions Give Birth to Quasars

Peering behind thick clouds of cosmic gas and dust, researchers believe they have finally determined the origins of quasars, the brightest and most powerful objects in the universe. X-ray and infrared observations of over 200 distant galaxies, coupled with images taken in visible light, reveal that quasars form when two galaxies smash into each other and their central black holes merge. The new observations also suggest that quasars were more common in the early universe than previously thought.	研究人员将视线深入到由宇宙气体和尘埃构成的浓密云团后面，这时他们认为终于确定了类星体的起源。类星体是宇宙间最为明亮、最具威力的天体。通过对200多个远方的星系进行X射线和红外线观测，结合在可见光状态下拍摄的图像，结果显示当两个星系互相碰撞、其中心的黑洞融合在一起的时候，类星体就形成了。这些新观测还表明，早期宇宙中的类星体更为常见，这一点是始料不及的。
Astronomers discovered quasars, short for quasi-stellar objects, in the 1950s. Only about the size of our solar system, quasars easily outshine entire galaxies and can burn for 100 million years. For decades, however, astronomers could not figure out what generated these cosmic beacons. The obvious suspects were supermassive black holes, which anchor the cores of practically all galaxies, can devour gigantic amounts of matter, and are known to produce huge jets of particles and energy. But many galaxies—including the Milky Way—host supermassive black holes yet don't generate quasars.	天文学家们在20世纪50年代发现了类星体，类星体是“类似恒星的天体”之缩略语。类星体仅仅大约相当于我们太阳系的大小，可是它们可以轻而易举地照亮整个星系，可以燃烧一亿年。然而几十年来，天文学家们不明白是什么创造了这些宇宙明灯。最明显的怀疑目标就是超大质量的黑洞，它们稳居在几乎所有星系的中心，可以吞噬大量的物质，而且据了解它们能够产生巨大的粒子和能量射流。但是很多星系——包括银河系在内——都拥有超大质量的黑洞，然而并没有产生类星体。
Perhaps the younger quasars were hiding. At least, that's what astronomers began to suspect in the late 1990s when they noticed that some galaxies whose central cores were obscured by huge clouds of dust seemed to emit the same kind of radiation and produced similar levels of energy as quasars.	或许，较为年轻的类星体隐匿起来了！至少在20世纪90年代末期，天文学家们就开始那样认为了。当时，他们注意到一些星系的核心被巨大的尘埃云团所遮掩，但所释放出的射线似乎跟类星体一样，散发出的能量水平也近似于类星体。
To peek behind the dust, astronomer Ezequiel Treister of the University of Hawaii, Honolulu, and colleagues first selected about 200 candidate galaxies out of 100,000 images taken by the Hubble Space Telescope. Then they trained the Chandra and Spitzer space telescopes, which can see in x-ray and infrared light, respectively, on the galaxies' obscured cores. The candidates ranged out to a distance of about 11 billion light-years—back to a time when the universe was only 2.7 billion years old. The new observations revealed hidden quasars in every one of the galaxies, the team reports online today in Science. What's more, by studying the shape of the galaxies in the Hubble images, the astronomers found that they all had arisen from the mergers of two massive galaxies and their central black holes.	为了观察到尘埃后面的情况，檀香山夏威夷大学的天文学家伊齐基尔·特雷斯特及同事，首先从哈勃太空望远镜拍下的10万张图像中选择了大约200张图像。然后，他们将钱德拉太空望远镜和斯必泽太空望远镜对准这些星系被遮挡的核心。这两个望远镜分别可以在X射线和红外线状态下进行拍摄。这些目标星系的距离范围延伸到大约110亿光年之外——返回到宇宙仅有27亿岁的时候。研究小组今天在《科学》在线版上报道说：这些新的观测显示，每个星系中都有隐藏的类星体。况且，通过研究哈勃太空望远镜拍下的图像中显示出来的星系形状，天文学家们发现类星体全部产生于两个巨型星系及其中心黑洞的合并。
Taken together, the data paint a picture of how quasars form. "Suddenly it all made sense," says astronomer and co-author Priyamvada Natarajan of Yale University. When two big galaxies merge, she explains, their supermassive black holes combine. The new black hole, which weighs as much as billions of suns, begins devouring everything in its vicinity. For up to 100 million years, this activity is shrouded by the dust kicked up by the galaxy merger. But eventually, the black hole consumes enough of the dust that the powerful and brilliant particle jets blasting out from its poles can break into the open. After another 100 million years, the quasar exhausts its fuel and goes dark, becoming just a garden-variety, billion-solar-mass or so black hole.	综合考虑，这些资料描绘了类星体形成的画面。“突然间，这一切变得很有道理，”耶鲁大学的天文学家、研究论文的合作者普里亚姆瓦达·那塔拉印说。她解释说，当两个大型星系合并之时，它们的超大质量黑洞也结合在一起。新形成的黑洞质量相当于几十亿个太阳，开始吞噬附近的一切。在长达一亿年的时间里，这种活动被星系合并激起的尘埃所遮蔽。但是，黑洞最终吞噬了足够的尘埃，从两极爆发出高能而明亮的粒子射流，射流破尘而出，进入太空。再过一亿年之后，类星体耗尽自己的燃料，逐渐变暗，成为质量大约相当于十亿个太阳的普通黑洞。
In the early universe, collisions—and hence quasars—occurred more often because galaxies were packed much more closely together. For many years, Treister says, astronomers doubted that obscured quasars were very common. "Now," he says, "we are seeing them everywhere."	在早期宇宙中，星系碰撞以及因此而形成类星体的情况发生得更加频繁，因为当时星系之间的距离要接近得多。特雷斯特表示，多年来天文学家们一直不相信被遮挡的类星体是很普遍的。“现在看来，”他说，“它们随处可见。”
The findings provide "powerful insights into how quasars form and evolve," says astronomer Amy Barger of the University of Wisconsin, Madison. The notion that obscured quasars are the byproduct of galaxy mergers has been around for quite a while, adds astronomer Christopher Reynolds of the University of Maryland, College Park. "But it is amazing that the fraction of obscured quasars tracks the galaxy-merger rate so well," he says. "It means that there isn't much room for other mechanisms to produce obscured quasars."	威斯康星大学麦迪逊分校的天文学家艾米·巴杰说，这些发现在类星体的形成和演化方面提供了深刻的认识。马里兰大学帕克分校的天文学家克里斯托佛·雷诺补充说：被遮挡的类星体是星系合并的副产品，这个观念已经存在了好长时间。“然而，通过那些被遮挡的类星体总是能够跟踪到星系的合并，”他说，“这意味着被遮挡的类星体不会拥有多少其他的形成机制。”