黑洞风中的尘埃(Astronomers Find Dust in the Wind of Black Holes)

http://www.spitzer.caltech.edu/Media/releases/ssc2007-16/release.shtml

For Release: October 9, 2007

                               
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The hit song that proclaimed, "All we are is dust in the wind," may have some cosmic truth to it. New findings from NASA's Spitzer Space Telescope suggest that space dust -- the same stuff that makes up living creatures and planets -- was manufactured in large quantities in the winds of black holes that populated our early universe.
The findings are a significant new clue in an unsolved mystery: where did all the dust in the young universe originate?
"We were surprised to find what appears to be freshly made dust entrained in the winds that blow away from supermassive black holes," said Ciska Markwick-Kemper of the University of Manchester, U.K. Markwick-Kemper is lead author of a new paper appearing in an upcoming issue of the Astrophysical Journal Letters. "This could explain where the dust came from that was needed to make the first generations of stars in the early universe."
Space dust is essential to the formation of planets, stars, galaxies and even life as we know it. The dust in our corner of the universe was piped out by dying stars that were once a lot like our sun. But, when the universe was less than a tenth of its present age of 13.7 billion years, sun-like stars hadn't been around long enough to die and make dust. So, what produced the precious substance back when the universe was just a toddler?
Theorists have long-postulated that short-lived, massive exploding stars, or supernovae, might be the source of this mysterious dust, while others have proposed that a type of energetic, growing supermassive black hole, called a quasar, could be a contributing factor. A quasar consists of a supermassive black hole surrounded by a dusty doughnut-shaped cloud that feeds it. Theoretically, dust could form in the outer portion of the winds that slowly blow away from this doughnut cloud.
"Quasars are like the Cookie Monster," said co-author Sarah Gallagher of the University of California at Los Angeles, who is currently a visiting astronomer at the University of Western Ontario, Canada. "They are messy eaters, and they can consume less matter than they spit out in the form of winds."
Nobody has found conclusive proof that either quasar winds or supernovae can create enough dust to explain what is observed in the early universe. Markwick-Kemper and her team decided to test the former theory and investigate a quasar, called PG2112+059, located in the center of a galaxy about 8 billion light-years way. Although this particular quasar is not located in the early universe, because it is closer, it is an easier target for addressing the question of whether quasars can make dust. The team used Spitzer's infrared spectrograph instrument to split apart infrared light from the quasar and look for signs of various minerals.
They found a mix of the ingredients that make up glass, sand, marble and even rubies and sapphires. While the mineral constituting glass was expected, the minerals for sand, marble and rubies were a surprise. Why? These minerals are not typically detected floating around galaxies, suggesting they could have been freshly formed in the winds rushing away from the quasar.
For instance, the ingredient that makes up sand, crystalline silicate, doesn't survive for long free-floating in space. Radiation from stars zaps the minerals back to an amorphous, glass-like state. The presence of crystalline silicate therefore suggests something -- possibly the quasars winds -- is churning out the newly made substance.
Markwick-Kemper and her team say the case of the missing dust is not firmly shut. They hope to study more quasars for further evidence of their dust-making abilities. Also, according to the astronomers, quasars may not be the only source of dust in the early universe. "Supernovae might have been more important for creating dust in some environments, while quasars were more important in others," said Markwick-Kemper. "For now, we are very excited to have identified the different species of dust in a quasar billions of light-years away."
Other authors of this paper include Dean Hines of the Space Science Institute, Boulder, Colo., and Jeroen Bouwman of the Max Planck Institute for Astronomy, Heidelberg, Germany. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared spectrograph was built by Cornell University, Ithaca, N.Y. Its development was led by Jim Houck of Cornell.

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.

jpl2007-114
ssc2007-16

上海网上天文台译文：
http://www.astron.sh.cn/yiwen/2007/yw071019-bhdust.htm

有一首流行歌曲唱道：“我们都是风中之尘”。歌词中可能蕴涵着宇宙真理。NASA斯必泽太空望远镜的新近发现说明，太空尘埃（与造就生命和行星的物质相同）在宇宙早期的黑洞风中大量产生。
对于一个未解之谜来说，这一发现提供了一条明显的新线索：宇宙早期的所有尘埃来自何方？

                               
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艺术家笔下类星体风中的尘埃颗粒。（图片提供：NASA/JPL-Caltech）

英国曼彻斯特大学的Ciska Markwick-Kemper说：“能发现新形成的尘埃被特大质量黑洞吹出的风带走，我们非常惊讶。”Markwick-Kemper是下一期《天体物理学通报》上某篇新论文的第一作者。“这就解释了尘埃的来源，这些尘埃是第一代恒星在早期宇宙中诞生时所必需的。”
正如我们已经知道的，对于行星、恒星、星系甚至是生命的形成来说，宇宙尘埃是至关重要的。曾经类似于太阳的濒死恒星一度将我们的邻近区域撒满了尘埃。但当宇宙年龄到达现在（137亿年）的十分之一时，这里已经长期没有过类太阳恒星死亡并产生尘埃了。那么，在宇宙的童年，是什么因素挽回了宝贵的尘埃成分呢？
理论家一直认为，短寿命的大质量爆发恒星（也就是超新星）也许是神秘的尘埃来源，同时有些人也认为，某种活跃增长着的特大质量黑洞（类星体）也许起了重要作用。类星体由一个特大质量黑洞连带周围的富尘环形云团组成，后者为黑洞提供物质来源。从理论上看，从环形云团缓慢吹出的黑洞风外围可以形成尘埃。
论文的合作者、加州大学洛杉矶分校的Sarah Gallagher说：“类星体就好象是曲奇怪兽（Cookie Monster，译注：《芝麻街》中的角色）。它们是邋遢的吞噬者，而它们消耗的物质数量还比不上以星际风的形式吐出的物质。”Sarah Gallagher当下是加拿大西安大略大学的访问天文学家。
不论是类星体风还是超新星，没有人能找到决定性的证据说明它们是否能产生早期宇宙中所见的足量尘埃。Markwick-Kemper和她的小组决定检验前一种理论，并研究了一个编号为PG2112+059的类星体，它位于银心方向，距离我们80亿光年。虽然该类星体由于较近的距离，并不属于早期宇宙的一份子，但对于回答类星体可否产生尘埃这一问题来说，它却是个易于研究的目标。小组使用斯必泽的红外光谱仪来分解类星体的红外辐射，以寻找各种无机物质的踪迹。
他们发现了玻璃、砂石、大理石甚至红蓝宝石的组分混合物。虽然玻璃的组分是预料到的，但砂石、大理石和红宝石的成分却让人吃了一惊。为什么呢？因为这些并非在星系周围探测到的典型物质，也就是说，它们是在类星体喷涌而出的星际风中新近形成的。
举例来说，砂石和晶体硅的组分不能在宇宙空间中自由飘荡很久。恒星的辐射会将这些物质破坏，让它们复归于无定形态的玻璃状。因此晶体硅的出现说明，某些机制（也许就是类星体风）正在艰难地制造新物质。
Markwick-Kemper和她的小组认为，这个失落尘埃的例子并不是十分坚实。他们希望能研究更多的类星体，以寻找产尘能力的进一步证据。另外，根据这些天文学家的观点，类星体可能不是早期宇宙中尘埃的唯一来源。Markwick-Kemper说：“在某些环境中，超新星可能是最重要的产尘机制，而其他情况下类星体更为重要。现在，能够在几十亿光年开外的类星体中鉴定出不同类型的尘埃，我们非常激动。”
本论文的其他作者包括科罗拉多Boulder空间科学研究所的Dean Hines和德国海德堡马普天文所的Jeroen Bouwman。加州帕萨迪纳的NASA喷气推进实验室是加州理工学院的机构，为NASA位于华盛顿的科学任务理事会管理着斯必泽太空望远镜。望远镜的科研运转由帕萨迪纳的加州理工学院斯必泽科学中心负责。加州理工学院为NASA管理着喷气推进实验室。斯必泽的红外光谱仪由纽约州伊萨卡的康乃尔大学制造，研发工作的领导者是康乃尔的Jim Huck。
关于斯必泽望远镜的更多图片和资料，可以访问http://www.spitzer.caltech.edu/spitzer和http://www.nasa.gov/spitzer。
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好文,多謝LZ提供!