文章出处： http://www.spitzer.caltech.edu/features/articles/20060303.shtml
本人翻译。仅供个人学习、参考，探讨翻译技巧，不得用作商业用途。请合理使用。
A Shocking Surprise in Stephan's Quintet
Written by Michelle Thaller, Spitzer Science Center
March 3, 2006
When astronomers using NASA's Spitzer Space Telescope turned their attention to a well-known cluster of galaxies called Stephan's Quintet, they were, quite simply, shocked at what they saw. There in the middle of the cluster, invisible to our eyes, lurked one of the biggest shock waves, or "sonic booms," ever seen. Astronomers hope that observing how galaxies generate such huge shock waves will lead to a better understanding of what powers the most luminous galaxies in the universe.
For some time now, astronomers have known that four galaxies in this distant cluster (and a 5th unrelated foreground galaxy -- hence the name "Quintet"), located about 300 million light-years away from Earth, are involved in a violent collision. In visible light, the galaxies are clearly distorted, indicating that they have experienced gravitational encounters in the past. But that, as it turns out, is only part of the drama.
One of the four galaxies, called NGC 7318b, for reasons not fully understood, is currently falling toward the others at high speed, generating a giant bow shock in front of it which can be seen in X-ray, infrared, and radio light. Amazingly, the surface of the shock wave is larger than our own Milky Way galaxy, and stretches across intergalactic space between the colliding galaxies.
Shock waves occur when an object moves faster than the speed of sound through any kind of medium -- from water to intergalactic gas. One of the most commonly observed forms of shock waves is the sonic boom produced by high-speed aircraft. As a supersonic jet exceeds the speed of sound (or Mach 1), it catches up with its own sound waves. The sound waves become compressed together into a cone-shaped "shock" which travels outwards towards the ground and produces the familiar "sonic boom" when a supersonic jet flies by. The sonic shock is usually invisible to us, but in one case, a Navy ensign was able to capture a spectacular photograph of a shock wave extending behind a low-flying jet over the ocean. As water vapor in the air was compressed by the shock wave, it condensed into droplets and formed a conical cloud behind the tail of the jet (see image).
Dr. Philip Appleton (Caltech) and collaborators turned Spitzer's sensitive Infrared Spectrograph toward the location of the giant shock wave, a visibly dark area between the galaxies, hoping to discover more about what was going on. Unlike an optical telescope, Spitzer has the ability to detect infrared light from invisible materials, like dust grains or molecules.
To their surprise they discovered the telltale fingerprint of extremely powerful molecular-hydrogen. "The strength of the emission and the fact that it shows the gas to be Doppler-broadened (astronomer-speak for "highly disturbed") was a huge surprise to us." said Appleton. "We expected to see the spectral signature of dust grains -- but instead we saw an almost pure laboratory-like spectrum of hydrogen molecules and almost nothing else. It was quite unlike anything we had seen before in a distant galaxy system."
Spectrographs have the ability to break light down into its component wavelengths, where the chemical signatures of the material that produced it can be seen as spectral lines. The width of these lines allows astronomers to determine the velocity of the gas, with wider lines indicating gas at a higher velocity. Appleton and his team measured the widest lines ever observed for hot hydrogen molecules, corresponding to turbulent gas motions of 870 kilometers per second (2 million mph)!
"In Stephan's Quintet," says Appleton, "the shock wave is due to the intruder galaxy (NGC 7318b) traveling at speeds greater than Mach 100 as it plows into intergalactic gas within the cluster. Hydrogen molecules are apparently forming either in or behind the shock, similar to the water droplets condensing in the jet image -- but on an enormous scale!"
In a way, the discovery of something so new and unusual was not such a big surprise to the team, as the dynamics of this cluster are far from understood. Dr. C. Kevin Xu (Caltech), one of the team members says: "Stephan's Quintet is such a unique object that every time it is looked at with a new instrument, it reveals totally unexpected things. No exception this time."
This discovery may lead to a better understanding of the most infrared-luminous galaxies in the universe, the mysterious Ultra-luminous Infrared Galaxies. "Ultra-luminous Infrared Galaxies typically have infrared luminosities 100 to 1,000 times greater than the Milky Way, and their numbers increase as you look out to higher and higher red-shifts," says Appleton. "We know that these galaxies are also involved in vast mergers and collisions. It's possible that some of the emission we see from them is created not by stars, but by vast shocks in the gas between colliding galaxies."
The observations presented by Appleton and fellow team members C.K. Xu and W. Reach at Caltech, as well as other astronomers in the US, Germany and Australia, provide a new diagnostic for studying conditions in merging and colliding galaxies in the early universe.
"Observing a relatively nearby densely populated galaxy group immersed in a thick gas cloud, gives us a local view of what might have been going on about 10 billion years ago, soon after the first galaxies formed, when the density of the intergalactic medium and the density of galaxies was much greater than today. In that respect these observations are a bit like stepping into a time machine," said Drs. Cristina Popescu and Richard Tuffs, two other team members from the Astrophysics Department of the Max-Planck Institute in Heidelberg, Germany.
But this discovery has implications even for our own galaxy, the Milky Way. Though far in the future, it is likely that in about two billion years from now, we will collide with the slightly larger Andromeda Galaxy, creating shocks of our own. In that case, our future descendents will have a ring-side sea.
Image Credit: Composite- NASA/JPL-Caltech/Max-Planck Institute/ P. Appleton (SSC/Caltech); 8-Micron (IRAC): NASA/JPL-Caltech/J. Houck (Cornell)
当天文学家用NASA的Spitzer空间红外望远镜将注意力集中到著名的Stephan’s 五重星系群时，他们完全被他们发现的情景所震惊（shocked）了。在星系群中，埋伏着人类迄今所“见到过”（当然，实际上是通过不可见光观测）的最强大的激波（shock wave或“声爆”）。天文学家希望通过研究星系产生如此强大激波的原理，来增进我们对宇宙中最明亮的星系能量来源的认识。
天文学家已经知道，这一距离我们约30亿光年远的星系群中4个成员（第5个是背景星系，和它们没有物理联系）正置身于一场激烈的冲撞中不能自拔。在可见光波段，这些星系的外观明显扭曲变形，说明它们在过去经历过巨大的引力相互作用。但是，现在的观测证明，这仅仅是一出大戏中的一小折。
因为某种尚未探明的原因，这4个星系中的一员——NGC7318b——目前正以很高的速度冲向其它星系，同时产生一个极为庞大的弓状激波，在X射线、红外和射电波段都能观测到。这个弓状激波实在是太惊人了，它的波前比我们的银河系还要大，横跨碰撞星系之间的整个空间。
激波产生于物体运动速率高于其所在介质的声速时——这介质可以是各种物质，可以是水，也可以是星系际气体。最常见的激波是高速飞行的飞机产生的声爆。当喷气机的速度超过音速（或说1马赫）时，它将赶上自己激起的声波，将其压缩成一个锥状的冲击面。飞机飞过后，这个激波到达地面，产生我们所熟悉的“音爆”。激波通常是看不见的。但是有一次，一位美国海军准尉拍到了一幅给人深刻印象的激波照片。照片上可见激波在一架低飞于海面上的喷气机后面延伸的锥状击波。空气中的水蒸气被击波压缩时会变成液滴，在飞机后面形成一条可见的锥状云团（如图）。
Credit: Courtesy of the U.S. Navy
加州理工大学的Philip Appleton博士和同事用Spitzer望远镜敏感的红外摄谱仪对准了巨型击波所在的位置：星系间一块在可见光波段显示为黑暗的天区。他们希望发现关于这些星系的更多情况。与光学望远镜不同，Spitzer可以探测不可见物体（如尘埃微粒或气体分子）发出的红外线。
他们惊喜地发现了高能分子氢被泄露的行踪。“辐射的强度，还有气体谱线被多普勒展宽（意味着它们被激烈扰动）的现象，这些让我们大为吃惊。”Appleton说。“我们本希望看到尘埃微粒的光谱踪迹，可是我们实际上看到的是氢分子光谱——简直和完美的实验室样本一模一样——除此之外几乎别无它物。这和我们在遥远星系系统看到过的所有样本都不一样。”
摄谱仪的作用是把光分解为不同波长的组份，这样可以观察到发光物质不同化学组份在光谱中的体现。谱线的宽度使得天文学家能够测定气体的速率，谱线宽则说明气体速率高。Appleton 的小组在Stephan’s 星系群中观测到了分子氢迄今已知最宽的谱线，这对应着以870千米/秒疾速运动的气体！
“在Stephan’s五重星系里，”Appleton 说，“激波是入侵的星系NGC7318b造成的。它以100马赫以上的速率运动，像一把犁一般切入星系际气体。很明显，分子氢就是在激波表面或后方形成的，类似于喷气机那张照片中凝结的水滴——不过规模要大得多！”
某种程度上，这次发现的如此新奇的现象对他们并不能算一个太大的惊喜，毕竟我们对这个星系群的动力学特征了解得还太少。研究组成员之一，加州理工的C. Kevin Xu博士说：“Stephan’s五重星系太特殊了，每当用一种新的仪器观测它，都能看到完全出乎意料的新事物。这次也不例外。”
这些发现可能有助于我们更好地理解宇宙中那些红外光度最高的星系——神秘的Ultra-luminous Infrared Galaxies（暂不知中文译名，下简称ULIG）。“ULIGs的红外光度通常比银河系高100到1000倍，而且它们的数量随着红移值（也就是说，距离）的增大而增大”Appleton说。“我们还发现，这些星系也都处于剧烈的碰撞或融合过程中。很有可能，它们发出的强大辐射不全是恒星产生的，而是产生于碰撞星系间气体的激波。”
Appleton、Xu、Reach以及其他天文学家作出的观测为研究宇宙早期的碰撞星系提供了一个新的标本。
“观测这样一个邻近的、致密的、浸于浓密气体云中的星系群，使得我们可以看到大约100亿年前第一批星系形成之初可能发生过的事情。当时星系际介质和星系的密度都远高于今天。从这个角度说，对Stephan’s 星系群的这些观测有点像是走进了时间机器。”德国Max Planck研究所的观测者Cristina Popescu 和 Richard Tuffs说。
不过这一发现对我们自己的银河系也有启示意义。我们星系在20亿年后很可能与仙女座大星系相撞，虽说这是离我们还很遥远的事情。相撞的后果可能是产生我们自己的巨型激波。要是这样的话，我们的后代就要置身于一个激波的海洋之中了。
-------------------------------------------------------------------------------------------
大家还可以参考http://www.mpg.de/english/illust ... 20060303/index.html，内容类似。我翻译的这篇内容较浅。他们的专业论文见
P.N. Appleton, K.C. Xu , W. Reach , M.A. Dopita , Y. Gao , N. Lu , C.C. Popescu, J.W. Sulentic, R.J. Tuffs, and M.S. Yun
Powerful High-Velocity Dispersion Molecular Hydrogen Associated with an Intergalactic Shock Wave in Stephan's Quintet
The Astrophysical Journal, 639 51-L54, 10 March 2006
值得指出的是作者之一的Gao是中国紫金山天文台的。
[已经订正] |
|
置顶卡
变色卡
千斤顶
显身卡