OVI Emission in the Halos of Edge-on Spiral Galaxies

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光的衍射英文作文

光的衍射英文作文Light DiffractionLight is a fundamental aspect of our physical world, and its behavior has been the subject of intense study and fascination for centuries. One of the most intriguing and complex phenomena associated with light is diffraction, which refers to the bending and spreading of light waves as they encounter obstacles or apertures. This phenomenon has profound implications in various fields, from optics and quantum mechanics to biology and technology.At its core, diffraction is a wave-like property of light, where the interaction between light and the physical structures it encounters leads to the interference and redistribution of the light waves. This process is governed by the principles of wave interference, where the constructive and destructive interference of light waves result in patterns of light and dark regions, known as diffraction patterns.The fundamental principles of diffraction can be understood by considering the wave nature of light. Light, like other forms of electromagnetic radiation, can be described as a wave, with a specific wavelength and frequency. When light encounters an obstacle or anaperture, the waves are forced to bend and spread out, creating a diffraction pattern. The specific characteristics of this pattern are determined by factors such as the size and shape of the obstacle or aperture, as well as the wavelength of the light.One of the most well-known examples of diffraction is the phenomenon of single-slit diffraction. When light passes through a narrow slit, the resulting diffraction pattern consists of a central bright region, known as the central maximum, surrounded by alternating bright and dark regions, known as diffraction fringes. The spacing and intensity of these fringes are directly related to the wavelength of the light and the width of the slit.Another important aspect of diffraction is the concept of the Fraunhofer diffraction, which describes the diffraction pattern observed at large distances from the aperture or obstacle. In this case, the diffraction pattern is characterized by a series of bright and dark spots, known as the Fraunhofer diffraction pattern. This pattern is particularly useful in applications such as optical imaging, spectroscopy, and the design of diffraction-based optical devices.Diffraction also plays a crucial role in the behavior of light in various natural and man-made systems. For example, the diffraction of light through small apertures or slits is responsible for the characteristic patterns observed in the interference of light, such as those seen inYoung's double-slit experiment. Additionally, the diffraction of light around the edges of objects or through small openings is responsible for the phenomena of diffraction fringes, which can be observed in various optical devices and natural phenomena, such as the colorful patterns seen in the wings of some insects or the halos and glories observed around the Sun or Moon.The study of diffraction has also led to the development of numerous applications in science and technology. In optics, diffraction is used in the design of various optical devices, such as diffraction gratings, which are used in spectroscopy and other analytical techniques. In the field of quantum mechanics, the wave-like nature of particles, as described by the de Broglie hypothesis, has led to the observation of diffraction patterns in the behavior of subatomic particles, such as electrons and neutrons.Furthermore, the understanding of diffraction has been instrumental in the development of modern imaging techniques, such as X-ray crystallography, where the diffraction of X-rays by the atoms in a crystal is used to determine the arrangement and structure of the atoms within the crystal. Similarly, the diffraction of light by various biological structures, such as the compound eyes of insects or the structures found in the wings of some butterflies, has inspired the development of biomimetic materials and devices.In conclusion, the phenomenon of light diffraction is a fundamental and fascinating aspect of our physical world. It is a testament to the wave-like nature of light and the complex interplay between light and the physical structures it encounters. The study of diffraction has led to numerous insights and advancements in various fields, and its continued exploration promises to yield further discoveries and innovations that will shape our understanding of the universe and the technology we use to interact with it.。

2013职称英语理工类阅读理解原文答案译文之17

2013职称英语理工类阅读理解原文答案译文之17A Sunshade for the PlanetEven with the best will1 in the world, reducing our carbon emissions is not going prevent global warming. It has become clear that even if we take the most strong measures to control emissions, the uncertainties in our climate models still leave open the possibility of extreme warming and rises in sea level. At the same time, resistance by governments and special interest groups makes it quite possible that the actions suggested by climate scientists might not be implemented soon enough.Fortunately, if the worst comes to the worse2, scientists still have a few tricks up their sleeves3. For the most part they have strongly resisted discussing these options for fear of inviting a sense of complacency that might thwart efforts to tackle the root of the problem. Until now, that is. A growing number of researchers are taking a fresh look at large-scale “geoengineering” projects that might b e used to counteract global warming. “I use the analogy of methadone4,” says Stephen Schneider, a climate researcher at Stanford University in California who was among the first to draw attention to global warming. “If you have a heroin addict, the correct treatment is hospitalization, and a long rehab. But if they absolutely refuse, methadone is better than heroin.Basically the idea is to apply “sunscreen” to the whole planet. One astronomer has come up with a radical plan to cool Earth: launch trillions of feather-light discs into space, where they would form a vast cloud that would block the sun’s rays.It’s controversial, but recent studies suggest there are ways to deflect just enough of the sunlight reaching the Earth’s surface tocounteract the warming produced by the greenhouse effect. Global climate models show that blocking just 1. 8 per cent of t he incident energy in the sun’s rays would cancel out the warming effects produced by a doubling of greenhouse gases in the atmosphere. That could be crucial, because even the most severe emissions-control measures being proposed would leave us with a doubling of carbon dioxide by the end of this century, and that would last for at least a century more.注释:1. the best will:昀好的愿望2. if the worst comes to the worst:如果昀昀糟糕的事情发生了。

光的衍射英语作文

The Phenomenon of Light Diffraction: AScientific MarvelIn the vast and enigmatic realm of physics, the phenomenon of light diffraction stands as a testament tothe wave-like nature of light. This remarkable occurrence, which manifests when light waves encounter obstacles or apertures, is not only fascinating but also holds immense significance in various fields of science and technology.Diffraction, simply put, is the bending of light waves around the edges of an obstacle or through a small aperture. This bending is a direct consequence of the wave-likenature of light, which differs from the particle-like behavior exhibited by matter. When light waves encounter an obstacle, they spread out in a characteristic pattern known as a diffraction pattern. Similarly, when light passes through a small aperture, it spreads out in a similar pattern, known as a diffraction fringe.The diffraction pattern observed is unique to the shape and size of the obstacle or aperture. For instance, a circular obstacle will produce a characteristic ring-shaped diffraction pattern, while a rectangular aperture willproduce a pattern with distinct vertical and horizontal fringes. This characteristic behavior of light allows scientists to determine the shape and size of objects using diffraction techniques.The phenomenon of diffraction has found numerous applications in various fields of science and technology. In optics, diffraction gratings are used to split lightinto its constituent colors, a principle that underlies the operation of spectrometers and monochromators. In microscopy, diffraction-limited imaging techniques are employed to achieve higher resolution images, enabling scientists to observe finer details than ever before.Diffraction also plays a crucial role in quantum mechanics, where it is used to probe the atomic and molecular structure of matter. Techniques like electron diffraction and neutron diffraction provide insights into the internal structure of crystals and molecules,揭示物质内部的原子排列和相互作用。

外星文明猜想(英文作文)

Certainly! Here’s an essay exploring the conjectures about extraterrestrial civilizations, delving into the scientific, philosophical, and speculative aspects of the topic. Extraterrestrial Civilizations: The Great Beyond and Our Place in the CosmosThe universe, vast and ancient, stretches its arms across 93 billion light-years of observable space, containing billions of galaxies, each with billions of stars. Within this cosmic tapestry, the question of whether we are alone has captivated human minds for centuries. This essay explores the conjectures surrounding extraterrestrial civilizations, from the scientific theories to the speculative musings that fuel our imaginations.The Drake Equation: A Mathematical Framework for SpeculationAt the heart of the search for extraterrestrial intelligence (SETI) lies the Drake equation, formulated by astronomer Frank Drake in 1961. This mathematical framework attempts to estimate the number of active, communicative civilizations in the Milky Way galaxy. Variables include the rate of star formation, the fraction of stars with planetary systems, the number of planets capable of supporting life, the fraction of those planets where life actually emerges, the fraction of those life-bearing planets that develop intelligent life, the fraction of those that develop a civilization with technology, and the length of time such civilizations release detectable signals into space. While many of these variables remain unknown, the Drake equation serves as a tool for structured speculation and highlights the immense challenge in estimating the likelihood of extraterrestrial life.The Fermi Paradox: Where Are They?The Fermi paradox, named after physicist Enrico Fermi, poses a compelling question: Given the vastness of the universe and the high probability of habitable worlds, why have we not encountered any evidence of extraterrestrial civilizations? This paradox has led to numerous hypotheses. Perhaps civilizations tend to destroy themselves before achieving interstellar communication. Or, advanced civilizations might exist but choose to avoid contact with less developed species, adhering to a cosmic form of the “prime directive” seen in science fiction. Alternatively, the distances between stars could simply be too great for practical interstellar travel or communication, making detection exceedingly difficult.The Search for TechnosignaturesIn the quest for extraterrestrial intelligence, scientists have focused on detecting technosignatures—signs of technology that might indicate the presence of a civilization elsewhere in the universe. These include radio signals, laser pulses, or the dimming of stars due to megastructures like Dyson spheres. SETI projects, such as the Allen Telescope Array and Breakthrough Listen, scan the skies for anomalous signals that could be attributed to alien technology. While no definitive technosignatures have been found to date, the search continues, driven by advances in technology and a growing understanding of the cosmos.Astrobiology: Life Beyond EarthAstrobiology, the study of the origin, evolution, distribution, and future of life in the universe, offers insights into the conditions necessary for life. Research in astrobiology has revealed that life can thrive in extreme environments on Earth, suggesting that the conditions for life might be more widespread in the universe than previously thought. The discovery of exoplanets in the habitable zones of their stars, where liquid water can exist, increases the probability of finding environments suitable for life.Continued exploration of our solar system, particularly of Mars and the icy moons of Jupiter and Saturn, holds promise for uncovering signs of past or present microbial life. The Philosophical ImplicationsThe possibility of extraterrestrial civilizations raises profound philosophical questions about humanity’s place in the universe. Encountering another intelligence would force us to reevaluate our understanding of consciousness, culture, and ethics. It could lead to a new era of global unity as humanity comes together to face the challenges and opportunities of interstellar diplomacy. Conversely, it might also highlight our vulnerabilities and prompt introspection on our stewardship of the planet and our responsibilities as members of the cosmic community.Concluding ThoughtsWhile the existence of extraterrestrial civilizations remains a conjecture, the pursuit of answers has expanded our understanding of the universe and our place within it. The search for life beyond Earth is not just a scientific endeavor; it is a philosophical journey that challenges us to consider our origins, our destiny, and our role in the vast cosmic drama unfolding around us. Whether we find ourselves alone or part of a galactic community, the quest for knowledge about the universe and our place in it is one of humanity’s most enduring and inspiring pursuits.This essay explores various aspects of the conjectures surrounding extraterrestrial civilizations, from the scientific frameworks used to estimate their likelihood to the philosophical implications of their existence. If you have specific areas of interest within this broad topic, feel free to ask for further elaboration! If you have any further questions or need additional details on specific topics related to extraterrestrial life or astrobiology, please let me know!。

2013年考研外语阅读理解第三篇全文翻译

2013年考研外语阅读理解第三篇全文翻译注释：未来总是隐藏在迷雾中，借助已有的知识推测未来贯穿于整个已知的和我可以预见的人类文明史，尤其在工业化和信息化革命大幅度提升人类改造自然的能力并带来相当严重的后果之后，注重实证的西方科学体系甚至产生了过去干脆叫神棍的未来学。

人类的未来如何，甚至于人类能否存在到下个世纪甚至下个十年，争议一直不断，但我始终相信一点，在明天，在黑暗之后，太阳照常升起the sun rises as usual。

我在本文翻译中大量使用了意译，并在后面给出了直译，强烈提示，意译是一种很高的翻译技巧，按照考研判分的标准，如果采用意译，基本可以肯定是要么满分要么零分，如果自己的水平没有足够的把握，绝不要轻易使用。

Up until a few decades ago, our visions of the future were largely - though by no means uniformly - glowingly positive. Science and technology would cure all the ills of humanity, leading to lives offulfillment and opportunity for all.曾经，人类的未来似乎一片光明（意译，直译为前景辽阔，蒸蒸日上，只是发展各异）。

科技的发展必定能治愈顽疾，满足需求，提供契机。

Now utopia has grown unfashionable, as we have gained a deeper appreciation of the range of threats facing us, from asteroid strike to epidemic flu and to climate change. You might even be tempted to assume that humanity has little future to look forward to.然而如今梦想已成灰（utopia乌托邦，unfashionable过时的），我们要面对的是更可怕的现实，彗星撞地球、重度流感，甚至气候变更。

关于惠更斯原理的英文作文

关于惠更斯原理的英文作文The Wave Nature of Light and Huygens' PrincipleLight is a fundamental aspect of our universe, and understanding its behavior has been a central focus of scientific inquiry for centuries. One of the key principles that helps explain the wave-like properties of light is Huygens' Principle, named after the Dutch physicist and astronomer Christiaan Huygens.Huygens' Principle states that every point on a wavefront can be considered as a new source of secondary wavelets that spread out in all directions with the same speed as the original wave. The combined effect of these secondary wavelets determines the shape of the wavefront at a later time. This principle helps explain a variety of optical phenomena, including reflection, refraction, and diffraction.To understand Huygens' Principle in more detail, let's consider the case of a plane wave traveling through a medium. Imagine a flat wavefront, such as the surface of a still pond when a stone is dropped in. According to Huygens' Principle, each point on thiswavefront can be considered as a new source of secondary wavelets that spread out in all directions. As time passes, the combined effect of these secondary wavelets creates a new wavefront that maintains the same shape as the original, but has moved forward in the direction of propagation.This principle can also be applied to the case of reflection and refraction. When a wave encounters a boundary between two different media, such as the surface of a mirror or the interface between air and water, Huygens' Principle can be used to predict the behavior of the reflected and refracted waves. The secondary wavelets generated at the boundary interfere with each other, resulting in the familiar patterns of reflection and refraction that we observe in everyday life.One of the key implications of Huygens' Principle is that light can be considered as a wave phenomenon, rather than a stream of particles. This wave-like behavior of light was a significant departure from the prevailing particle-based theories of light that had dominated scientific thought for centuries. Huygens' Principle provided a powerful framework for understanding the propagation of light and the various optical phenomena that we observe.The wave nature of light has important practical applications in a wide range of fields, from telecommunications to medical imaging.For example, the principles of wave optics are fundamental to the design and operation of fiber-optic communication systems, which rely on the propagation of light through waveguides. Similarly, the wave-like properties of light are exploited in technologies such as lasers, which generate highly coherent and directional beams of light.In the field of medical imaging, the wave-like behavior of light is utilized in techniques such as ultrasound imaging and optical coherence tomography (OCT). In ultrasound imaging, high-frequency sound waves are used to create detailed images of the body's internal structures, while in OCT, the interference of low-coherence light is used to generate high-resolution images of biological tissues.Huygens' Principle has also had a profound impact on our understanding of the nature of light and its relationship to other forms of electromagnetic radiation. The wave-like behavior of light, as described by Huygens' Principle, is a fundamental aspect of the broader electromagnetic spectrum, which includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.The wave nature of light has also been crucial in the development of quantum mechanics, which has revolutionized our understanding of the behavior of matter and energy at the atomic and subatomic scales. In quantum mechanics, light is often described as a particle-wave duality, with both particle-like and wave-like properties. This dual nature of light has led to a rich and complex understanding of the fundamental nature of the universe.In conclusion, Huygens' Principle is a powerful and influential concept in the study of optics and the wave-like behavior of light. By considering each point on a wavefront as a new source of secondary wavelets, Huygens' Principle provides a framework for understanding a wide range of optical phenomena, from reflection and refraction to diffraction and interference. The wave-like nature of light, as described by Huygens' Principle, has had far-reaching implications for our understanding of the physical world and has led to numerous technological advancements in fields as diverse as telecommunications, medical imaging, and quantum mechanics.。

英译汉。高英

Lesson 11)The one I am thinking of particularly is entered by Gothicarched gateway of aged brick and stone .You pass from the heat and glare of a big, open square into a cool, dark cavern which extends as far as the eye can see, losing itself in the shadowy distance.此时此刻我要说的集市的入口是一座古老的砖石结构的哥特式拱门，年代非常久远。

当你穿过一个烈日暴晒的大型露天广场，然后走进一个凉爽、幽暗的洞穴。

这洞穴一直一直向前延伸，一眼望不到尽头，最终消失在远处的阴影里。

2)It is a point of honor with the customers not to let the shopkeeper guess what it is she really likes and want until the last moment. 对于顾客来说，至关重要的一点是，不到最后一刻是不能让店主猜到她心里究竟中意哪样东西、想买哪样东西的.3)The seller ,on the other hand ,makes a point of protesting that the price he is charging is depriving him of all profit, and that he is sacrificing this because of his personal regard for the customer. 而在店主那一方来说，则是竭尽全力的让顾客相信，他开出的价钱使他根本无利可图，而他之所以愿意这样做完全是出于本人对顾客的敬重。

霍金讣文翻译

霍金讣文翻译Friends and colleagues from the University of Cambridge have paid tribute to Professor Stephen Hawking, who died today at the age of 76.Widely regarded as one of the world’s most brilliant minds, he was known throughout the world for his contributions to science, his books, his television appearances, his lectures and through biographical films. He leaves three children and three grandchildren.Professor Hawking broke new ground on the basic laws which govern the universe, including the revelation that black holes have a temperature and produce radiation, now known as Hawking radiation. At the same time, he also sought to explain many of these complex scientific ideas to a wider audience through popular books, most notably his bestseller A Brief History of Time.He was awarded the CBE in 1982, was made a Companion of Honour in 1989, and was awarded the US Presidential Medal of Freedom in 2009. He was the recipient of numerous awards, medals and prizes, including the Copley Medal of the Royal Society,the Albert Einstein Award, the Gold Medal of the Royal Astronomical Society, the Fundamental Physics Prize, and the BBVA Foundation Frontiers of Knowledge Award for Basic Sciences. He was a Fellow of The Royal Society, a Member of the Pontifical Academy of Sciences, and a Member of the US National Academy of Sciences.He achieved all this despite a decades-long battle with motor neurone disease, with which he was diagnosed while a student, and eventually led to him being confined to a wheelchair and to communicating via his instantly recognisable computerised voice. His determination in battling with his condition made him a champion for those with a disability around the world.Professor Hawking came to Cambridge in 1962 as a PhD student, and rose to become the Lucasian Professor of Mathematics, a position once held by Isaac Newton, in 1979. In 2009, he retired from this position and was the Dennis Stanton Avery and Sally Tsui Wong-Avery Director of Research in the Department of Applied Mathematics and Theoretical Physics until his death. He was also a member of the University's Centre for TheoreticalCosmology, which he founded in 2007. He was active scientifically and in the media until the end of his life.Professor Stephen Toope, Vice-Chancellor of the University of Cambridge, paid tribute, saying, “Professor Hawking was a unique individual who will be remembered with warmth and affection not only in Cambridge but all over the world. His exceptional contributions to scientific knowledge and the popularisation of science and mathematics have left an indelible legacy. His character was an inspiration to millions. He will be much missed.”。

tpo54三篇阅读原文译文题目答案译文背景知识

托福阅读tpo54全套解析阅读-1 (2)原文 (2)译文 (4)题目 (5)答案 (9)背景知识 (10)阅读-2 (10)原文 (10)译文 (12)题目 (13)答案 (18)背景知识 (20)阅读-3 (25)原文 (26)译文 (27)题目 (28)答案 (33)背景知识 (35)阅读-1原文The Commercialization of Lumber①In nineteenth-century America, practically everything that was built involved wood.Pine was especially attractive for building purposes.It is durable and strong, yet soft enough to be easily worked with even the simplest of hand tools.It also floats nicely on water, which allowed it to be transported to distant markets across the nation.The central and northern reaches of the Great Lakes states—Michigan, Wisconsin, and Minnesota—all contained extensive pine forests as well as many large rivers for floating logs into the Great Lakes, from where they were transported nationwide.②By 1860, the settlement of the American West along with timber shortages in the East converged with ever-widening impact on the pine forests of the Great Lakes states. Over the next 30 years, lumbering became a full-fledged enterprise in Michigan, Wisconsin, and Minnesota. Newly formed lumbering corporations bought up huge tracts of pineland and set about systematically cutting the trees. Both the colonists and the later industrialists saw timber as a commodity, but the latter group adopted a far more thorough and calculating approach to removing trees. In this sense, what happened between 1860 and 1890 represented a significant break with the past. No longer were farmers in search of extra income the main source for shingles, firewood, and other wood products. By the 1870s, farmers and city dwellers alike purchased forest products from large manufacturingcompanies located in the Great Lakes states rather than chopping wood themselves or buying it locally.③The commercialization of lumbering was in part the product of technological change. The early, thick saw blades tended to waste a large quantity of wood, with perhaps as much as a third of the log left behind on the floor as sawdust or scrap. In the 1870s, however, the British-invented band saw, with its thinner blade, became standard issue in the Great Lakes states' lumber factories.Meanwhile, the rise of steam-powered mills streamlined production by allowing for the more efficient, centralized, and continuous cutting of lumber. Steam helped to automate a variety of tasks, from cutting to the carrying away of waste. Mills also employed steam to heat log ponds, preventing them from freezing and making possible year-round lumber production.④For industrial lumbering to succeed, a way had to be found to neutralize the effects of the seasons on production. Traditionally, cutting took place in the winter, when snow and ice made it easier to drag logs on sleds or sleighs to the banks of streams. Once the streams and lakes thawed, workers rafted the logs to mills, where they were cut into lumber in the summer. If nature did not cooperate—if the winter proved dry and warm, if the spring thaw was delayed—production would suffer. To counter the effects of climate on lumber production, loggers experimented with a variety of techniques for transporting trees out of the woods. In the 1870s, loggers in the Great Lakes states began sprinkling water on sleigh roads, giving them an artificial ice coating to facilitate travel. The ice reduced the friction and allowed workers to move larger and heavier loads.⑤But all the sprinkling in the world would not save a logger from the threat of a warm winter. Without snow the sleigh roads turned to mud. In the 1870s, a set of snowless winters left lumber companies to ponder ways of liberating themselves from the seasons. Railroads were one possibility.At first, the remoteness of the pine forests discouraged common carriers from laying track.But increasing lumber prices in the late 1870s combined with periodic warm, dry winters compelled loggers to turn to iron rails. By 1887, 89 logging railroads crisscrossed Michigan, transforming logging from a winter activity into a year-round one.⑥Once the logs arrived at a river, the trip downstream to a mill could be a long and tortuous one.Logjams (buildups of logs that prevent logs from moving downstream) were common—at times stretching for 10 miles—and became even more frequent as pressure on the northern Midwest pinelands increased in the 1860s. To help keep the logs moving efficiently, barriers called booms (essentially a chain of floating logs) were constructed to control the direction of the timber. By the 1870s, lumber companies existed in all the major logging areas of the northern Midwest.译文木材的商业化①在19世纪的美国，几乎所有建筑材料都含有木材。

散发光成为光英语作文

散发光成为光英语作文Radiating Light: The Luminary of the Universe.In the vast expanse of the cosmos, countless celestial bodies emit radiant energy, illuminating the darkness and illuminating our understanding of the universe. Among these luminous celestial objects, stars reign supreme as the primary source of light, energy, and awe for observers both on Earth and beyond.Stars, the building blocks of galaxies, are incandescent beacons of plasma held together by their own gravitational forces. Within their nuclear furnaces, the fusion of hydrogen atoms into helium releases prodigious amounts of energy, a process that sustains their brilliance for billions of years. This energy manifests as electromagnetic radiation, which travels through space as a spectrum of light waves.The light emitted by stars encompasses a vast range ofwavelengths, from short-wavelength gamma rays to long-wavelength radio waves. However, the human eye is only capable of perceiving a narrow band within this spectrum, known as visible light. Visible light ranges from violet to red, with each wavelength corresponding to a different color.Stars exhibit a remarkable diversity in their light output, ranging from faint and barely visible to dazzling and brilliant. The brightness of a star, as perceived by an observer on Earth, depends on several factors, includingits size, temperature, and distance from Earth.Large stars, with greater masses and hence more fuel to burn, typically emit more light than smaller stars. Temperature also plays a crucial role in determining astar's luminosity. Hotter stars emit blue and white light, while cooler stars radiate yellow or red light.The distance between a star and Earth also influences its apparent brightness. Stars that are closer to Earth appear brighter than those that are farther away. This isbecause the inverse square law of light dictates that the intensity of light decreases with the square of the distance from the source.The light of stars serves as a valuable tool for astronomers and astrophysicists. By analyzing the spectrum of light emitted by stars, scientists can determine their temperature, chemical composition, and other physical characteristics. This information helps us understand the evolution of stars, the nature of stellar populationswithin galaxies, and the history of the universe itself.Moreover, the light of stars provides a celestial beacon for navigators and explorers. For centuries, seafarers relied on the positions of stars to guide their ships across vast oceans. Even today, spacecraft venturing into the depths of space utilize star charts and celestial navigation to determine their location and trajectory.Beyond its practical applications, the light of stars also holds profound aesthetic and philosophical significance. Throughout human history, stars have capturedthe imagination of poets, artists, and philosophers. Their twinkling radiance has inspired countless works of art, literature, and music. Stars have also been associated with spirituality, divinity, and the pursuit of knowledge and enlightenment.In conclusion, the light of stars permeates our existence, providing both practical and profound benefits. It illuminates the darkness, guides our paths, and fuels our understanding of the universe. As we continue to explore the cosmos and unravel its mysteries, the light of stars will forever remain a constant and awe-inspiring source of wonder and inspiration.。

高中英语一轮复习语基默写选择性必修第四册UNIT 12 INNOVATION (学生版)

UNIT 12INNOVATION(限时：20分钟)Ⅰ.阅读单词1.vitamin tablets 维生素片bour－saving machine 节省劳力的机器3.discover the causes of diseases under microscopes在显微镜下发现疾病的起因4.correspond with my friends by letters与我的朋友通信unch satellites into orbit发射卫星至轨道6.the smallest particle of matter in the universe宇宙中最小的物质微粒7.one of the 20th century’s premier scientists20世纪最重要的科学家之一8.the theory of relativity相对论9.an evolutionary biologist who studies insects一位研究昆虫的进化生物学家10.a blue mould in the dish 盘子里蓝色的霉菌11.the natural form of penicillin青霉素的天然形式12.be dressed in a smart navy blue suit穿一身漂亮的海军蓝制服13.improve the accuracy of their missiles提高他们导弹的准确度14.an outstanding physicist一位杰出的物理学家15.thousands of circuits成千上万的电路16.what a coincidence真是个巧合17.after an incubation period 经过孵化期之后18.his brilliant work in cosmology他在宇宙学方面的杰出工作19.lift a kettle top 移去壶盖20.the Industrial Revolution工业革命21.bamboo fibre竹纤维22.mechanical clocks 机械钟23.subsequently ad v.后来，随后Ⅱ.核心单词1.health and well－being健康和幸福2.garbage gathering device垃圾收集装置3.how to split the atom 如何分裂原子4.this outspoken young man这个直言不讳的年轻人5.the finding of a recent survey最近一项调查的发现6.perceive the world differently用不同的方式感知世界lions of people 数百万人8.remain humble about the amazing outcome of his discovery对他发现的惊人结果保持谦虚9.problems begin to emerge问题开始出现10.overcome the difficulty 克服困难11.his grand design for the economic future他对未来经济的宏伟计划12.under the influence of gravity在重力的影响下13.the steam engine 蒸汽机1.instant adj.立刻的，马上的；即食的n.片刻；瞬间unch v t.发射；发动，发起，开始从事3.orbit n.轨道v i. & v t.沿轨道运行4.constant adj.恒久不变的；持续不断的，经常发生的n.常量，恒量5.tube n.圆管，管子；电视显像管，阴极射线管6.decline v i.减少，降低v i. & v t.谢绝n.下降，衰退7.bonus n.意外收获，额外的好处；奖金；红利8.mass n.大量，大宗；堆；块adj.大量的Ⅲ.派生单词1.entertain v t.招待；使快乐；娱乐→entertaining adj.有趣的→entertainment n.娱乐活动，娱乐节目2.gift n.天赋；天才；才能；礼物→gifted adj.有天赋的，有才华的3.produce v t.生产→product n.产品；产物→productive adj.多产的；丰饶的；富有成效的→production n.生产；产量4.science n.科学→scientific adj.科学(上)的→scientist n.科学家5.donate v i. & v t.捐献(器官)，献(血)；捐赠，捐献→donation n.捐赠；捐赠物→donor n.捐赠者6.accurate adj.正确无误的；精确的；准确的→accuracy n.准确性；精准度；正确，准确→accurately ad v.准确地7.join v t.参加；加入→joint adj.联合的，共同的，共有的→jointly ad v.共同地；联合地8.efficiency n.效率→efficient adj.效率高的→efficiently ad v.有效率地，高效能地→inefficient adj.效率低的9.propose v t.提出(某观点、方法等)→proposal n.提议；建议；求婚10.clue n.线索；提示→clueless adj.一窍不通的，一无所知的11.evaluate v t.评价；评估→evaluation n.评价，评估12.theory n.理论；学说→theoretical adj.理论的13.boil v i. & v t.(使)沸腾，煮沸；(用开水)煮n.沸腾→boiling adj.很热的；炽热的→boiled adj.煮沸的，煮熟的14.improve v t.改善；改进→improvement n.改善，改进；改进之处15.available adj.可利用的；可获得的→availability n.可能性16.explode v i. & v t.(使)爆炸；急剧增长→explosion n.爆炸→explosive adj.易爆炸的17.sail v i.航行；起航→sailor n.水手，海员→sailing n.帆船运动；航行18.illustrate v t.说明，阐明；给(书籍、文章等)加插图→illustration n.图表；图解；实例，示例19.regulate v t.控制，管理→regulation n.规则；章程；管理20.preserve v t.保存(食物)，腌制；维护，保护n.腌菜，果酱→preservation n.维护；保存21.specific adj.特有的；具体的；明确的→specifically ad v.特定地，专门地22.electric adj.用电的，带电的，电动的→electrical adj.电的；用电→electronic adj.电子的→electricity n.电；电能1.not to mention our knowledge of the world and space2.更不用说我们对世界和空间的了解了2.courses range from cooking to computing课程从烹饪到计算机应用都有3.figure out what to do with the plastic弄清楚如何处理塑料4.single out a few pioneers of the 20th century挑选几个20世纪的先驱5.turn to my teacher for help 向我的老师求助6.do the job in a joint effort共同努力做这项工作7.the bedroom light went out after a moment卧室的灯片刻后熄灭了8.be dedicated to improving the quality of human life致力于提高人类生活质量9.be expected to drop to 400 or below预计会降到400或更低10.be reduced to zero 减少到零11.in spite of the bad weather 尽管天气恶劣12.reflect on your successes and failures反思一下你的成功和失败1.If you had to choose the single most important discovery of the 20th century，you would have a real problem on your hands.(if引导虚拟条件句)如果你必须在20世纪重大发现中选出最重要的一项，你将会真正地陷入进退两难的境地。

X-ray Emission from Haloes of Simulated Disc Galaxies

a r X i v :a s t r o -p h /0201529v 1 31 J a n 2002Mon.Not.R.Astron.Soc.000,1–8(2001)Printed 1February 2008(MN L A T E X style ﬁle v2.2)X-ray Emission from Haloes of Simulated Disc GalaxiesS.Toft,1⋆,J.Rasmussen 1,J.Sommer-Larsen 2and K.Pedersen,11Astronomical Observatory,Copenhagen University,Juliane Maries Vej 30,DK-2100Copenhagen Ø,Denmark2TheoreticalAstrophysics Center,Juliane Mariesvej 30,DK-2100Copenhagen Ø,DenmarkABSTRACTBolometric and 0.2-2keV X-ray luminosities of the hot gas haloes of simulated disc galaxies have been calculated at redshift z =0.The TreeSPH simulations are fully cosmological and the sample of 44disc galaxies span a range in characteristic circular speeds of V c =130-325km s −1.The galaxies have been obtained in simulations with a considerable range of physical parameters,varying the baryonic fraction,the gas metallicity,the meta-galactic UV ﬁeld,the cosmology,the dark matter type,and also the numerical resolution.The models are found to be in agreement with the (few)relevant X-ray observations available at present.The amount of hot gas in the haloes is also consistent with constraints from pulsar dispersion measures in the Milky Way.Forthcoming XMM and Chandra observations should enable much more stringent tests and provide constraints on the physical parameters.We ﬁnd that simple cooling ﬂow models over-predict X-ray luminosities by up to two orders of magnitude for high (but still realistic)cooling eﬃciencies relative to the models presented here.Our results display a clear trend that increasing cooling eﬃciency leads to decreasing X-ray luminosities at z =0.The reason is found to be that increased cooling eﬃciency leads to a decreased fraction of hot gas relative to total baryonic mass inside of the virial radius at present.At gas metal abundances of a third solar this hot gas fraction becomes as low as just a few percent.We also ﬁnd that most of the X-ray emission comes from the inner parts (r <∼20kpc)of the hot galactic haloes.Finally,we ﬁnd for realistic choices of the physical parameters that disc galaxy haloes possibly were more than one order of magnitude brighter in soft X-ray emission at z ∼1,than at present.Key words:methods:N-body simulations –cooling ﬂows –galaxies:evolution –galaxies:formation –galaxies:halos –galaxies:spiral –X-rays:galaxies1INTRODUCTIONIn disc galaxy formation models infall of halo gas onto the disc due to cooling is a generic feature.However,the gas accretion rate and hot gas cooling history are at best uncer-tain in all models so far.It is thus not clear to which extent the gas cooling out from the galaxy’s halo is replenishing that which is consumed by star formation in the disc.Such continuous gas infall is essential to explain the extended star formation histories of isolated spiral galaxies like the Milky-Way and the most likely explanation of the “G-dwarf prob-lem”—see,e.g.,Rocha-Pinto &Marciel (1996)and Pagel (1997).At the virial temperatures of disc galaxy haloes the dominant cooling mechanism is thermal bremsstrahlung plus atomic line emission.The emissivity,increasing strongly with halo gas density,is expected to peak fairly close to the disc and decrease outwards,and if the cooling rate is signif-⋆E-mail:toft@astro.ku.dkicant the X-ray ﬂux may be visible well beyond the optical radius of a galaxy.Recently,Benson et al.(2000)compared ROSAT ob-servations of three massive,nearby and highly inclined disc galaxies with predictions of simple cooling ﬂow models of galaxy formation and evolution.They showed that these models predict about an order of magnitude more X-ray emission from the galaxy haloes (speciﬁcally from a 5-18arcmin annulus around the galaxies)than observational de-tections and upper limits.In this paper we present global X-ray properties of the haloes of a large,novel sample of model disc galaxies at red-shift z =0.The galaxies result from physically realistic grav-ity/hydro simulations of disc galaxy formation and evolution in a cosmological context.We ﬁnd that our model predic-tions of X-ray properties of disc galaxy haloes are consistent with observational detections and upper limits.Given the results of the theoretical models of Benson et al.we list the most important reasons why simple cooling ﬂow modelsc2001RAS2S.Toft et al.-60-40-200204060x [kpc]34567l o g T [K]Figure 1.The ﬁgure shows the temperature of the SPH gas particles in a typical disc galaxy from our simulations versus their x coordinate (one of the axes in the disc).The “cold”(log T <4.5)gas which is primarily situated in the disc is removed from the catalogues since it does not contribute to the X-ray ﬂux.over-predict the present day X-ray emission of disc galaxy haloes.In section 2we give a very short description of the disc galaxy simulations.In section 3we brieﬂy describe the X-ray halo emission calculations and in section 4the results obtained.Section 5constitutes the discussion and section 6the conclusion.2DISC GALAXY SIMULATIONSWe have in recent years developed novel models of formation and evolution of disc galaxies.The model disc galaxies result from ab initio ,fully cosmological (ΩM =1or ΩM +ΩΛ=1),gravity/hydro simulations.These simulations are started at a suﬃciently high redshift (z i =20-40)that the density per-turbations are still linear and are then evolved through the entire non-linear galaxy formation regime to the present epoch (z =0).The code uses a gridless,fully Lagrangian,3-D TreeSPH code incorporating the eﬀects of radiative cooling and heating (including the eﬀects of a meta-galactic UV ﬁeld),inverse Compton cooling,star formation,and ener-getic stellar feedback processes.A major obstacle in forming realistically sized disc galaxies in such simulations is the so-called “angular mo-mentum problem”(e.g.,Navarro &White 1994,Sommer-Larsen et al.1999).We overcome this problem in two dif-ferent ways:a)Using cold dark matter (CDM)+stellar feedback processes (Sommer-Larsen et al.2002)or b)Us-ing warm dark matter (WDM)(Sommer-Larsen &Dolgov 2001).A total of 44such disc galaxy models with charac-teristic circular velocities in the range V c =130–325km s −1form the basis of the predictions presented in this paper.The simulations initially consist of 30000-400000SPH+DM particles and in the majority of them some of the SPH par-ticles are turned into star particles over the course of theFigure 2.Bolometric luminosity as a function of characteristic circular speed.Small symbols :Flat ΩM =1.0cosmology:Open symbols:baryon fraction f b =0.05,ﬁlled circles f b =0.1.Triangles:without UV ﬁeld,non-triangles:with a UV ﬁeld of the Efstathiou (1992)type.Connected symbols are the same galaxies run with medium (open circles)and high (open circles with crosses)res-olution.All simulations represented by small symbols have pri-mordial rge symbols :Flat (ΩΛ,ΩM )=(0.7,0.3)cosmology:Open symbols:f b =0.05,ﬁlled symbols f b =0.1.Cir-cles correspond to primordial abundance and with a Haardt &Madau (1996)UV ﬁeld,squares correspond to Z =1/3Z ⊙(us-ing the cooling function of Sutherland &Dopita 1993,which does not include eﬀects of a UV ﬁeld).The curves are the L X,bol -V c relationship for the simple cooling ﬂow models (for ΛCDM NFW haloes)described in Sec.5.The curves represent diﬀerent bary-onic fractions (solid curves have f b =0.1,dotted curves have f b =0.05)and abundances (thick curves:primordial abundances,thin curves:Z =1/3Z ⊙).simulation —for details about the TreeSPH simulations we refer the reader to the above quoted references.Most of the simulations were run with primordial gas composition (76%H and 24%He by mass)under the as-sumption that the inﬂowing,hot gas is fairly unenriched in heavy elements.To test the eﬀects of metal abundance eight ΛCDM simulations (four with “universal”baryonic fraction f b =0.05and four with f b =0.10)were run with a gas abundance of 1/3solar (speciﬁcally [Fe/H]=-0.5).This is the metal abundance of the intracluster medium and can probably be considered a reasonable upper limit to the metal abundance of the hot gas in disc galaxy haloes.3X-RAY HALO EMISSION CALCULATIONFor each of the simulated galaxies at z =0we create a cata-logue of SPH gas particle positions,densities,temperatures and masses.For each catalogue a box of size (1000kpc)3centered on the galaxy is retained and all gas particles with temperatures log(T)<4.5are cut away (see Fig.1)sincec2001RAS,MNRAS 000,1–8X-ray Emission from Haloes of Simulated Disc Galaxies3Figure 3.0.2-2keV band luminosity as a function of character-istic circular speed.Symbols as in Fig.2.they will not contribute to the X-ray ﬂux (these are mainly gas particles which have cooled onto the disc).The density and temperature of each particle is then averaged over itself and its ﬁve nearest neighbors using a spherical smoothing kernel proposed by Monaghan &Lattanzio (1985)1,and a volume is assigned to the particle given its mass and ing the average temperatures and densities,each SPH particle is treated as an optically thin thermal plasma,and the associated X-ray luminosity is calculated at the rele-vant position in a given photon energy band with the meka plasma emissivity code (Mewe et al.1986).X-ray luminosi-ties are then computed by summation over all particles in the volume of interest.The bolometric X-ray luminosity is calculated using the 0.012−12.4keV band.4RESULTSIn Fig.2the total bolometric X-ray luminosities L X,bol of the 44simulated disc galaxies in our sample are plotted versus their characteristic circular speed V c ,deﬁned as the circu-lar velocity in the disc at R 2.2=2.2R d ,where R d is the disc scale length –see Sommer-Larsen &Dolgov (2001)for details.The X-ray luminosities derived from the simulations are up to two orders of magnitude below values derived from simple cooling ﬂow models which are described in Sec.5.1.As can be seen from the ﬁgure L X,bol ∼1040erg s −1for a Milky Way sized galaxy.As expected from the simple models,the simulated1It is important to note that we average over the original den-sities from the TreeSPH simulations.One can show that if the cold gas particles are cut away and the densities of the remaining gas particles are then recalculated using the full SPH procedure the X-ray luminosities will be underestimated due to resolution problems at the disc–halo interface.1020304050|z| [kpc]10-1810-1710-1610-1510-1410-1310-12S (z ) [ e r g s -1 a r c m i n -2 c m -2]305 < V C < 325202 < V C < 240Figure 4.Mean bolometric X-ray surface brightness proﬁles per-pendicular to the plane of the disc of three high V c galaxies (solid curve)and four Milky Way sized galaxies (dashed curve).The physically most important parameters for these galaxies are f b =0.10and primordial gas abundance.The proﬁles were calcu-lated by binning the emission in 40kpc wide,5kpc high slices parallel to the disc.galaxies display an L X,bol -V c relation,but with a signiﬁcant scatter.Part of this scatter arises from the diﬀerent condi-tions under which the simulations have been run (see Sec.5.3)and part of it is a “real”scatter arising from the diﬀer-ent geometries and cooling histories of the individual galaxy haloes.This “real”scatter can be estimated by inspection of the large ﬁlled circles in Fig.2which represent simulations run with the same,physically important parameters (see the ﬁgure caption).These data points display a rms dispersion of about 50%around the mean.There is a tendency for galaxies formed in simulations with baryon fraction f b =0.05and primordial gas abundance to have systematically higher L X,bol (by about a factor of two)than galaxies formed in similar simulations with f b =0.10.Also,galaxies formed in simulations with gas abun-dance Z =1/3Z ⊙tend to have systematically lower L X,bol than the ones with primordial gas.We discuss these trends in Sec.5.Fig.3shows the 0.2-2keV band X-ray luminosities of the 44sample disc galaxies versus V c .The systematic trends mentioned above are also seen in this plot,in particular is the diﬀerence between the f b =0.05and 0.10simulations (with primordial gas)even more pronounced than in Fig.2.We also discuss this in Sec.5.Most (but not all)of the X-ray emission originates from regions of the hot gas halo fairly close to the disc:95%of the emission typically originates within about 20kpc of the disc.This is illustrated in Fig.4where we plot the mean surface brightness proﬁles perpendicular to the disc of three high V c galaxies and four Milky Way sized galaxies.5DISCUSSIONc2001RAS,MNRAS 000,1–84S.Toft et al.5.1Comparison to simple coolingﬂow modelsIn order to compare our results with previous work,we cal-culated a family of simple coolingﬂow models similar to those considered by Benson et al.(2000).In these models it is assumed that the cooling occurs in a static potential and that the gas initially was in place and traced the dark matter(DM).Gas is assumed toﬂow from the cooling radius(at which the cooling time equals the age of the universe)to the disc(settling there as cold gas)on a time-scale much shorter than the Hubble time.The bolo-metric X-ray luminosity can then be approximated simply as the mass accretion rate,˙M cool=4πr2coolρgas(r cool)˙r cool, times the gravitational potential diﬀerence,soL X,bol=˙M cool(r cool) r cool r optical V2c(r)X-ray Emission from Haloes of Simulated Disc Galaxies5L X=45±4.4×1040erg s−1for NFW haloes(and even more for isothermal sphere haloes).However,the above result is in excellent agreement with the expectation from our simu-lations,as illustrated in Fig.5where we plot the bolometric luminosity in the considered annulus(assuming a distance of13.8Mpc)of all the galaxies in our simulation sample ver-sus V c.We note that our three“data”points at V c=300-325 km s−1are for simulations with baryon fraction f b=0.10and primordial gas abundance.For a given V c we would expect the bolometric luminosities to be about twice as much for simulations with f b=0.05-see section5.3.4.Such a low f b is unlikely given determinations of the“universal”baryon fraction,f b≈0.10(h/0.7)−3/2,derived from galaxy clusters (Ettori&Fabian1999)and galaxy clustering(Percival et al. 2001),but in any case the X-ray luminosities would still be consistent with the NGC2841measurement.Moreover,the (more realistic)inclusion of some level of enrichment in the gas and the use of the more realistic meta-galactic UVﬁeld of Haardt&Madau(1996)would tend to lower the X-ray lu-minosities.We also note that the total X-ray luminosities of our three galaxies at V c=300-325km s−1are“only”a factor of3-5lower than the predictions of the simple coolingﬂow models of Benson et al.and ours(the latter for f b=0.10and primordial gas).Hence an important part of the reason why our models match the observed bolometric luminosity of the 5-18arcmin annulus around NGC2841is that most of the X-rays are emitted from the inner20kpc of our model disc galaxy haloes(5arcmin correspond to20kpc at a distance of13.8Mpc).In other words our“geometric correction”is considerably larger than the one used by Benson et al.The observational constraints on the bolometric lumi-nosity of NGC4594and NGC5529are weaker than for NGC 2841.The upper limits on these are again about an order of magnitude less than expected from the simple models,but in agreement with the expectation from our simulations.The diﬀuse X-ray luminosity of the Milky Way’s hot halo has recently been estimated:Pietz et al.(1998)es-timate a0.1-2keV luminosity of7·1039erg s−1and Wang (1997)a0.5-2keV luminosity of3·1039erg s−1.Assuming a temperature of0.15keV(Georgantopoulus et al.1996;Par-mar et al.1999)this translates into0.2-2keV luminosities of5and7·1039erg s−1respectively.These estimates(which should probably be seen as upper limits)are consistent with ourﬁndings from the simulations for V c≃220km s−1—see Fig.3.5.3Eﬀects of numerical resolution and physicalparametersThe galaxies in our sample have been compiled from a number of simulations which have been run with diﬀer-ent cosmological and environmental parameters.These in-homogeneities in our simulation sample may introduce some scatter in the L X−V c diagram.On the other hand,this allows us to investigate trends when varying the physical parameters.In general,varying a parameter has impact on the present day X-ray luminosity of a given galaxy if it signif-icantly alters the ability of the hot halo gas to cool during its life time.In the simple coolingﬂow models,increasing the cooling eﬃciency leads to an increase in L X,while this is not necessarily the case in more realistic simulations.If the cooling eﬃciency is increased,more gas has cooled out on the disc at z=0.This results in an increase of the char-acteristic circular speed of the disc as its dynamics become more baryon(and less DM)dominated,and usually a de-crease in the X-ray luminosity of the halo since there is less hot gas left in the halo to cool and contribute to the X-ray emission.In the following we brieﬂy discuss how the derived re-sults depend on the resolution of the simulation,the pres-ence of an external UVﬁeld,the assumed gas metallicity, the baryon fraction f b,the cosmology,the dark matter type and whether or not star formation is incorporated in the simulations.5.3.1Eﬀects of resolutionAn important test of all numerical simulations is to check whether the results depend on the resolution.This can be done from Fig.2by comparing the connected symbols. These represent the same galaxy,run with normal(open symbols)and8times higher mass+2times higher force resolution(open symbols with crosses).It can be seen that this signiﬁcant increase in resolution only leads to a very modest increase of19±64%in the X-ray luminosity rela-tive to the mean L X−V c relation(i.e.taking the eﬀect of the change of V c with resolution into account).A similar result is inferred from Fig.3.5.3.2Eﬀects of a meta-galactic UVﬁeldThe main eﬀects of a hard UV photonﬁeld is to ionize the gas,signiﬁcantly reducing its ability to cool by colli-sional excitation(line-cooling)mechanisms(Vedel,Hellsten and Sommer-Larsen1994).This eﬀect is evident in Fig.2 where the set of4small triangles and the set of4small open circles represent the same4haloes,run under exactly the same conditions,except that for the latter eﬀects of a meta-galactic,redshift-dependent UVﬁeld were included in the cooling/heating function.There is a tendency for the galaxies without an external UV-ﬁeld to have lower L X,bol and higher V c than the galaxies with external UV-ﬁeld:The former have a median bolometric luminosity of55±16%of the latter(again taking into account the change of V c).So in this case an increase in the cooling rate leads to a decrease in L X,bol at the present epoch since a smaller amount of hot gas is left in the halo to produce the emission –see Fig.6.Note however that in the above simulations with UVﬁeld we used aﬁeld of the Efstathiou(1992)type which is too hard and intense compared to the more realistic one of Haardt&Madau(1996)—see also Sec.5.3.5.Hence,the suppression in cooling eﬃciency and the related increase in L X,bol for disc galaxy haloes formed in simulations with a Efstathiou type UVﬁeld is somewhat too large.5.3.3Eﬀects of gas metallicityThe eﬀects of the metallicity of the gas on the derived L X,bol can be investigated by comparing the squares in Fig.2(which have Z=1/3Z⊙)with the rest of the symbols(which have primordial abundance).In the simple coolingﬂow models, an increase of the metal abundance leads to an increase inc 2001RAS,MNRAS000,1–86S.Toft etal.Figure 6.Bolometric luminosity relative to the one expected for the simple cooling ﬂow models versus hot gas fraction -see text for details.Symbols as in Fig.2.the cooling rate and L X,bol (compare the thick and thin curves in Fig.2);however this is not what we ﬁnd from our simulations.The galaxies with Z =1/3Z ⊙have systemat-ically lower L X,bol than the galaxies with primordial abun-dance,by about a factor of 3-4for the same f b .This is in agreement with the argument that increasing the cooling ef-ﬁciency leads to a decrease in L X,bol at z =0.Note that for the Z =1/3Z ⊙simulations we used the cooling function of Sutherland &Dopita (1993)which does not include the eﬀect of a UV ﬁeld.So we can not completely disentangle the eﬀects of gas metal abundance versus lack of UV ﬁeld on the X-ray luminosities,but Fig.6strongly hints that the former is the most important.5.3.4Eﬀects of baryon fractionComparing in Fig.2L X,bol for simulations run with primor-dial gas and with f b =0.05and 0.10,respectively,the former are more luminous on average by about a factor of two.Yet again we see how increased cooling eﬃciency leads to a de-creased L X,bol at z =0.Summarizing the above results we ﬁnd no statistically sig-niﬁcant dependence of the X-ray luminosities of the simu-lated galaxies on numerical resolution.With respect to cool-ing eﬃciency there is a general trend of higher cooling ef-ﬁciency over the course of the simulation to result in less hot gas left in the halo at z =0to cool,yielding a lower X-ray luminosity.This is qualitatively demonstrated in Fig.6,which shows the bolometric luminosity relative to the one expected for the simple cooling ﬂow models versus the hot gas fraction f (hot gas )=M (hot gas )(<r vir )/(f b M vir ),where M (hot gas )(<r vir )is the mass of hot gas (log(T )>4.5)insideof the virial radius r vir and M vir is the total mass (baryonic +DM)inside r vir .It is seen that only for hot gas frac-tions f (hot gas )>∼0.4-0.5,requiring a physically implausible parameter combination of f b =0.05,primordial gas and an unrealistically hard and intense UV ﬁeld,can our models match the L X,bol predicted by the simple cooling ﬂow mod-els.Pulsar dispersion measures can be used to place ob-servational upper limits on the amount of hot gas in the halo of the Milky Way.We ﬁnd from our simulations that Milky Way sized galaxies formed in primordial gas simu-lations have about 109M ⊙of hot gas inside of 50kpc at z =0and the ones from the Z =1/3Z ⊙simulations about 108M ⊙.Both values are consistent with the observational upper limits of about 2·109M ⊙from pulsar dispersion mea-sures to the Magellanic Clouds and the globular cluster M53(Moore &Davis 1994;Rasmussen 2000).The diﬀerence between the f b =0.05and 0.10cases is even more pronounced for the 0.2-2keV X-ray luminosities,as shown in Fig.3.The reason is that at a given charac-teristic circular speed V c the dynamics of the inner galaxy (where V c is determined)are more baryon dominated for f b =0.10than for f b =0.05.This in turn means that the hot halo is smaller and cooler for the f b =0.10case than for the f b =0.05case.This is demonstrated in Fig.7,which shows the average temperature of the central hot halo gas (inside of 20h −1kpc)for the 44simulations.At a given V c the tem-perature of the inner,hot halo is systematically shifted to lower values for f b =0.10as compared to f b =0.05.Hence for the relevant,relatively low temperatures (T <∼0.3keV,orequivalently,V c <∼300km s −1)less of the emitted radia-tion has energies above 0.2keV for the former than for the latter case.On the issue of baryon dominance of the inner galaxy dynamics note that for a given DM halo,f b =0.10(as compared to f b =0.05)leads to a larger V c and a smaller d v c (R )/d R in the outer parts of the disc,where v c (R )is the rotation curve –this is in line with the ﬁndings of Persic,Salucci &Stel (1996)on the basis of a large observational sample of disc galaxy rotation curves.Finally,as mentioned in Sec.5.2,the temperature of the Milky Way’s inner halo is about 0.15keV corresponding to 1.5-2·106K in agreement with Fig.7.5.3.5Eﬀects of dark matter type,cosmology and star formationWe do not ﬁnd any dependence on the DM type,i.e.whether the simulations are of the WDM or CDM +feedback type.Neither do we ﬁnd any indications of systematic trends with cosmology (SCDM/WDM versus ΛCDM/WDM)although more overlap in ﬁgures 2and 3between the two cosmologies would have been desirable (the reason for this lack of overlap is that rather diﬀerent cosmological volumes were sampled in the two cosmologies —the box size was ∼40h −1Mpc for the ΩM =1cosmology and 10h −1Mpc for the Λ-cosmology).Note also that in Fig.6the results for the diﬀerent cosmolo-gies fall along the same continuous sequence.The reason why the disc galaxies formed in primordial gas simulations for the Ωm =1cosmology tend to have slightly higher relative bolometric luminosity and f (hot gas )than the Λ-cosmology ones is most likely due to the two diﬀerent models of thec2001RAS,MNRAS 000,1–8X-ray Emission from Haloes of Simulated Disc Galaxies7Figure 7.Average inner halo hot gas temperature versus char-acteristic circular speed -see text for details.Symbols as in Fig.2.meta-galactic UV ﬁeld used:For the former we used the one suggested by Efstathiou (1992)(see Vedel et al.1994),whereas for the latter we used the more realistic one from Haardt &Madau (1996).The former has z reionization =∞and is considerably harder and,at z >∼2,more intense than the latter which has z reionization ≃6.Finally,we do not ﬁnd any dependence on whether star formation is included or not in the simulations.5.4Mass accretion ratesIn Sommer-Larsen et al.(2002)disc gas accretion rates due to cooling-out of hot gas are determined for the ΛCDM +feedback model disc galaxies considered there (a sub-set of the sample considered here obtained in simulations with f b =0.10,primordial gas composition and the Haardt &Madau UV ﬁeld).For Milky-Way sized galaxies accretion rates in the range 0.3-0.6h −1M ⊙yr −1(h =0.65)are found at z =0.One can show that the rate at which hot halo gas cools out is proportional to L X ,bol ·<1T>is the emissivity weighted inverse temperature of the hot gas.As mentioned previously we ﬁnd in the current work that L X ,bol is proportional to about the ﬁfth power of V c ,so asT ∝V 2c we would expect ˙M∝V 3c .This is indeed found by Sommer-Larsen et al.(2002)and is sensible since the I -band (and hence approximately mass)Tully-Fisher relation has a logarithmic slope of about three (Giovanelli et al.1997).Given the trends of L X with various environmental pa-rameters discussed in section 5.3we would expect galaxies formed in simulations with f b =0.05and primordial abun-dance to have about twice as large mass accretion rates,whereas galaxies formed in Z =1/3Z ⊙simulations will have accretion rates 3-4times lower than the similar primor-dial gas ones.Hence we expect at z =0a fairly strong trend of the ratio of present to average past accretion rate decreas-ing with increasing cooling eﬃciency.Indeed Sommer-Larsen et al.(2002)ﬁnd for their ΛCDM simulations accretion rates at z =1which are an order of magnitude larger than the ones at z =0,so disc galaxies may have been considerably more X-ray luminous in the past than they are today.A detailed analysis of mass accretion rates and high-z X-ray properties of our sample of simulated disc galaxies will be presented in a forthcoming paper.5.5Future X-ray observational testsFrom the predicted X-ray surface brightness proﬁles (Fig.4)and the halo temperatures (Fig.7)we have estimated the feasibility for detecting halo emission with XMM-Newton and Chandra using the most recent instrument responses.In order to avoid confusion with X-ray emission originating in the disc we aim at detecting halo emission from nearly edge-on disc galaxies at (vertical)disc heights of 10-15kpc.Count rates for a 5kpc high and 40kpc wide slice (parallel to the disc)at such disc heights were calculated assuming a column density of absorbing neutral hydrogen in the disc of the Milky Way of n H =2.5·1020cm −2(corresponding to the typical value for galactic latitudes of |b |∼60◦).For galaxies with circular speeds in excess of 300km s −1and distances d <∼50Mpc XMM-Newton,should be able to obtain a 5σde-tection at such disc heights in a 10ksec exposure.Although Chandra has a smaller collecting area than XMM-Newton the Chandra background is generally lower and its superior spatial resolution allows for more eﬃcient removal of con-taminating point sources.We thus expect that only slightly longer exposures are required for Chandra detection of halo emission than for XMM-Newton.However,the curve in Fig.4for the V c >300km s −1galaxies represents an optimistic case since the underly-ing simulations were run with primordial abundances and a strong external UV-ﬁeld,both increasing the present day X-ray luminosity.As mentioned in sections 5.3.2and 5.3.3,in more realistic simulations,including metals and a weaker external UV-ﬁeld,the halo ﬂux is lower by a factor of about 3.In this case,XMM-Newton as well as Chandra should still obtain a 5σdetection for V c >300km s −1galaxies within 25Mpc in about 25ksecs.For Milky Way sized galaxies,due to their much lower surface brightness (e.g.Fig.4)and lower halo temperature (the latter making these more sensitive to absorption)the predicted XMM-Newton and Chandra halo count rates are two orders of magnitude lower than for the V c >300km s −1galaxies.Detection of X-ray haloes for Milky Way sized galaxies at vertical disc heights of 10-15kpc will thus have to await future X-ray observatories with much larger collecting areas (Constellation-X and XEUS).6CONCLUSIONWe have presented X-ray properties of the hot gas haloes of disc galaxies derived from a large sample of physically realistic gravity/hydro simulations of galaxy formation and evolution.The simulated galaxies follow an L X,bol -V c relation with approximately the same slope as expected from simple cool-ing ﬂow models (L X,bol ∝V 5c ),but shifted to lower L X,bol ,c2001RAS,MNRAS 000,1–8。

tpo40三篇托福阅读TOEFL原文译文题目答案译文背景知识

tpo40三篇托福阅读TOEFL原文译文题目答案译文背景知识阅读-1 (2)原文 (2)译文 (5)题目 (8)答案 (17)背景知识 (17)阅读-2 (20)原文 (20)译文 (23)题目 (25)答案 (35)背景知识 (35)阅读-3 (38)原文 (38)译文 (41)题目 (44)答案 (53)背景知识 (54)阅读-1原文Ancient Athens①One of the most important changes in Greece during the period from 800 B.C. to 500 B.C. was the rise of the polis, or city-state, and each polis developed a system of government that was appropriate to its circumstances. The problems that were faced and solved in Athens were the sharing of political power between the established aristocracy and the emerging other classes, and the adjustment of aristocratic ways of life to the ways of life of the new polis. It was the harmonious blending of all of these elements that was to produce the classical culture of Athens.②Entering the polis age, Athens had the traditional institutions of other Greek protodemocratic states: an assembly of adult males, an aristocratic council, and annually elected officials. Within this traditional framework the Athenians, between 600 B.C. and 450 B.C., evolved what Greeks regarded as a fully fledged democratic constitution, though the right to vote was given to fewer groups of people than is seen in modern times.③The first steps toward change were taken by Solon in 594 B.C., when he broke the aristocracy's stranglehold on elected offices by establishing wealth rather than birth as the basis of office holding, abolishing the economic obligations of ordinary Athenians to the aristocracy, and allowing the assembly (of which all citizens were equal members) to overrule the decisions of local courts in certain cases. The strength of the Athenian aristocracy was further weakened during the rest of the century by the rise of a type of government known as a tyranny, which is a form of interim rule by a popular strongman (not rule by a ruthless dictator as the modern use of the term suggests to us). The Peisistratids, as the succession of tyrants were called (after the founder of the dynasty, Peisistratos), strengthened Athenian central administration at the expense of the aristocracy by appointing judges throughout the region, producing Athens’ first national coinage, and adding and embellishing festivals that tended to focus attention on Athens rather than on local villages of the surrounding region. By the end of the century, the time was ripe for more change: the tyrants were driven out, and in 508 B.C. a new reformer, Cleisthenes, gave final form to the developments reducing aristocratic control already under way.④Cleisthenes' principal contribution to the creation of democracy at Athens was to complete the long process of weakening family and clanstructures, especially among the aristocrats, and to set in their place locality-based corporations called demes, which became the point of entry for all civic and most religious life in Athens. Out of the demes were created 10 artificial tribes of roughly equal population. From the demes, by either election or selection, came 500 members of a new council, 6,000 jurors for the courts, 10 generals, and hundreds of commissioners. The assembly was sovereign in all matters but in practice delegated its power to subordinate bodies such as the council, which prepared the agenda for the meetings of the assembly, and courts, which took care of most judicial matters. Various committees acted as an executive branch, implementing policies of the assembly and supervising, for instance, the food and water supplies and public buildings. This wide-scale participation by the citizenry in the government distinguished the democratic form of the Athenian polis from other less liberal forms.⑤The effect of Cleisthenes’ reforms was to establish the superiority of the Athenian community as a whole over local institutions without destroying them. National politics rather than local or deme politics became the focal point. At the same time, entry into national politics began at the deme level and gave local loyalty a new focus: Athens itself. Over the next two centuries the implications of Cleisthenes’ reforms were fully exploited.⑥During the fifth century B.C. the council of 500 was extremely influential in shaping policy. In the next century, however, it was the mature assembly that took on decision-making responsibility. By any measure other than that of the aristocrats, who had been upstaged by the supposedly inferior "people", the Athenian democracy was a stunning success. Never before, or since, have so many people been involved in the serious business of self-governance. It was precisely this opportunity to participate in public life that provided a stimulus for the brilliant unfolding of classical Greek culture.译文古雅典①在公元前800年到公元前500年期间，希腊最重要的变化之一是城邦的崛起，并且每个城邦都发展了适合其情况的政府体系。

光的衍射英语作文

光的衍射英语作文Diffraction of Light。

Light is a fascinating phenomenon that plays a crucial role in our everyday lives. One of the most intriguing aspects of light is its ability to diffract, or bend, when it encounters an obstacle or passes through a narrow opening. This phenomenon, known as light diffraction, has been studied and observed for centuries, leading to a better understanding of the nature of light and its behavior.When light waves encounter an obstacle or a slit that is comparable in size to the wavelength of the light, diffraction occurs. This causes the light waves to bend around the edges of the obstacle or slit, creating a pattern of light and dark fringes on a screen placed behind the obstacle. This pattern, known as a diffraction pattern, is a result of the interference of the diffracted light waves.The diffraction of light can be observed in various everyday situations. For example, when light passes through a small opening, such as a pinhole or the aperture of a camera, it diffracts and creates a blurry image. This is why pinhole cameras produce images with a soft focus and why the edges of shadows appear fuzzy.In addition to being a fascinating phenomenon, light diffraction also has practical applications in various fields. In optics, diffraction gratings are used to disperse light into its component colors, allowing scientists to study the spectral properties of light. In astronomy, diffraction is used to analyze the light emitted by stars and galaxies, providing valuable information about their composition and temperature.Furthermore, the study of light diffraction has led to the development of technologies such as holography, which relies on the interference patterns created by diffracted light waves to produce three-dimensional images. Holograms are used in security features on credit cards, passports, and other important documents, as they are difficult to replicate or counterfeit.In conclusion, the phenomenon of light diffraction is a fascinating and important aspect of the behavior of light. By studying and understanding the principles of light diffraction, scientists and researchers have been able to make significant advancements in various fields, from optics to astronomy to technology. The study of light diffraction continues to be an exciting and fruitful area of research, with new discoveries and applications constantly emerging.。

火山的爆发：地球的脉动英语作文

Volcanic Eruptions:The Pulse of the EarthVolcanic eruptions,one of nature's most powerful and awe-inspiring phenomena,serve as a vivid reminder of the Earth's dynamic inner workings.These natural events,characterized by the explosive release of magma,gases,and ash from beneath the Earth's crust,play a crucial role in shaping our planet's landscape and atmosphere.This essay explores the significance of volcanic eruptions,highlighting their role in the Earth's geological processes,their impact on the environment and human societies,and the importance of monitoring and studying these natural phenomena.The Geological Significance of Volcanic EruptionsVolcanic eruptions are a key component of the Earth's geology,acting as a mechanism for the planet to release internal heat and pressure.The movement of tectonic plates often triggers these eruptions,especially at divergent and convergent boundaries where plates move apart or collide. As magma rises to the surface,it cools and solidifies,forming new crust and shaping the Earth's topography.Over millions of years,volcanic activity has created mountains,islands,and entire landmasses, demonstrating the planet's ever-changing nature.Environmental Impact and FertilityWhile volcanic eruptions can be destructive,they also play a vital role in replenishing the Earth's soils with nutrients.The ash and lava released during an eruption are rich in minerals that,over time,contribute to soil fertility,supporting plant growth and agriculture.For instance,the fertile soils of the Italian countryside and the Hawaiian Islands owe their richness to centuries of volcanic activity.Additionally,volcanic gases such as carbon dioxide contribute to the greenhouse effect,influencing the Earth's climate and atmospheric composition.Hazards and Human SocietyThe immediate effects of volcanic eruptions can be devastating for nearby communities,causing loss of life,destruction of property,and displacement of va flows,ash falls,and pyroclastic flows can obliterate everything in their path.Moreover,the release of volcanicash into the atmosphere can lead to respiratory health issues,disrupt air travel,and affect climate patterns globally.The eruption of Mount Tambora in1815,for example,led to the"Year Without a Summer," causing widespread crop failures and famine across the globe. Monitoring and ResearchGiven the potential hazards associated with volcanic eruptions, monitoring and research are crucial for predicting and mitigating their impact.Volcanologists use a variety of tools,including seismographs,gas sensors,and satellite imagery,to monitor volcanic activity and provide early warnings to at-risk communities.Understanding the signs of an impending eruption,such as increased seismic activity or changes in gas emissions,can save lives and minimize economic damage. ConclusionVolcanic eruptions are a testament to the Earth's vitality,reminding us of the planet's constant evolution and the forces that shape our natural world.While they pose significant risks to human society,they also enrich our environment,contributing to the cycle of renewal that sustains life on Earth.By studying and monitoring volcanic activity,we can better appreciate the complexity of our planet and learn to coexist with these powerful natural phenomena.In embracing the pulse of the Earth,we acknowledge our place within a much larger,ever-changing system.。

流浪地球的450字作文

流浪地球的450字作文英文回答：In the vast expanse of the cinematic universe, "The Wandering Earth" (2019) emerges as a poignant and thought-provoking masterpiece that explores the indomitable spirit of humanity in the face of impending cosmic catastrophe. Framed against the backdrop of a dying sun and an uninhabitable Earth, the film weaves a complex tapestry of personal sacrifices, scientific ingenuity, and the unyielding hope that fuels our collective existence.The film's narrative transports us to a future where the Earth, facing an imminent collision with the expanding sun, embarks on an audacious mission to relocate its entire population to a distant star system. This colossal undertaking, the Wandering Earth Project, involves igniting thousands of planetary engines scattered across the globe, propelling the Earth away from its doomed orbit.At the heart of this extraordinary endeavor lies a cast of compelling characters, each navigating their own personal struggles amidst the impending global crisis. Liu Peiqiang (Wu Jing), a veteran astronaut, must confront the weight of his responsibility as mission commander, while his son Liu Qi (Qu Chuxiao) grapples with the realities of life on a migrating Earth.As the countdown to the Earth's departure approaches, the film delves into the complexities of scientific achievement and the ethical dilemmas it poses. Thesacrifice of countless lives and the potential for unforeseen consequences weigh heavily on the minds of scientists and engineers, forcing them to confront the limits of human knowledge and ingenuity.Ultimately, "The Wandering Earth" transcends its genre as a mere science fiction epic. It becomes a testament to the enduring power of the human spirit, our capacity for resilience, and the unwavering hope that propels us forward even in the face of unimaginable adversity.中文回答：《流浪地球》这部电影以一个濒死的太阳和一个不再宜居的地球为背景，讲述了一段感人至深、发人深省的故事。

Possible Negative Pressure States in the Evolution of the Universe

ible matter: the vacuum energy (also known by such names as dark energy, quintessence, x-matter,
become unbound, while the nongravitationally bound systems remain bound.
Meanwhile, it is convenient to express the mean densities ρi of various quantities in the Universe in terms of their fractions relative to the critical density: Ωi = ρi/ρcrit. The theory of cosmological
system of equations. Their results showed that even for very slow growth of Λ (which satisﬁes all
the conditions on the variation of GN ), in the distant future the gravitationally bound systems
1E-mail: chukh0581@ 2The second address : P.O. Box 30-15, Shanghai 200030, PR China.
–2–
term have been presented before. For example, we can start from the Einstein action describing the gravitational forces in the presence of the cosmological constant (Padmanabhan 2003)

英语想象科幻作文

英语想象科幻作文Title: A Journey Beyond the Stars。

In the vast expanse of the cosmos, where galaxies twirl in a cosmic dance and stars flicker like distant beacons, lies a realm of endless possibilities. It is a place where the boundaries of imagination blur with the fabric of reality, where dreams take flight on the wings of innovation and exploration. Welcome to the realm of science fiction, where the impossible becomes possible and the mundane transforms into the extraordinary.Imagine a world where humanity has unlocked the secrets of interstellar travel, where starships soar through the void of space like majestic birds in flight. These vessels, powered by exotic energies and guided by the brilliance of human ingenuity, traverse the cosmic highways, charting courses to distant worlds beyond our wildest dreams.On board one such starship, the Odyssey, a diverse crewof explorers embarks on a mission that will redefine the very essence of human existence. Their destination? A remote corner of the galaxy known only as Epsilon Prime, a fabled world rumored to hold the key to unlocking the mysteries of the universe itself.As the Odyssey hurtles through the vastness of space, its crew faces myriad challenges, from cosmic storms that threaten to tear their ship asunder to encounters withalien civilizations whose motives remain shrouded in mystery. Yet, fueled by their unwavering determination and boundless curiosity, they press onward, driven by the promise of discovery that awaits them on the horizon.At long last, the Odyssey arrives at its destination, greeted by the sight of a world unlike any they have ever seen. Epsilon Prime is a world of wonders, where towering spires of crystal pierce the sky and rivers of liquid light flow through verdant valleys teeming with life. But amidst the beauty lies a secret, a truth that will shake the very foundations of everything the crew holds dear.For Epsilon Prime is not simply a planet; it is a sentient being, a living consciousness that has traversed the cosmos since time immemorial. It speaks to the crew in whispers of stardust and echoes of distant galaxies, revealing to them the true nature of the universe and their place within it.As the crew grapples with this revelation, they come to understand that they are not merely explorers seeking knowledge, but participants in a grand cosmic tapestry woven from the fabric of existence itself. And as they bid farewell to Epsilon Prime and set course for home, they carry with them not only the memories of their journey but the realization that the greatest adventure of all is the one that lies within.So, dear reader, as you gaze up at the stars and ponder the mysteries of the universe, remember that the boundaries of imagination are but illusions, and that within each of us lies the spark of a thousand suns, waiting to ignite the fires of discovery and illuminate the darkness of the unknown. For in the end, it is not the destination thatmatters, but the journey itself, and the countless worlds that await us beyond the stars.。

《一位哲学家在太阳仪面前的演讲》英语作文

《一位哲学家在太阳仪面前的演讲》英语作文全文共3篇示例，供读者参考篇1A Philosopher's Speech in Front of the SundialAs I approached the ancient sundial in the center of the university quad, I saw a small crowd had already gathered. An elderly man with a long white beard and wearing a tweed jacket stood next to the sundial, leaning slightly on a cane. This was Professor Emerson, one of the most renowned philosophers of our time. I had been lucky enough to get into his seminar on Existential Ethics this semester, and he had announced he would be giving an impromptu lecture outdoors today. I found a spot towards the front of the crowd and waited eagerly for him to begin.Professor Emerson closed his eyes briefly, seeming to collect his thoughts, before beginning to speak in his trademark soft, yet commanding voice. "My dear students, we find ourselves here today in the presence of one of humanity's oldest tools for measuring the inexorable march of time - the sundial. Since the dawn of civilization, we have sought to quantify the periods ofday and night, the seasons, the years. To regulate our lives by the movements of the heavenly bodies."He gestured towards the sundial's gnomon, the upright piece that cast its shadow across the markings. "This simple device allowed our ancestors toSection the day into manageable increments. Yet unlike our modern orderly clocks and calendars, the sundial's graduations are uneven. The hours shrink and expand, shifting with the seasons, ephemeral as a cloud's shape morphing in the wind."The professor smiled wistfully. "What a wonderful metaphor this presents for the very nature of human experience. We constantly strive to systematize our lives, to predict and control. We make our plans and schedules. But life will not be contained so tidily. Like those wandering sundial hours, our moments overflow and bleed into each other, defying quantization."He began slowly pacing around the small gathering, making eye contact with students one by one. "We university folk are particularly susceptible to this delusion of orderliness. We divide knowledge into disciplines, classify everything into rigid categories. We pursue our degrees and credentials along an approved path, checking off requirements. We become adept atliving our lives by the syllabus, counting out experience in credits and semesters."Professor Emerson shook his head. "But for all our studies, we often miss the fundamental truth that life and time are irreducibly messy. The divisions we impose are fictions, a cobweb of conceptual structure spun over the continuous tumbling flow of existence. That audacious alpha student who has perfectly planned their future career?"He fixed his gaze at a young woman in the front row who blushed. "One day, an undisciplined passion will set their soul alight and they will be consumed. That sullen underachiever," he nodded towards a young man wearing headphones, "may be transformed by a single profound experience into a driven visionary."The professor chuckled. "Do not misunderstand me - I am not advocating chaos or dereliction! By all means, embrace the structures and disciplines that aid in understanding. The great edifice of human knowledge is also a wondrous achievement. But never forget - it is a fiction we have constructed over the raw, anarchic entropy of being."He returned to stand next to the sundial, tracing its markings with his hand. "Look at this record of solar time, shifting andadjusting to nature's rhythms. This is the reality - ceaseless flow and flux, with our imposed order floating atop it like a reed riding the current of a river."Professor Emerson's eyes twinkled as he seemed to gather his thoughts towards a conclusion. "So study hard, my friends, but study with an open mind. Earn your accolades, but never let them become blinders to the marvelous unruliness of the world. Make your plans diligently, but always be prepared to cast them aside when Being beckons you toward richer adventure!""Let the timepiece in your palm be your phone's sterile digital display. But let the timepiece in your heart be this wild sundial, dancing to the unpredictable music of heaven's spheres. For our moments are meant to be savored and surrendered to, not merely accumulated like merit badges."With that, Professor Emerson nodded, took up his cane, and began slowly walking away, leaving us all stunned and provoked by his words. I found myself both freshly inspired towards my studies and yet wistfully pondering how I might open myself more to life's untamed rhythms.As I watched the professor's small figure disappearing towards the tree-lined path, I saw the shadow had shifted across the ancient sundial, marking another lopsided hour. And I smiledat the beautiful contradiction that in that very moment, I had encountered something transcendent and outside of time.篇2A Philosopher's Speech Before the SundialAs I stand here before this ancient sundial, a relic of humanity's quest to measure and understand the passage of time, I am struck by the profound weight of the subject matter I am about to address. Time itself is a vast and enigmatic concept that has puzzled philosophers, scientists, and thinkers throughout the ages. It is a force that governs our very existence, shaping our experiences, memories, and aspirations, yet it remains an elusive and almost mystical phenomenon.From the earliest civilizations, mankind has grappled with the nature of time, seeking to quantify and delineate its boundaries. The sundial before us serves as a poignant reminder of this enduring pursuit, a testament to our ancestors' ingenuity and their desire to impose order on the seemingly chaotic flow of existence. Yet, as we have advanced in our understanding, we have come to realize that time is far more intricate and paradoxical than we could have ever imagined.The great Greek philosopher Heraclitus famously stated, "No man ever steps in the same river twice, for it's not the same river and he's not the same man." This profound observation encapsulates the essence of time's fluidity and our own transient nature within its currents. We are constantly in motion,ever-changing, and the river of time carries us forward, never allowing us to truly revisit the past or dwell in the present for more than a fleeting moment.The notion of time as a linear construct, an arrow stretching from the past to the future, has long been challenged by modern physics and the revelations of Einstein's theory of relativity. Time, it seems, is intimately intertwined with space, forming a fabric that bends and warps under the influence of gravity and motion. The boundaries between past, present, and future blur, and our perception of time becomes a subjective experience, shaped by our individual circumstances and frames of reference.Philosophers have grappled with the metaphysical implications of time, questioning whether it is an objective reality or merely a construct of our minds. Some have proposed that time is an illusion, a byproduct of our consciousness, while others argue that it is a fundamental aspect of the universe, woven into the very fabric of existence. The great thinkers ofantiquity, such as Parmenides and Zeno, challenged our notions of time with their paradoxes, forcing us to confront the limitations of our rational understanding.In the realm of existentialism, time takes on an even more profound significance. Thinkers like Kierkegaard and Heidegger explored the concept of temporality, the lived experience of time, and how it shapes our sense of being and purpose. They posited that our awareness of our own mortality, the finite nature of our existence, imbues time with a sense of urgency and meaning, compelling us to embrace the present and live authentically.The study of time has also been inextricably linked to the realm of consciousness and subjective experience. Philosophers like Henri Bergson and Edmund Husserl delved into the phenomenological aspects of time, exploring how our perception of its passage is shaped by our internal states, memories, and anticipations. They challenged the notion of time as a purely objective, quantifiable phenomenon, emphasizing the rich tapestry of our lived experiences as they unfold within the temporal realm.As we stand here, contemplating the enigma of time, we are confronted with a paradox: the more we seek to understand and quantify it, the more elusive and enigmatic it becomes. Thesundial before us serves as a poignant reminder of our ancient quest to tame and measure time, yet it also represents the limitations of our attempts to fully comprehend its nature.Perhaps the true wisdom lies not in seeking to conquer time, but in embracing its fluidity and accepting our transient existence within its currents. For it is in the fleeting moments, the ephemeral experiences, that we find the richness and depth of what it means to be human. It is in the embrace of the present, the acceptance of the ever-changing nature of existence, that we can find true meaning and purpose.As we ponder these profound questions, let us be humbled by the vastness of the cosmos and the mysteries that still remain to be unraveled. Let us approach the study of time with a sense of wonder and curiosity, recognizing that each new discovery may reveal further depths and complexities that challenge our preconceptions.And let us not forget the enduring wisdom of the ancients, who understood that time is not merely a construct to be measured, but a force that shapes our very existence. For it is in the eternal dance between the fleeting and the enduring, the transient and the timeless, that we glimpse the true nature of our reality and our place within the grand tapestry of existence.篇3A Philosopher's Speech Before the SolariumAs I approached the solarium for the lecture that crisp autumn morning, my footsteps crunched over the fallen leaves coating the brick path. The stained glass dome ahead glowed warmly, backlit by the rising sun. Though I had attended many classes and seminars in this verdant enclosure over the years, today felt different - weighted with significance. For today, we would be honored by the words of Dr. Miriam Abramson, one of the foremost moral philosophers of our age.My fellow students and I found our usual seats along the curved stone benches fanning out from the small raised platform at the center. The solarium's high glass ceilings allowed the fragrance of the surrounding gardens to waft in on the cool breeze. As we waited, I gazed up at the colorful abstract panes depicting the cosmic dance of planets and stars in whimsical stained glass. How fitting, I mused, that a philosopher who pondered the deepest questions of existence would speak beneath this celebratory canopy of the cosmos.At last, a hush fell over the crowd as Dr. Abramson made her way to the lectern, a tiny figure swallowed up by the enormity ofthe space around her. She adjusted the microphone and cleared her throat."Good morning," she began, her voice resonant despite her unimposing stature. "It is an honor to be invited to share some thoughts with all of you here today, in this splendid solarium bathed in the golden light of a new day's dawning."She paused, sweeping her gaze across the crowd with eyes that seemed to laser into each of us individually despite the audience's size."Throughout human history, we have gazed upwards and asked: What is our place in this vast universe? What roles do we play in the grand cosmic drama? What duties, if any, do we owe to one another, to other forms of life, to this fragile planet we call home?"Dr. Abramson took a breath, allowing her words to hang heavy in the hushed air. I could sense the students around me leaning slightly forward, hanging on her every phrase."The questions of ethics, of how we ought to live and treat one another, have captivated the greatest minds since the dawn of reason. From the ancient traditions of Buddhism, Confucianism, and the Greek philosophies, to the Enlightenmentthinkers who laid the groundwork of secular modern ethics, to contemporary philosophers grappling with the moral implications of technology and globalization - all have pondered what constitutes a good life. What paths should we walk as individuals and societies to cultivate virtue, justice, and human flourishing?"She paced slowly across the small platform, her gaze firmly holding our attention as if she were a gazelle warily eyeing a pack of lions."Yet for all of our lofty philosophical investigations, it becomes clear that ethics is not merely an intellectual exercise. It is, at its core, intensely personal and practical. For how we choose to live our lives, how we treat friends and strangers alike, what priorities we pursue - these are the tuning forks that reverberate louder than any academic discourse, sending ripples outward with consequences that shape our shared reality."Dr. Abramson's eyes crinkled upwards as she squinted into the morning sunlight slanting through the solarium's eastern windows."You are young. The paths before you spin off in a million different directions, with a million different possible futures awaiting. The choices are exhilarating...and daunting. Whichethical framework will guide your journey? What moral code will be your compass as you navigate the myriad crossroads of your lives?"She nodded slowly, seeming to ponder her own rhetorical questions. After a few beats of silence, she continued."There are no easy answers, no philosophical universal theory that can hand you a pre-programmed set of unambiguous instructions to follow. The great thinkers have bequeathed a toolbox of ethical ideas and frameworks that can aid your reasoning - consequentialism, deontology, virtue ethics, and so on. But in the end, the hard work of actually working out what is right and good in any given situation must be undertaken anew by each of us."We cannot simply work out the theory in our minds and remain at the level of abstraction. We must get our hands dirty, live according to our beliefs, and be prepared to wrestle with the real-life applications and grey areas that inevitably emerge when translating neat philosophical ideas into a messycomplexity of reality."Dr. Abramson's expression grew more intense, her eyes boring into ours as if willing us to take her words to heart."This is the great challenge I leave you with today: To not merely be students of ethics, but to fully embrace the struggle of lived ethics. To develop your own moral operating system through rigorous reflection and analysis, to be sure - but then to inscribe that code onto the vast keyboard of daily decision making and behavior. To strive at every turn to walk the talk, allowing your values to suffuse your words and deeds no matter how big or small the choice at hand."Her voice grew impassioned, rising in a crescendo over the solarium's polished floors and domed glass ceilings."This is the path to authentic ethics, to developing genuine wisdom rather than mere philosophicalacademia. It is to adopt an ethical life as an ongoing,ever-evolving practice. One misstep does not negate the entirety of one's progress - humans are flawed, and our moral compasses will wobble and stray at times. The key is to perpetually re-orient ourselves through intensive introspection, course corrections, and a continual refinement of our ethical beliefs and behavior."Dr. Abramson paused for a breath, her eyes alight with fervor. My pen hovered over my notebook, having long abandoned attempting to capture her every word in favor of simply drinking in the profound ethos of her exhortation."In doing so," she proclaimed, "in striving to manifest our inner moral codes as outer reality through painstaking growth and ethical praxis, we do not merely think lofty thoughts. We become architects sculpting the very fabric of human civilization through the chisel strokes of our choices and character."Her voice grew quieter now, allowing the weight of her words to reverberate across the solarium's sacred space."That is the great potential each of you carries within you. To construct the future, both personal and societal, through the patient building blocks of ethical living. It is a colossal responsibility, a staggering burden - and the most ennobling calling to which each of us can aspire."With that, Dr. Abramson fell silent. I sat stunned alongside my peers, caught in the wake of her powerful oration. As sunbeams danced through the stained glass surrounding us, I felt as if I had been simultaneously humbled by the magnitude of the ethical gauntlet she had thrown down...and elevated by the breathtaking vision of human potential she had cast.In that solarium awash in celestial light, I experienced a moral reckoning - a visceral felt-sense that the path of virtue she exhorted would be an arduous journey I nonetheless had to undertake. For in heeding Dr. Abramson's call, in committing torelentlessly manifest my ethical code through each choice and action, I could perhaps play some small part in humanity's ongoing striving towards the lofty ideals that have rung forth through the words of its greatest thinkers since the dawn of philosophy itself.。

高中英语时文拓展阅读48(新型传感器有助于预测火山爆发)

时文阅读|新型传感器有助于预测火山爆发从前，人们在面对突如其来的自然灾害时，只能无奈地接受它们带来的苦难，被迫流离失所，甚至付出生命代价。

如今，监测系统的发展使得科学家能够监测灾害来临前的迹象，这无疑给人们留出了应对灾害的时间，从而大大减少伤亡、降低损失。

现在，让我们去了解一种可以预测火山爆发的新型装置吧阅读短文并回答问题Compared to other causes of natural disasters, volcanoes offer clues only when they are about to erupt. Now, however, developments in monitoring systems have allowed scientists to develop sensors（传感器）to detect and consequently forecast eruptions, allowing authorities to plan for them with increasing precision.University of Cambridge volcanologist Marie Edmonds says that scientists are now able to use very precise sensors that monitor the gases volcanoes emit, which can give clues on the location of the magma（岩浆）. The sensors help with prediction because different gases are emitted at various stages of an eruption. When magma rises, pressure is released along with gases. Carbon dioxide is released early on and then, as the magma goes higher, sulphur dioxide is released. The ratio（比例）of the two gases is used to detect the location of magma relative to the surface, telling researchers the imminence of the eruption.Edmonds is connected to an international group known as the Deep Carbon Observatory that has worked to put new gas sensors on fifteen of the most active and dangerous volcanoes to improve the forecasting of various types of eruptions. The gas sensors continually measure water vapor, sulphur dioxide, and carbon dioxide. They are placed inside large boxes with surface antennae（触角）and buried underground. Advances in electronics have increased their precision and lowered their cost, allowing more of them to be used worldwide.Putting these sensors atop active volcanoes is dangerous. Scientists wear reflective suits that protect against heat, plus gas masks for protection from corrosive gases. They sometimes hike long distances in remote regions to reach a site. However, according to Edmonds, the work they do to save people’s lives makes a dangerous job worth it. She enjoys doing something that helps people.Edmonds’ team has also attached sensors to drones to measure emissions from a Papua New Guinea volcano for a short time, a technique developed to gather “snapshots” of the activity. These snapshots help researchers to better understand the complexities of activities that lead to eruptions1.How do sensors detect and predict volcanic eruptions?A. By sending warnings to researchers.B. By measuring the heat underground.C. By tracking different gases released.D. By studying the surrounding emissions.2. What do we know about the gas sensors?A. They should be attached to the magma.B. They are available around the world.C. They can check various types of eruptions.D. They become more accurate and expensive.3. Why is it risky to place sensors atop active volcanoes?A. There is heat and dangerous gases.B. It’s hard to find the top of volcanoes.C. Scientists lack enough suits and masks.D. Active volcanoes may erupt at any time.4. What are the snapshots for?A. Predicting volcanic eruptions earlier.B. Attracting people’s attention to volcanoes.C. Avoiding the danger of the researchers’ work.D. Collecting more information for researchers.【参考答案】CBAD单词学习1. emit v. 发出；散发Sulphur gases were emitted by the volcano. 硫磺气体由火山喷发出来。

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a rXiv:as tr o-ph/3479v13Apr23accepted for publication in Astrophysical Journal Probing O VI Emission in the Halos of Edge-on Spiral Galaxies B.Otte 1,E.M.Murphy 2,J.C.Howk 3,Q.D.Wang 4,W.R.Oegerle 5,K.R.Sembach 6ABSTRACT We have used the Far Ultraviolet Spectroscopic Explorer to search for O VI λλ1031.926,1037.617emission in the halos of the edge-on spiral galax-ies NGC 4631and NGC 891.In NGC 4631,we detected O VI in emission toward a soft X-ray bubble above a region containing numerous H αarcs and ﬁlaments.The line-of-sight component of the motion of the O VI gas appears to match the underlying disk rotation.The observed O VI luminosities can account for 0.2−2%of the total energy input from supernovae (assuming a full O VI emitting halo)and yield mass ﬂux cooling rates between 0.48and 2.8M ⊙yr −1depending on the model used in the derivations.Based on these ﬁndings,we believe it is likely that we are seeing cooling,galactic fountain gas.No emission was detected from the halo of NGC 891,a galaxy in a direction with considerably high foreground Galactic extinction.Subject headings:ISM:general —ISM:individual (NGC 4631,NGC 891)—galaxies:general —galaxies:halos —galaxies:individual (NGC 4631,NGC 891)—galaxies:ISM1.INTRODUCTIONIn a seminal work,Spitzer(1956)predicted that the Milky Way is surrounded by a tenuous,hot corona.Provided that the temperature of such gas was below the escape temperature for a galaxy,he showed that the gas should extend several kiloparsecs above the midplane.Spitzer’s original arguments have been extended to a more general theory of the dynamical evolution of the gaseous disks of spiral galaxies.This so called“galactic fountain”model(Shapiro&Field1976;Bregman1980)postulates that the hot gas produced by multiple overlapping supernovae would be buoyant in the thin,cold disks of gas in spiral galaxies.As the hot gas rises from the thin disk into the halo,it cools,perhaps forming condensations of neutral material which then fall back towards the thin disk as high-or intermediate-velocity clouds.The existence of such gas in the Milky Way is now well documented through observa-tions of strong O VI absorption(e.g.Savage et al.2000)and X-ray emission(Burrows& Mendenhall1991).The1032/1038˚A doublet transition of Li-like oxygen(O VI)is an impor-tant tracer of hot,collisionally ionized material.At temperatures of∼3×105K,where gas of near-solar metallicity cools most eﬃciently,emission from the O VI doublet can be the primary coolant.Hence,observations of O VI emission provide a direct probe of the cooling rate of material near this temperature.Furthermore,because the cooling of gas at tempera-tures of a few times105K is so much more eﬃcient than that of material at higher(or lower) temperatures,O VI emission traces rapidly cooling material that may condense into clouds in the galactic fountain scenario.Observations of O VI from this“transition temperature”gas in the halos of galaxies provide fundamental information on the circulation of material within spiral galaxies.An O VI absorption line survey of the Milky Way(Savage et al.2003)showed that the observed O VI was best explained by a patchy disk corotating with the Galactic plane with signs of outﬂows.O VI observed in emission in the Milky Way has been associated with gas in or around the Local Bubble(Welsh et al.2002and references therein)and an outﬂow from the Perseus arm(Otte,Dixon,&Sankrit2003).With our improving understanding of the O VI distribution inside the Milky Way,it becomes necessary to search for O VI emission in other galaxies as well in order to be able to distinguish between peculiarities and common properties of galaxies.We analyzed spectra of two galaxies(NGC4631and NGC891)taken by the Far Ultraviolet Spectroscopic Explorer(FUSE).Both galaxies are nearly edge-on(i.e.allow clear distinction between disk and halo gas)and known for strong extraplanar emission in radio continuum,optical lines,and soft X-rays(SXR).Thus,they appear to be good candidates to search for evidence of galactic fountains.While the spectra of NGC891did not reveal any O VI emission,we detected the O VI doublet in NGC4631.This is theﬁrst detection of O VI in emission in a spiral galaxy other than the Milky Way and its companions.2.OBSERVATIONS AND DATA REDUCTION2.1.NGC4631We obtained spectra with the low resolution aperture(30′′×30′′)of FUSE at two positions above the disk of NGC4631(see Figure1)as part of Project P134.Position A is at the peak of the1/4keV X-ray emission detected by Wang et al.(1995)using ROSAT. Position B is between two Hαarcs that were believed to form the walls of an open chimney into the halo(Rand,Kulkarni,&Hester1992),although a recent Hubble Space Telescope Hαimage(Wang et al.2001)indicates that this feature may be a superposition of several loops.All FUSE observations were conducted in TTAG mode where the detectors record the arrival time,x-and y-position,and the pulse height of each event(Sahnow et al.2000). About2/3of each observation was conducted during orbital night.The O VI lines in the NGC4631spectra were extracted from the raw dataﬁles.Only events with a pulse height between4and15,inclusive,were used in the analysis.Nearby O I airglow lines were used to determine the height of the extraction windows,and the spectra were summed perpendicular to the dispersion direction.Emission free regions adjacent to the O VI lines in the dispersion direction were used to determine the background.Figure2shows a plot of the extracted spectra for both pointings toward NGC4631. The wavelength scale was determined by running the spectra through the FUSE data pipeline V2.0.5.Its uncertainty is about one pixel(0.007˚A).Both the redshifted1032˚A and1038˚A line were detected on the LiF1A channel.However,the latter was blended with the O Iλλ1039.23,1040.94,1041.69airglow triplet.The background at wavelengths>1040˚A was increased due to a stripe of scattered light within the satellite.The O VI emission lines were also seen on the LiF2B segment for Position B;however,the lower eﬀective area signif-icantly reduced the quality of the data.Therefore,we have used only data from the1032˚A emission line of segment LiF1A in the analysis.A comparison of day-plus-night versus night-only data yielded the sameﬂux in the O VIλ1032line and therefore ruled out the possibility that the emission arose from scattered solar O VI emission in the LiF channels.Deep integrations towards other lines of sight ruled out detector artifacts,scattered light,and stray light contamination at the positions of the detected lines.The background was not signiﬁcantly higher during the sunlit portion of the orbit;therefore,we have used the combined day and night data in our analysis.Theresulting signal-to-noise ratios for the integrated O VIλ1032intensities are4.5(Position A) and6.0(Position B).2.2.NGC891The data for NGC891were obtained as part of FUSE Guest Investigator Cycle2pro-gram B114.Two positions below the disk and one position above the bulge of NGC891 (see Figure3)were observed.The data reduction was identical to the data reduction for NGC4631above.Approximately85%of the observations were conducted during the night portion of the orbit.The observational data for both galaxies are listed in Table1.Given the large random errors in the intensities of NGC891,we do not believe that any of these detections are signiﬁcant.3.RESULTS3.1.NGC4631NGC4631is an Sc/SBd type galaxy at a distance of about7.6Mpc with an inclination of i≈85◦.The interaction between NGC4631and its two neighboring galaxies NGC4656 and NGC4627might have triggered the active star formation observed in NGC4631today. The galaxy possesses a complex system of Hαﬁlaments and bubbles(Wang et al.2001)and an extended radio halo with spurs up to10kpc above the disk(Hummel&Dettmar1990). X-ray observations of NGC4631revealed a SXR bubble on the north side of the disk(Wang et al.1995)and a halo of gas extending up to8kpc above the plane with temperatures of (2−7)×106K(Wang et al.2001).Both the radio and the X-ray halo appear asymmetric with the larger extent being on the north side of the disk.The two FUSE pointings for NGC4631were chosen to lie within the SXR bubble ob-served with ROSAT(Wang et al.1995,see Figure1).Assuming an eﬀective area of27.0cm2 for the LiF1A segment and900arcs2for the aperture area,we derive O VIλ1032intensi-ties of4600±1000photons s−1cm−2sr−1(Position A)and8000±1300photons s−1cm−2sr−1 (Position B).Table2lists the intensities(also converted to ergs s−1cm−2sr−1)and the cor-responding height above the disk.The O VIλ1032intensity at Position B is a factor ofﬁve smaller than the2σupper limit of Ferguson et al.(1995),who observed the southern portion of the galaxy with the Hopkins Ultraviolet Telescope on the Astro-2mission.The full extent of the O VI emitting gas is unknown.However,it is useful to consider two cases.In theﬁrst scenario,the O VI emission is conﬁned to the region of the SXR bubble on the north side ofthe disk.On the other hand,the O VI emission may extend across the full hot halo revealed by the Chandra observations of Wang et al.(2001).The former would correspond to a sin-gle large bubble breaking out into the halo,while the latter implies a full galactic fountain. Therefore,we will calculate the luminosity for both cases assuming that the average O VI sur-face brightness is equivalent to the average of Positions A and B.The O VIλ1032/O VIλ1038 line ratio approaches two in optically thin gas and is reduced in optically thick gas due to self-absorption.We assumed the upper limit for this line ratio of2in order to account for the O VIλ1038emission in our calculations.Ignoring extinction(see below)we calculated a lower limit for the total O VI luminosity of L OVI=(6.2±0.8)×1038ergs s−1for the bubble and L OVI=(2.3±0.3)×1039ergs s−1for the entire halo.3.1.1.Correction for ExtinctionThe Milky Way extinction toward NGC4631is E(B−V)=0.017(Schlegel,Finkbeiner, &Davis1998).Using a Galactic extinction curve by Cardelli,Clayton,&Mathis(1989),we derived an optical depthτ=0.25,i.e.the UVﬂux is attenuated by a factor of1.28.The corrected values for the luminosities are L OVI=(7.9±1.1)×1038ergs s−1for the bubble and L OVI=(3.0±0.4)×1039ergs s−1for the entire halo.The amount of extinction within NGC4631is uncertain.Long slit optical spectroscopy of the Balmer lines by Martin&Kern (2001)has revealed evidence for signiﬁcant and patchy extinction due to dust in the halo of NGC4631up to5kpc from the plane.However,another group found almost no extinction at a height of>1kpc on the northern side of the disk(Otte et al.2001).We obtained long slit spectra of NGC4631with the2.3m Steward Observatory on Kitt Peak,Arizona,on 2002April7–9.The slit was about4.′′5wide and orientated north-south to cross both FUSE positions.The measured Hα/Hβline ratios are comparable to those measured by Otte et al.(2001).We therefore assume that extinction within the halo of NGC4631is negligible.parison with X-ray LuminosityAs mentioned earlier,the O VI emission is an important coolant for hot ionized gas.In fact,for gas in heating-cooling equilibrium,the O VI emission cooling at intermediate tem-peratures(few×105K)should dominate the radiative cooling in the SXR emission of106K gas(Sutherland&Dopita1993).The SXR intensities in the0.1−2keV range measured by Chandra and integrated over the area of the FUSE positions are1.5×10−7ergs s−1cm−2sr−1 (Position A)and3.1×10−7ergs s−1cm−2sr−1(Position B)yielding SXR/O VI ratios of 1.2±0.3and1.3±0.2for Positions A and B,i.e.the intensities are more or less comparablebetween the O VI doublet and the SXR emission.However,the used O VIλλ1032,1038emis-sion was a lower limit(see paragraph3.1)and not corrected for Galactic extinction(because we did not know the Galactic extinction for the SXR emission);the SXR/O VI ratios did not account for any dust in NGC4631either.Thus,the quoted ratios are probably lower limits suggesting that the gas is not in equilibrium.The total O VI luminosity(corrected for Galactic extinction,see paragraph3.1.1),if assumed to cover the whole halo of NGC4631,is by a factor of9lower than the two com-ponentﬁt of the X-ray luminosity(2.7×1040ergs s−1,Wang et al.1995,corrected for diﬀerent distances).The corresponding temperature of this X-ray luminosity was approxi-mately6×105K,i.e.a temperature about twice as high as the one for the peak of the O VI cooling curve.This again implies that the gas is not cooling eﬃciently in O VI,i.e.it is not in equilibrium.However,the uncertainties here are higher than for the comparison above,as we now have compared observational data with a model using diﬀerent simpliﬁcations(e.g. for the shape and extent of the emitting halo).Wang et al.(1995)estimated that the mechanical energy input from supernovae is about (1.5−15)×1041ergs s−1,i.e.the observed O VI luminosity can account for at most∼2% of the total energy input from supernovae.If the gas is in equilibrium,then the remaining energy input has to be used to transport the gas from the disk into the halo against the gravitational potential of the galaxy and against the pressure of the magneticﬁeld lines and is partially lost due to cooling of∼106K hot gas by species other than oxygen.From the coolingﬂow models of Edgar&Chevalier(1986),we calculate that the full halo case is equivalent to a massﬂux of2.8M⊙yr−1for cooling at constant density and1.8M⊙yr−1for cooling at constant pressure.In case of a single cooling bubble the corresponding massﬂuxes are0.74M⊙yr−1(isochoric)and0.48M⊙yr−1(isobaric).A rough estimate for the massﬂux derived from ROSAT observations is∼1M⊙yr−1(Wang et al.1995).Despite the large uncertainties due to unknown extinction and the diﬀerences in the cooling models,the data suggest that the O VI emitting region is larger than the SXR bubble,but smaller(within a factor of∼2)than the X-ray halo observed with Chandra.This means that outﬂows are more signiﬁcant in the central region of the galaxy,as indicated by theﬁlamentary structure in the Hαimage(Wang et al.2001).If,in fact,there was strong,patchy extinction along our lines of sight,the actual O VI luminosities could be much higher than the observed luminosities and could account for a signiﬁcant fraction of the input energy from supernovae.On the other hand,if the O VI emission originated in the interface between cold clouds moving through the hot coronal gas,the relationship between SXR and O VI emission would become more complicated.3.1.3.Rotation VelocityWeﬁtted the observed O VIλ1032emission line by a convolution of a tophat(106km s−1 wide to represent theﬁlled FUSE30′′×30′′square aperture)and a Gaussian proﬁle(to represent the spectral resolution of FUSE).We usedχ2calculations to determine the best central wavelengths and intrinsic velocity dispersions for the emission lines at Positions A and B.In order to estimate1σuncertainties for these parameters,we varied each parameter independently until the corresponding minimumχ2increased by1relative to the bestﬁtχ2. The derived values areλc=1034.30±0.09˚A and FWHM=200±50km s−1for Position A andλc=1034.37±0.08˚A and FWHM=170±50km s−1for Position B.The corresponding radial velocities(geocentric)are v=690±30km s−1(Position A)and710±20km s−1 (Position B).An oﬀset of−9.1and−9.5km s−1,respectively,yields local standard of rest velocities.If the O VI emitting gas does notﬁll the LWRS aperture and is not centered in it,the maximum additional oﬀset in the line centroid is50km s−1(for a point source at the border of the aperture).The line broadening due to the satellite’s orbital motion is <8km s−1,i.e.well within the uncertainties of the FWHM.The intrinsic FWHMs,if assumed to be thermal widths,correspond to temperatures of (1.4±0.7)×107K(Position A)and(1.0±0.6)×107K(Position B).These temperatures are an order of magnitude higher than the hottest component of the SXR emission.This suggests that the line widths are caused by turbulent motion or diﬀerential rotation rather than thermal motion.From position-velocity diagrams(Rand1994),one can expect an oﬀset of about50–100km s−1between the systemic velocity(606km s−1)and the velocity of gas corotating in the disk at the distances of the FUSE positions projected onto the major axis.A combination of the size of the low resolution aperture(30′′or about1.1kpc)and diﬀerential rotation yields a velocity dispersion of up to200km s−1in the galactic plane near the center.The O VI velocity centroids and FWHMs closely match those of the H I gas in the disk of NGC4631.Thus,the line-of-sight component of the motion of the O VI emitting gas appears to follow the rotation of the underlying disk gas.Due to the galaxy’s high inclination,the velocity of the O VI emitting gas perpendicular to the disk is unknown. Although not conclusive,this strong relationship to the disk gas implies that the O VI emitting material originated in the disk.3.1.4.Scale HeightIf the gas has an exponential distribution in the vertical direction above the disk,we can calculate the density scale height of the gas from the two observations at Positions Aand B using:h =−2(z A −z B )I B )(1)where z A and z B are the vertical heights of the projected aperture above the plane of the galaxy,and I A and I B are the measured speciﬁc intensities at Positions A and B,respectively.The 2in Equation 1results from the fact that the intensity is proportional to the square of the ing the intensities and z heights from Table 2,we ﬁndh =8.3±4.1kpc .(2)The large uncertainty in the density scale height results from the large uncertainties in the measured speciﬁc intensities at Positions A and B.However,it appears that the scale height of O VI gas in NGC 4631is signiﬁcantly larger than the 2.3kpc scale height found by Savage et al.(2003)for the Milky ing the intensities from paragraph 3.1.2,we ﬁnd a scale height of about 6.6kpc for the SXR emission in NGC 4631,whereas Hummel,Beck,&Dahlem (1991)reported a scale height of about 8kpc for the magnetic ﬁeld.Scale heights derived from optical emission lines range between 0.9and 1.2kpc,not considering the roughly constant intensity at z >3kpc (Martin &Kern 2001).Given the uncertainties,the scale heights of the O VI and SXR emission and the magnetic ﬁeld appear to be comparable.3.2.NGC 891The Sb spiral galaxy NGC 891is at a distance of about 9.6Mpc with an inclination of i ≥88.6◦(Rupen et al.1987).We observed three positions in the halo of NGC 891and did not detect O VI in emission to a level greater than 3σ(Fig.4).The 3σintensity limits can be obtained from Table 2.The Milky Way extinction in this direction is E (B −V )=0.065(Schlegel et al.1998).Assuming a standard interstellar extinction curve (Cardelli et al.1989),the UV ﬂux is attenuated by a factor of 2.4(τ=0.88).If we assume that the O VI emitting halo of NGC 891is the same size as the X-ray emitting halo seen by Bregman &Pildis (1994),and that the average speciﬁc intensity of the halo in NGC 891is equal to the average at Positions 1and 2,then the upper limit to the O VI λ1032luminosity is L 1032<7.5×1038ergs s −1.Bregman &Houck (1997)found that the SXR (0.1–2.5keV)band luminosity is 2.45×1039ergs s −1with a cooling rate of 0.08M ⊙yr −1(corrected for d =9.6Mpc).This predicts an O VI λ1032luminosity (Edgar &Chevalier 1986)of 5.7×1037ergs s −1(isochoric)and 8.8×1037ergs s −1(isobaric),well within our limits.We have not attempted any correction for extinction in the halo of NGC 891itself.4.CONCLUSIONSNGC4631has a stronger radio halo than NGC891,but a comparable far-IR luminosity. Rand,Kulkarni,&Hester(1992)concluded from these observations that NGC4631probably has a larger star formation rate than NGC891.The Hubble Space Telescope images of Wang et al.(2001)clearly show loops,arcs,andﬁlaments as would be expected from galactic chimneys.ROSAT observations(Wang et al.1995)revealed the presence of a large region of SXR emission directly above the arcs andﬁlaments,indicating that some of the hot supernovae gas has vented into the halo.In this paper,we reported the detection of O VI emission at two locations above the arcs at the peaks of the SXR emission with a line-of-sight velocity similar to the disk gas and a FWHM broader than what might be expected from thermal motions of hot gas.The observation of O VI emitting gas suggests that the hot gas originated in the disk and is cooling.Due to the large uncertainties in the O VI intensities and luminosities and the range in the luminosities and massﬂux rates predicted by cooling ﬂow models,it is not clear whether the coolingﬂow in NGC4631is(yet)in equilibrium with the massﬂux derived from X-ray observations.However,given all this evidence,we believe that,because of its edge-on nature,NGC4631is theﬁrst spiral galaxy in which O VI has been detected in emission unambiguously from the galactic halo and in which all the expected components for a full cycle of gas in a galactic fountain have been observed.The lack of reliable O VI measurements for NGC891is likely explained with the large foreground extinction of the Milky Way and the amount of dust in NGC891itself.The O VI luminosities predicted from coolingﬂow models are well within our3σlimits for NGC891.The NGC4631data were obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University.Financial support to U.S. participants has been provided by NASA contract NAS5-32985.The NGC891data were obtained as part of the FUSE Cycle2Guest Investigator Program B114.Financial support to U.S.participants has been provided by NASA contract NAG5-10250.JCH recognizes support from NASA grant NAG5-10957.This work made use of the NASA Extragalactic Database(NED)and the NASA Astrophysics Data System(ADS).REFERENCESBregman,J.N.1980,ApJ,236,577Bregman,J.N.,&Houck,J.C.1997,ApJ,485,159Bregman,J.N.,&Pildis,R.A.1994,ApJ,420,570Burrows,D.N.,&Mendenhall,J.A.1991,Nature,351,629Cardelli,J.A.,Clayton,G.C.,&Mathis,J.S.1989,ApJ,345,245Edgar,R.J.,&Chevalier,R.A.1986,ApJ,310,L27Ferguson,H.C.,Dixon,W.V.,Davidsen,A.F.,&Dettmar,R.–J.1995,ApJ,454,L23 Howk,J.C.,&Savage,B.D.2000,AJ,119,644Hummel,E.,Beck,R.,&Dahlem,M.1991,A&A,248,23Hummel,E.,&Dettmar,R.-J.1990,A&A,236,33Martin,C.,&Kern,B.2001,ApJ,555,258Otte,B.,Dixon,W.V.,&Sankrit,R.2003,ApJL,586,in pressOtte,B.,Reynolds,R.J.,Gallagher,J.S.,&Ferguson,A.M.N.2001,ApJ,560,207 Rand,R.J.1994,A&A,285,833Rand,R.J.,Kulkarni,S.R.,&Hester,J.J.1992,ApJ,396,97Rupen,M.P.,van Gorkom,J.H.,Knapp,G.R.,Gunn,J.E.,&Schneider,D.P.1987,AJ, 94,61Sahnow,D.J.,et al.2000,ApJ,538,L7Sancisi,R.,&Allen,R.J.1979,A&A,74,73Savage,B.D.,et al.2000,ApJ,538,L27Savage,B.D.,et al.2003,ApJS,in pressSchlegel,D.J.,Finkbeiner,D.P.,&Davis,M.1998,ApJ,500,525Shapiro,P.R.,&Field,G.B.1976,ApJ,205,762Spitzer,L.1956,ApJ,124,20Sutherland,R.S.,&Dopita,M.A.1993,ApJS,88,253Wang,Q.D.,Immler,S.,Walterbos,R.,Lauroesch,J.T.,&Breitschwerdt,D.2001,ApJ, 555,L99Wang,Q.D.,Walterbos,R.A.M.,Steakley,M.F.,Norman,C.A.,&Braun,R.1995,ApJ, 439,176Welsh,B.Y.,Sallmen,S.,Sfeir,D.,Shelton,R.L.,&Lallement,R.2002,A&A,394,691Table1.OBSERVATIONSPosition Program Right Ascension Declination Exposure Time Events in LiF1A ID(J2000h m s)(J2000◦′′′)(s)1032˚A lineTable2.SPECIFIC O VIλ1032INTENSITIESPosition z I1032NGC4631–A 4.84600±10009±2NGC4631–B 2.58000±130015±3NGC891–1a 2.4<2000<3.8NGC891–2a 1.4<2200<4.2NGC891–3a 4.1<2800<5.5Fig. 1.—X-ray contours superimposed on an Hαimage of NGC4631(Wang et al.1995). Both FUSE positions are marked in the top panel,which shows the SXR bubble observed in the ROSAT0.15–0.3keV band.The bottom panel shows the hard X-ray contours of the 0.5–2.0keV band overlaid over the same Hαimage as in the top panel.Fig.2.—FUSE day-plus-night spectra of NGC4631at Positions A(top)and B(bottom). The spectra were binned by16pixels for display only.The O VIλλ1032,1038doublet is marked with arrows;airglow lines are marked with the Earth symbol.The dashed lines show the extraction window over which the O VIλ1032photons were counted.The continuum ﬁt for this emission line is plotted as a horizontal line.Its extend outside the dashed linesshows the region of the spectrum used to determine the continuum.Fig. 3.—BV image of NGC891(Howk&Savage2000).The three FUSE positions are marked.Fig. 4.—Same as Fig.2,but for NGC891at Positions1–3(from top to bottom).The positions of the arrows and extraction windows are based on the rotation curve by Sancisi&Allen(1979).。

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