Stellar populations in the nuclear regions of nearby radiogalaxies

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2024届高三下学期英语培优外刊阅读学案年度人物话题

高三英语培优外刊阅读班级：____________学号：____________姓名：____________外刊精选｜《时代》2023年度人物：泰勒·斯威夫特近日，美国流行音乐歌手泰勒·斯威夫特当选为《时代》杂志2023年度人物，成为近百年以来第一位单独拥有这一称号的艺人，也是该杂志第一位两次获评年度人物的女性。

2023年，泰勒·斯威夫特的时代（Eras）巡回演唱会在全球掀起追捧热潮，其个人净资产也高达11亿美元。

这位歌手到底有着怎样的魅力？她曾经历过怎样的人生低谷，又如何脱胎换骨，成为更强大的自己？Taylor Swift named Time Magazine's person of the yearBy Mark SavageTaylor Swift has capped off a stellar 2023 by being named Time Magazine's person of the year. The award goes to an event or person deemed to have had the most influence on global events over the past year.Already a superstar before 2023, her career has reached new heights thanks to the Eras tour -which sees the singer perform a career-spanning 45-song set every night. In Seattle, her concerts generated seismic activity equivalent to a 2.3 magnitude earthquake."It feels like the breakthrough moment of my career, happening at 33," she told Time. "And for the first time in my life, I was mentally tough enough to take what comes with that."Swift's imperial phase comes after a period where she was vilified for her positions on feminism - although her silence stemmed from nothing more sinister than a lack of confidence. After speaking out against Donald Trump and in favour of abortion rights, she hit a creative purple patch with the pandemic-era albums Folklore and Evermore.Time editor-in-chief Sam Jacobs said the US pop icon was "the rare person who is both the writer and hero of her own story", adding that Swift had "found a way to transcend borders and be a source of light".【词汇过关】请写出下面文单词在文章中的中文意思。

The Mass Assembly History for Galaxies with MaNGA

The Mass Assembly History for Galaxies with MaNGA Xue Ge;Hong-Tao Wang;Cheng-Long Lei;Yun-Jun Guo;Yi-Long Jiang;Xiao-Xiao Cao【期刊名称】《Research in Astronomy and Astrophysics》【年(卷),期】2024(24)3【摘要】How galaxies assemble masses through their own star formation or interaction with the external environment is still an important topic in the field of galaxy formation and evolution.We use Value Added Catalogs with galaxy features that are spatially and temporally resolved from Sloan Digital Sky Survey Data Release 17 to investigate the mass growth histories of early-type galaxies(ETGs)and late-type galaxies(LTGs).We find that the mass growth of ETGs is earlier than that of LTGs for massivegalaxies(M_(*)>10^(10)M_⊙),while low-mass(M_(*)≤10^(10)M_⊙)ETGs have statistically similar mass assembly histories as low-mass LTGs.The stellar metallicity of all massive galaxies shows a negative gradient and basically does not change with time.However,in low-mass galaxies,the stellar metallicity gradient of elliptical galaxies is negative,and the stellar metallicity gradient of lenticular and spiral galaxies evolves from positive to negative.ETGs are not all in a high-density environment,but exhibit mass dependence.As the tidal strength increases,the star formation rate of low-mass ETGs rapidly decreases.These results support a picture where massive galaxies exhibit inside-out quenching mode,while low-mass galaxies showoutside-in quenching mode.Environmental effects play an important role in regulating the mass assembly histories of low-mass ETGs.【总页数】12页(P58-69)【作者】Xue Ge;Hong-Tao Wang;Cheng-Long Lei;Yun-Jun Guo;Yi-Long Jiang;Xiao-Xiao Cao【作者单位】School of Physics and Information Engineering Second Normal University;Jiangsu Province Engineering Research Center of Basic Education Big Data Application Second Normal University;Key Laboratory of Modern Astronomy and Astrophysics University of Education;School of Science Normal University【正文语种】中文【中图分类】P15【相关文献】1.A thick-disk galaxy model and simulations of equal-mass galaxy pair collisions2.Radial stellar populations of AGN-host dwarf galaxies in SDSS-Ⅳ MaNGA survey3.Spatially resolved properties of supernova host galaxies in SDSS-IV MaNGA4.Velocity Dispersionσ_(aper)Aperture Corrections as a Function of Galaxy Properties from Integral-field Stellar Kinematics of 10,000 MaNGA Galaxies5.Measuring stellar populations,dust attenuation and ionized gas at kpc scales in 10010 nearby galaxies using the integral field spectroscopy from MaNGA因版权原因，仅展示原文概要，查看原文内容请购买。

TheMilkyWay“MilkyWay”-SFSUPhysics…

The Milky WayMilky Way probably looks likeAndromeda.The band of light we see is really 100 billion starsMilky WayBefore the 1920’s, astronomers used a “__________model” for the galaxyTried to estimate our location in the galaxy by counting stars in different __________Because some stars are _______ by dust, the true shape of this group of stars was unclear.A Globular ClusterFinding the Centerthe Solar System.The Milky WayParts of OurGalaxyDisk: The ____ Resides in theNuclear Bulge: The dense_______ regionHalo: Spherical regionsurrounding the disk where the_______ ________ live.Questions:How big is the Milky Way?Where are stars forming (or not forming)?How much mass is in the Milky Way?What’s going on at the center?so stars are still forming Car Headlights are standard candles:We use them to determine the car’s distanceHenrietta Leavitt Cepheid stars change in brightness. They pulsate in a very regular way. Large, bright Cepheids pulsate_____, while small, dim Cepheids pulsate _______.Milky Way Galaxy, we map out its structureA modern map of the Milky WayMeasuring the Mass of the Milky WayWe use the Sun’s ______around the center of the MilkyWayThe greater the mass insidethe orbit, the ______ the Sunhas move around the center.This way we can measure themass of the Milky Way.Total mass: about ___ _______ MThe Center of the Milky Wayat the center of the galaxy!Chapter 13Galaxies____)M 100NGC 300Less gas and dustAre generally ______ than spirals and ellipticals_______ Galaxies (E): Classified according to shape (E0-E9)_______ GalaxiesA Barred Spiral Galaxy with only 2 arms.Candles••Supernova in galaxy NGC4526 (HST Image)Hubble’s Original DataHubble Law/ Hd = vrClassifying Galaxies Lecture Tutorial: Page 127•Work with a partner or two•Read directions and answer all questions carefully. Take time to understand it now!•Discuss each question and come to a consensus answer you all agree on before moving on to the next question.•If you get stuck, ask another group for help.•If you get really stuck, raise your hand and I will come around.。

高考英语考前突破阅读理解能力科普类人福岛核电站地下水受到放射污染素材

福岛核电站地下水受到放射污染High levels of a toxic radioactive isotope have been found in groundwater at Japan's Fukushima nu clear plant, its operator says.福岛核电站附近的地下水中发现高剂量有毒的放射性同位素。

Tokyo Electric Power Company (Tepco) said tests showed strontium-90 was present at 30 times the legal rate.The radioactive isotope tritium has also been detected at elevated levels.The plant, crippled by the 2011 earthquake and tsunami, has recent ly see n a series of water leaks and power failures.The tsunami knocked out cooling systems to the reactors, which melted down.Water is now being pumped in to the re actors to cool them but this has left Tepco with the problem of how to safely store the cont aminated water.There have been several reports of leaks from storage tanks or pipes.Sea samplesStrontium-90 is for med as a by-product of nuclear fission. Tests showed that levels of stront ium in grou ndwater at the Fukushima plant had increased 100-fold since the end of last year, Toshihiko Fukuda, a Tepco official, told media.Mr Fukuda said Tepco believed the elevated levels originated from a lea k of contaminated water in April 2011 from one of the reactors."As it's near where the leak from reactor number two happened and taking into account the situation at the time, we believe that water left over from that time is the highest possibility," he said.Tritium, used in glow-in-the-dark watc hes, was found at eight times the allowable level.Mr Fukuda said that samples from the sea showed no rise in either substance and the company believed the groundwater was being contained by concrete foun dations."When we look at the impact that is having on the ocean, the levels seem to be within past trends and so we don't believe it's having an effect."But the discovery is another set-back for Tepco's plan to pump groundwater from the plant into the sea, correspondents say.Nuclear chemist M ichiaki Furukawa told Reuters news agency that Tepco should not release contaminated water into the ocean."They have to keep it somewhere so that it can't escape outside the plant," he said. "Tepco needs to carry out more regular testing in specific areas and disclose everything they find."The Fukushima power plant has faced a series of problems this ye ar. Early this month, radioactive water was found leaking from a storage tank.The plant also suffered three power failures in five weeks earlier this year. A leak of radioactive water from one of the plant's underground storage pools was also detected in April.。

中考有关核污染的英语作文题目

中考有关核污染的英语作文题目全文共3篇示例，供读者参考篇1Nuclear Pollution - A Looming Threat to Our PlanetAs a high school student, the issue of nuclear pollution is one that deeply concerns me and my generation. We are the inheritors of this planet, and the actions we take today will shape the world we live in tomorrow. Nuclear pollution, a silent and insidious threat, has the potential to cause catastrophic damage to our environment, our health, and our very existence.At the core of this issue lies the nuclear power industry and the management of radioactive waste. While nuclear power has been touted as a clean and efficient source of energy, the reality is far more complex and fraught with risks. The process of generating nuclear power produces a vast amount of highly radioactive waste, which must be stored and managed with the utmost care and precision.One of the most significant challenges we face is thelong-term storage of this radioactive waste. Many countries around the world have opted for temporary storage solutions,such as above-ground facilities or deep geological repositories. However, these solutions are far from perfect. Leaks, accidents, and natural disasters can all lead to the release of radioactive materials into the environment, with devastating consequences.The effects of nuclear pollution on human health arewell-documented and deeply troubling. Exposure to ionizing radiation can cause a wide range of health issues, including cancer, genetic mutations, and birth defects. Even low-level exposure over an extended period can increase the risk of developing these conditions. This is a particular concern for communities living near nuclear facilities or in areas affected by nuclear accidents, such as the infamous disasters at Chernobyl and Fukushima.But the impact of nuclear pollution extends far beyond human health. The release of radioactive materials into the environment can have far-reaching and long-lasting effects on entire ecosystems. Radioactive contamination can accumulate in the soil, water, and air, affecting plant and animal life in ways that we are still trying to fully understand. The consequences of this contamination can ripple through food chains, disrupting the delicate balance of nature and threatening entire species with extinction.As a student, I am deeply troubled by the prospect of inheriting a world marred by the scars of nuclear pollution. It is a burden that my generation did not create, but one that we may be forced to bear. We must ask ourselves difficult questions about the true costs of nuclear power and whether the risks outweigh the benefits.Equally concerning is the potential for nuclear materials to fall into the wrong hands. The specter of nuclear terrorism looms large, and the consequences of such an attack are almost too horrific to contemplate. We must remain vigilant and take every possible measure to secure nuclear facilities and prevent the proliferation of these deadly materials.However, it would be remiss of me not to acknowledge the complexities of this issue. Nuclear power, for all its risks, has played a role in meeting our ever-increasing energy demands and reducing our reliance on fossil fuels. The challenge lies in striking a balance between meeting our energy needs and ensuring the safety and well-being of our planet and its inhabitants.As we grapple with these challenges, it is clear that a multifaceted approach is required. We must invest in research and development of safer and more sustainable energy sources,such as renewable energy technologies. At the same time, we must work tirelessly to improve the management and storage of existing nuclear waste, employing the best available science and technology to minimize the risks of contamination.Education and public awareness are also crucial components of this effort. It is essential that we, as students and as members of society, understand the risks and implications of nuclear pollution. We must engage in open and honest dialogue, free from political or corporate influence, to make informed decisions about the role of nuclear power in our future.Ultimately, the issue of nuclear pollution is one that transcends borders and generations. It is a global challenge that demands a global response. We must work together, as nations and as individuals, to find solutions that protect our planet and safeguard the future for generations to come.As a student, I am filled with a sense of urgency and a determination to be part of the solution. We cannot afford to be complacent or to ignore the warning signs. The time to act is now, before the consequences of nuclear pollution become irreversible.I implore my fellow students, educators, policymakers, and leaders to join me in this fight. Let us be the generation thattakes a stand against nuclear pollution, that works tirelessly to protect our planet and our future. Together, we can create a world where the promise of a clean and sustainable future is not just a dream, but a reality.篇2Nuclear Pollution: A Threat to Our PlanetAs a student deeply concerned about the welfare of our planet, I can't help but feel a sense of urgency when it comes to the issue of nuclear pollution. The effects of radioactive contamination pose a grave threat not only to the environment but also to the health and well-being of all living beings, including humans. It's a complex and multifaceted problem that demands our immediate attention and action.To begin with, it's crucial to understand the sources of nuclear pollution. One of the primary culprits is the nuclear power industry. While nuclear energy has been touted as a clean alternative to fossil fuels, the process of generating it carries significant risks. Nuclear power plants, despite stringent safety measures, can experience accidents or malfunctions that lead to the release of radioactive materials into the environment. Thecatastrophic events at Chernobyl in 1986 and Fukushima in 2011 serve as grim reminders of the devastation that can ensue.Another major contributor to nuclear pollution is the production and testing of nuclear weapons. The detonation of these weapons, even for experimental purposes, releases vast amounts of radioactive particles into the atmosphere, soil, and water sources. The long-lasting effects of radiation can persist for decades, posing a continuous threat to the surrounding ecosystems and human populations.The consequences of nuclear pollution are far-reaching and alarming. Radioactive contamination can wreak havoc on the delicate balance of ecosystems, causing genetic mutations, birth defects, and even the extinction of certain species. Plants and animals exposed to high levels of radiation can suffer from various health issues, including cancer, immunodeficiency, and reproductive problems.s are not immune to the dangers of nuclear pollution either. Exposure to radioactive materials can lead to a host of severe health implications, such as an increased risk of cancer, genetic abnormalities, and organ damage. Children and fetuses are particularly vulnerable, as radiation can interfere with their developmental processes and potentially cause long-term health problems.Furthermore, nuclear pollution has a lasting impact on the environment itself. Radioactive particles can contaminate soil, water sources, and air, making entire regions uninhabitable for extended periods. The cleanup and decontamination efforts required in the aftermath of a nuclear accident are often lengthy, costly, and pose additional risks to the workers involved.As global citizens, it is our collective responsibility to address this critical issue head-on. We must demand stricter regulations and oversight for the nuclear industry, ensuring that safety protocols are rigorously enforced and constantly updated. Additionally, we should advocate for increased investment in alternative energy sources, such as solar, wind, and hydroelectric power, which offer cleaner and more sustainable solutions.On a personal level, we can also contribute to mitigating the effects of nuclear pollution. Educating ourselves and others about the risks and consequences is crucial. By raising awareness and encouraging open dialogue, we can foster a deeper understanding of the issue and inspire collective action.Moreover, supporting organizations and initiatives dedicated to environmental protection, nuclear disarmament, and the development of safer nuclear technologies can make a significant difference. Our voices, combined with those ofmillions of others, can influence policymakers and drive positive change.In conclusion, nuclear pollution is a grave threat that cannot be ignored. Its consequences extend far beyond our current generation, affecting the very future of our planet. As students, we have a responsibility to educate ourselves, raise awareness, and demand action from those in positions of power. By working together and embracing sustainable solutions, we can mitigate the risks of nuclear pollution and create a safer, cleaner world for ourselves and future generations.篇3Nuclear Pollution: A Looming Threat to Our PlanetAs a student deeply concerned about the well-being of our planet, I can't help but feel a sense of unease when I think about the issue of nuclear pollution. It's a topic that has been weighing heavily on my mind, and one that I believe deserves our utmost attention and action.Nuclear pollution is a silent, invisible threat that has the potential to wreak havoc on our environment and the health of all living beings. It's a byproduct of the nuclear industry, a consequence of our reliance on nuclear power and thedevelopment of nuclear weapons. And while the benefits of nuclear energy are undeniable, the risks associated with it are simply too great to ignore.What exactly is nuclear pollution, you might ask? It's the release of radioactive materials into the environment, either through accidents, leaks, or the improper disposal of nuclear waste. These radioactive materials can contaminate the air we breathe, the water we drink, and the soil that sustains our crops. And the effects of this contamination can be devastating.One of the most alarming aspects of nuclear pollution is its ability to cause genetic mutations and cancer in living organisms. Exposure to high levels of radiation can damage cells and DNA, leading to the development of various types of cancers, including leukemia, thyroid cancer, and lung cancer. Children and fetuses are particularly vulnerable, as their cells are rapidly dividing and growing.But the effects of nuclear pollution don't stop there. It can also have a profound impact on entire ecosystems, disrupting the delicate balance of nature and endangering countless species. Radioactive materials can accumulate in the food chain, causing birth defects, stunted growth, and even sterility inanimals. Entire populations of wildlife can be decimated, and once-thriving habitats can become barren wastelands.Perhaps one of the most haunting examples of the devastating consequences of nuclear pollution is the Chernobyl disaster of 1986. On April 26th of that year, a catastrophic explosion occurred at the Chernobyl Nuclear Power Plant in Ukraine, then part of the Soviet Union. The accident released massive amounts of radioactive materials into the environment, contaminating vast areas of Ukraine, Belarus, and even parts of Western Europe.The aftermath of the Chernobyl disaster was nothing short of horrific. Thousands of people were evacuated from the surrounding areas, and the long-term effects on human health and the environment were staggering. Cancer rates skyrocketed, and entire towns and villages were abandoned, left as ghostly reminders of the price we pay for our reliance on nuclear power.But Chernobyl is not an isolated incident. Nuclear accidents have occurred in various parts of the world, from Three Mile Island in the United States to Fukushima in Japan. Each time, the consequences have been severe, with radioactive contamination spreading across vast areas and affecting countless lives.It's important to note that nuclear pollution isn't just a threat during accidents or disasters. Even the routine operation of nuclear power plants can release small amounts of radioactive materials into the environment, and the storage and disposal of nuclear waste pose significant challenges. Improper handling of these materials can lead to leaks and contamination, putting entire communities at risk.So, what can we do to combat this looming threat? First and foremost, we need to acknowledge the severity of the issue and take proactive measures to reduce our reliance on nuclear power. While nuclear energy may seem like a clean and efficient alternative to fossil fuels, the risks associated with it are simply too great to ignore.Instead, we should focus our efforts on developing and investing in truly renewable and sustainable energy sources, such as solar, wind, and hydroelectric power. These technologies have the potential to meet our energy needs without the risk of nuclear pollution or the emission of greenhouse gases.Additionally, we need to prioritize the safe and responsible handling of existing nuclear waste. This means investing in advanced storage solutions and exploring innovative methodsfor decontaminating and disposing of radioactive materials in a way that minimizes their impact on the environment.But perhaps most importantly, we need to raise awareness about the dangers of nuclear pollution and educate ourselves and future generations about the importance of environmental stewardship. Only by understanding the gravity of this issue can we truly appreciate the urgency of taking action.As students, we have a unique opportunity to be agents of change. We can use our voices to advocate for policies that prioritize environmental protection and the development of sustainable energy sources. We can participate in community initiatives that promote awareness and education about the risks of nuclear pollution. And we can make conscious choices in our daily lives to reduce our environmental footprint and support companies and organizations that share our values.The path ahead may be challenging, but it is one that we must embrace with determination and resolve. Nuclear pollution is a global issue that transcends borders and affects us all. It's a threat that looms over our planet, casting a shadow on the future we hope to build for ourselves and generations to come.But if we act now, if we unite our efforts and channel our collective will, we can overcome this challenge. We can create aworld where nuclear pollution is a thing of the past, where our reliance on harmful and dangerous energy sources is replaced by clean, sustainable alternatives that respect and nurture the delicate balance of our planet.It won't be easy, but it's a fight worth fighting. For the sake of our health, our ecosystems, and the very future of our planet, we must confront the issue of nuclear pollution head-on. We must be the generation that says "enough is enough" and takes decisive action to protect the world we call home.So let us embrace this challenge with courage and conviction. Let us be the voices that echo through the halls of power, demanding change and accountability. And let us never forget that the future of our planet lies in our hands, and it is our responsibility to ensure that it remains a place where life can thrive for generations to come.。

银河系漫游指南英文版pdf

银河系漫游指南英文版pdfHere is the English essay with a word count of over 1000 words, as requested:The Milky Way Galactic OdysseyEmbark on a captivating journey through the vast expanse of the Milky Way Galaxy, a celestial wonder that has captivated the human imagination for millennia. As we delve into the mysteries and marvels of this galactic realm, prepare to be awestruck by the sheer scale and beauty of the cosmos that lies beyond our earthly confines.Let us begin our odyssey by venturing to the heart of the Milky Way, where the supermassive black hole known as Sagittarius A* resides. This gravitational behemoth, nearly 4 million times the mass of our Sun, anchors the center of our galaxy and exerts a powerful influence on the surrounding stars and stellar matter. As we approach this enigmatic cosmic phenomenon, we will witness the intricate dance of stars and gas clouds as they are drawn inexorably towards the event horizon, their fate forever sealed within the crushing grip of the black hole.Venturing outwards from the galactic center, we will encounter the diverse and vibrant neighborhoods that make up the Milky Way. Spiral arms, such as the Orion Arm in which our Solar System resides, are vast regions of star formation, with newborn stars and stellar nurseries dotting the landscape. We will marvel at the brilliant nebulae, glowing clouds of gas and dust that serve as the birthplaces of these young celestial bodies, their ethereal hues and intricate structures a testament to the dynamic processes that shape the galaxy.As we traverse the spiraling arms, we will come across the globular clusters – ancient, densely packed collections of stars that orbit the galactic center. These spherical assemblages, some of the oldest objects in the Milky Way, harbor valuable insights into the early history and evolution of our galaxy, their stars dating back to a time when the universe was a mere fraction of its current age.Amidst the stellar tapestry, we will discover the diverse array of stellar populations that call the Milky Way home. From the towering red giants, their brilliant crimson hues a testament to their advanced age and increased size, to the compact and enigmatic neutron stars, the collapsed remnants of once-mighty suns. Each type of star, with its unique properties and life cycle, contributes to the rich tapestry of the galactic landscape.But the Milky Way is not merely a collection of stars – it is a dynamic and ever-changing system, influenced by the complex interplay of gravity, stellar evolution, and the ever-present threat of cosmic catastrophes. We will explore the regions where massive stars meet their explosive demise, supernovae that briefly outshine entire galaxies and leave behind the dense, spinning neutron stars known as pulsars. These cataclysmic events not only shape the galactic environment but also provide the building blocks for new generations of stars and planets.As we venture deeper into the Milky Way, we will encounter the harrowing regions where the fabric of space-time is stretched and distorted by the intense gravitational fields of neutron stars and black holes. Here, we will witness the bizarre and mind-bending phenomena predicted by Einstein's theory of general relativity, from the warping of spacetime to the accretion disks that feed these cosmic monsters.Throughout our journey, we will be in awe of the sheer scale and majesty of the Milky Way. The galaxy, spanning nearly 100,000 light-years in diameter, is home to an estimated 200 to 400 billion stars, each one a unique and fascinating world unto itself. We will ponder the possibility of life elsewhere in this vast cosmic tapestry, wondering if intelligent civilizations have arisen on distant worlds and if they, too, gaze up at the night sky, marveling at the splendorof our shared galactic home.As our odyssey draws to a close, we will reflect on the profound impact that the study of the Milky Way has had on our understanding of the universe. From the groundbreaking work of pioneering astronomers to the cutting-edge research conducted with the most advanced observational tools, the Milky Way has been a constant source of fascination and discovery. And as we look to the future, we know that there are countless more secrets and mysteries waiting to be unveiled, beckoning us to continue our exploration of this awe-inspiring celestial realm.So let us embark on this Milky Way galactic odyssey, armed with a sense of wonder and a thirst for knowledge. For in unraveling the mysteries of our galactic home, we may just find the answers to some of the most profound questions that have puzzled humanity since the dawn of time.。

Stellar Populations in Ten Clump-Cluster Galaxies of the Ultra Deep Field

1.
Intபைடு நூலகம்oduction
A high fraction of young galaxies show an irregular clumpy structure from blue emission that is concentrated into clusters much larger and more massive than any found locally. Chain galaxies, for example (Cowie & Hu 1995; van den Bergh et al. 1996; Moustakas et al. 2004) are dominated by several giant blue clumps strung along a line; there are no exponential light proﬁles or central red bulges as in spiral galaxies today (Elmegreen, Elmegreen, & Sheets 2004, hereafter Paper I). Dalcanton & Schectman (1996) suggested that chains are edge-on low surface brightness galaxies. O’Neil et al. (2000) considered high-redshift disk galaxies in general, many of which have bulges and exponential proﬁles, and concluded from the distribution of axial ratios that chains are edge-on disks. Reshetnikov, Dettmar, & Combes (2003) showed that the axial ratios of 34 edge-on galaxies in the Hubble Deep Field are larger than for local galaxies, implying thicker disks, and noted that half of their sample, including some face-on galaxies, have non-exponential proﬁles like the chains. Edge-on local galaxies resemble chain galaxies too when seen in projection at ultraviolet wavelengths, although the local star forming regions are much smaller than in chain galaxies (Smith et al. 2001). Elmegreen, Elmegreen & Hirst (2004a, hereafter Paper II) studied a large sample of chain and non-exponential clumpy galaxies in a deep Advanced Camera for Surveys image and conﬁrmed that they are members of a new class of young galaxies with thick, non-exponential disks dominated by ∼ 109 M⊙ blue clusters. They have no regular bulge structures, like other disk galaxies at the same redshifts (Paper I), but sometimes have irregular or oﬀset bars with a bulge-like size (Elmegreen, Elmegreen, & Hirst 2004b, hereafter Paper III). An objective name for this type was suggested to be a “clump cluster,” because without kinematic data it was not known whether each is a conglomeration of pre-existing small galaxies, like a tiny Stephan’s Quintet, or a single coherent galaxy. A lack of clear rotation in some linear

我心目中的英雄医生英语范文

我心目中的英雄医生英语范文英文回答：The Physician Who Inspires Me.In the realm of healthcare, there are countless individuals who dedicate their lives to alleviating suffering and promoting well-being. Among them, one physician stands out as a beacon of hope and inspiration in my eyes. Her unwavering compassion, exceptional medical expertise, and unwavering dedication to serving the underserved have left an enduring mark on my life and the lives of countless others.Dr. Emily Carter, a renowned cardiologist, has spent her career tirelessly advocating for equitable access to healthcare for all. Raised in a working-class family, she witnessed firsthand the disparities in medical care faced by marginalized communities. This experience ignited within her a deep-seated determination to break down thesebarriers and ensure that everyone has the opportunity to live a healthy life.Dr. Carter's medical journey began with a stellar academic career. She graduated magna cum laude from Harvard Medical School and went on to complete her residency in cardiology at Johns Hopkins Hospital. Throughout her training, she immersed herself in research, publishing groundbreaking papers on the diagnosis and treatment of cardiovascular disease. Her passion for scientific inquiry has led to advancements in the field that have improved the outcomes of countless patients.Beyond her clinical practice, Dr. Carter is an ardent advocate for healthcare reform. She has testified before Congress, written op-eds in major publications, and worked tirelessly to raise awareness about the importance of universal healthcare. She firmly believes that healthcareis a fundamental human right and that no one should be denied access to quality medical care due to financial or social circumstances.As an educator, Dr. Carter is equally passionate about nurturing the next generation of physicians. She holds a professorship at the University of California, San Francisco, where she mentors medical students and residents, instilling in them the values of empathy, inclusivity, anda commitment to social justice. Her dedication to teaching extends beyond the classroom, as she frequently volunteers her time to provide medical care to underserved populations in both domestic and international settings.What truly sets Dr. Carter apart is her unwavering empathy for her patients. She treats each individual with dignity and respect, regardless of their background orability to pay. Her compassionate bedside manner andability to connect with patients on a personal level create a healing environment that fosters trust and understanding.The impact of Dr. Carter's work has been profound. She has not only saved countless lives but has also transformed the healthcare landscape for the better. Her advocacy for equitable access to healthcare has inspired others to join the fight for social justice, and her mentorship has shapedthe careers of countless future physicians.Dr. Emily Carter is a true hero in my eyes. Her unwavering compassion, exceptional medical expertise, and unwavering dedication to serving the underserved are an inspiration to me and to countless others. Her work has made a tangible difference in the lives of her patients,and her legacy will continue to inspire generations to come.中文回答：我心目中的英雄医生。

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a r X i v :a s t r o -p h /0103059v 1 4 M a r 2001Mon.Not.R.Astron.Soc.000,000–000(0000)Printed 1February 2008(MN L A T E X style ﬁle v1.4)Stellar populations in the nuclear regions of nearbyradiogalaxiesItziar Aretxaga 1,Elena Terlevich 1⋆,Roberto J.Terlevich 2†,Garret Cotter 3,´Angeles I.D´ıaz 41Instituto Nacional de Astrof´ısica,´Optica y Electr´o nica,Apdo.Postal 25y 216,72000Puebla,Pue.,Mexico 2Institute of Astronomy,Madingley Road,Cambridge CB30HA,U.K.3Cavendish Laboratory,Univ.of Cambridge,Madingley Road,Cambridge CB30HE,U.K.4Dept.F´ısica Te´o rica C-XI,Univ.Aut´o moma de Madrid,Cantoblanco,Madrid,Spain.1February 2008ABSTRACTWe present optical spectra of the nuclei of seven luminous (P 178MHz >∼1025W Hz −1Sr −1)nearby (z <0.08)radiogalaxies,which mostly correspond to the FR II class.In two cases,Hydra A and 3C 285,the Balmer and λ4000˚A break indices constrain the spectral types and luminosity classes of the stars involved,re-vealing that the blue spectra are dominated by blue supergiant and/or giant stars.The ages derived for the last burst of star formation in Hydra A are between 7and 40Myr,and in 3C 285about 10Myr.The rest of the narrow-line radiogalaxies (four)have λ4000˚A break and metallic indices consistent with those of elliptical galaxies.The only broad-line radiogalaxy in our sample,3C 382,has a strong featureless blue continuum and broad emission lines that dilute the underlying blue stellar spectra.We are able to detect the Ca II triplet in absorption in the seven objects,with good quality data for only four of them.The strengths of the absorptions are similar to those found in normal elliptical galaxies,but these values are both consistent with single stellar populations of ages as derived from the Balmer absorption and break strengths,and,also,with mixed young+old populations.Key words:galaxies:active –galaxies:starbursts –galaxies:stellar content1.INTRODUCTIONIn recent years new evidence that star formation plays an important role in Active Galactic Nuclei (AGN)has been gathered:•The presence of strong Ca II λλ8494,8542,8662˚A triplet (CaT)absorptions in a large sample of Seyfert 2nuclei has provided direct evidence for a population of red supergiant stars that dominates the near-IR light (Terlevich,D´ıaz &Terlevich 1990).The values found in Seyfert 1nuclei are also consistent with this idea if the dilution produced by a nuclear non-stellar source is taken into account (Terlevich,D´ıaz &Terlevich 1990,Jim´e nez-Benito et al.2000).The high mass-to-light ratios L (1.6µm)/M inferred in Seyfert 2nuclei also indicate that red supergiants dominate the nuclear light⋆Visiting Fellow at IoA,UK†Visiting Professor at INAOE,Mexico (Oliva et al.1995),but a similar conclusion does not holdfor Seyfert 1nuclei.•The absence of broad emission lines in the direct optical spectra of Seyfert 2nuclei which show broad lines in polar-ized light can be understood only if there is an additional central source of continuum,most probably blue stars (Cid Fernandes &Terlevich 1995,Heckman et al.1995).This con-clusion is further supported by the detection of polarization levels which are lower in the continuum than in the broad lines (Miller &Goodrich 1990,Tran,Miller &Kay 1992).•Hubble Space Telescope imaging of the Seyfert Mrk 447reveals that the central UV light arises in a resolved region of a few hundred pc,in which prominent CaT absorption and broad He II λ4686˚A emission lines reveal the red super-giant and Wolf Rayet stars of a powerful starburst.The stars dominate the UV to near-IR light directly received from the nucleus (Heckman et al.1997).At least 50per cent of the light emitted by the nucleus is stellar,as a conservative es-timate.Mrk 447is not a rare case:a large sample of nearby bright Seyfert 2s and LINERs show similar resolved star-c0000RAS2I.Aretxaga,E.Terlevich,R.Terlevich,G.Cotter,A.I.D´ıazburst nuclei of80to a few hundred pc in size(Colina et al. 1997,Gonz´a lez-Delgado et al.1998,Maoz et al.1995,1998), with some of the Seyfert2containing dominant Wolf-Rayet populations(Kunth&Contini1999,Cid Fernandes et al. 1999).A starburst–AGN connection has been proposed in at least three scenarios:starbursts giving birth to massive black holes(e.g.Scoville&Norman1988);black holes being fed by surrounding stellar clusters(e.g.Perry&Dyson1985,Peter-son1992);and also pure starbursts without black holes(e.g. Terlevich&Melnick1985,Terlevich et al.1992).The evi-dence for starbursts in Seyfert nuclei strongly supports some kind of connection.However,it is still to be demonstrated that starbursts play a key role in all kinds of AGN.One of the most stringent tests to assess if all AGN have associated enhanced nuclear star formation is the case of lobe-dominated radio-sources,whose host galaxies have relatively red colours when compared to other AGN vari-eties.In this paper we address the stellar content associated with the active nuclei of a sample of FR II radiogalaxies, the most luminous class of radiogalaxies(Fanaroﬀ&Ri-ley1974)which possess the most powerful central engines and radio-jets(Rawlings&Saunders1991).The presence of extended collimated radio-jets,which fuel the extended radio structure over>∼108yr,strongly suggests the exis-tence of a supermassive accreting black hole in the nuclei of these radiogalaxies.This test addresses the question of whether AGN that involve conspicuous black holes and ac-cretion processes also contain enhanced star formation.In section2we introduce the sample and detail the data acquisition and reduction processes.In section3we provide continuum and line measurements of the most prominent features of the optical spectra of the radiogalaxies.In sec-tion4we discuss the main stellar populations responsible for the absorption and continuum spectra.In section5we oﬀer notes on individual objects.A sumary of the main con-clussions from this work is presented in section6.2.DATA ACQUISITION AND REDUCTION Our sample of radiogalaxies was extracted from the3CRR catalogue(Laing,Riley and Longair1983)with the only selection criteria being edge-brightened morphology,which deﬁnes the FR II class of radiogalaxies(Fanaroﬀ&Ri-ley1974),and redshift z<0.08.This last condition was imposed in order to be able to observe the redshifted CaT at wavelengths shorter thanλ9300˚A,where the atmo-spheric bands are prominent.Six out of a complete sam-ple of ten FR II radiogalaxies that fulﬁll these require-ments were randomly chosen.In addition to this sub-sample of FR IIs,we observed the unusually luminous FR I ra-diogalaxy Hydra A(3C218).This has a radio luminosity of P178MHz=2.2×1026W Hz−1Sr−1,which is an order of magnitude above the typical FR I/FR II dividing luminos-ity.Spectroscopic observations of a total of seven radio-galaxies,one normal elliptical galaxy to serve as reference andﬁve K III stars to serve as velocity calibrators were per-formed using the double-arm spectrograph ISIS mounted in the Cassegrain Focus of the4.2m William Herschel Tele-scope‡in La Palma during two observing runs,in1997 November7–8and1998February19–20.Theﬁrst run was photometric but the second was not,being partially cloudy on the20th.The seeing,as measured from the spatial di-mension of spectrophotometric stars,was between0.7and0.8arcsec throughout the nights.A slit width of1.2arcsec centered on the nucleus of galaxies and stars was used.We oriented the slit along the radio-axis for all radiogalaxies,except for Hydra A,for which the orientation was perpendicular to the radio-axis.An R300B grating centered atλ4500˚A with a2148x4200 pixel EEV CCD and an R316R grating centered atλ8800˚A with a1024x1024pixel TEK CCD were used in the1998 run.The projected area on these chips is0.2arcsec/pixel and0.36arcsec/pixel respectively.This conﬁguration pro-vides the spectral resolution necessary to resolve the Mg b and CaT features and,at the same time,oﬀers a wide spec-tral span:λ3350˚A—λ6000˚A at5.1˚A resolution in the blue andλ7900˚A—λ9400˚A at3.5˚A resolution in the red.In the 1997run,in which we assessed the viability of the project, we used the R600B and R600R gratings instead.This setup covers theλ3810˚A—λ5420˚A andλ8510˚A—λ9320˚A range in the blue and red arm,at2.6and1.7˚A resolution respectively. Just one radiogalaxy(DA240)was observed with this alter-native setup.The dichroics5700and6100were used in1997 November and1998February,and in both runs we used a ﬁlter to avoid second order contamination in the spectra.We obtainedﬂux standards(HZ44and G191-B2B)for the four nights and gratings,except in1998Feb20,when we were unable to acquire the red spectrum of the correspond-ing standard due to a technical failure.One calibration lamp CuAr+CuNe exposure per spectral region and telescope po-sition was also obtained for all objects.The total integration times for the radiogalaxies(from1 to3hr)were split into time intervals of about1200or1800s in order to diminish the eﬀect of cosmic rays on individual frames and allow to take lampﬂat-ﬁelds with the red arm of the spectrograph between science exposures.The TEK CCD has a variable fringing pattern at the wavelengths of interest, such that the variations are correlated with the position at which the telescope is pointing.Sinceﬂat-ﬁelding is crucial for the reddest wavelengths,where the sky lines are most prominent,after every exposure of20to30min we acquired aﬂat-ﬁeld in the same position of the telescope as the one for which the galaxies were being observed.We followed this procedure with all galaxies except with DA240.The same procedure was also used in the case of the elliptical galaxy, splitting its total integration time in two.Table1summarizes the journal of observations,where column1gives the name of the object;column2the radio-power at178MHz;column3the redshift;column4the integrated V magnitude of the galaxy;column5identiﬁes whether the object is a radiogalaxy(RG),a normal elliptical (E)or a star(S);column6gives the date of the beginning of the night in which the observations were carried out;col-umn7the position angle(PA)of the slit;column8the total ‡The William Herschel Telescope is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrof´ısica de Canariasc 0000RAS,MNRAS000,000–000Stellar populations in the nuclear regions of nearby radiogalaxies3exposure time;column9the grating used;and column10 the corresponding linear size to1arcsec at the redshift of the galaxies(for H0=50km s−1Mpc−1).The data for the radiogalaxies were extracted from the3C Atlas(Leahy,Bri-dle&Strom,/atlas/)and for the host galaxy of Hydra A from the3CR Catalogue(de Vaucouleurs et al.1991).The data were reduced using the IRAF software pack-age.The frames wereﬁrst bias subtracted and thenﬂat-ﬁeld corrected.In the case of the red arm spectra,the diﬀerent ﬂats obtained for a single object were combined when no sig-niﬁcant diﬀerences were detected between them.However,in several cases the fringing pattern shifted positions that ac-counted for diﬀerences of up to20per cent.In these cases we corrected each science frame with theﬂat-ﬁeld acquired immediately before and/or afterwards.Close inspection of the faintest levels of theﬂat-ﬁelded frames showed that the fringing had been successfully eliminated.Wavelength and ﬂux calibration were then performed,and the sky was sub-tracted by using the outermost parts of the slit.The atmospheric bands,mainly water absorption at λ8920˚A—λ9400˚A,aﬀect the redshifted CaT region of sev-eral radiogalaxies.The bands have been extracted using a template constructed from the stellar spectra obtained each night.The template was built averaging the normalizedﬂux of spectrophotometric and velocity standard stars,once the stellar absorption lines had been removed.The atmospheric bands were eliminated from the spectra of the galaxies di-viding by theﬂux-scaled template.This reduces the S/N of the region under consideration,especially since the bands are variable in time and one of our observing nights was par-tially cloudy.However,the technique allows the detection of the stellar atmospheric features.The CaT of the elliptical galaxy is not aﬀected by atmospheric absorption.Figure1shows the line spectrum of the sky and,as an example,the atmospheric absorption template of1998 Feb19.Water-band correction proved to be critical for the detection of the CaT lines when the atmospheric conditions were most adverse.Figure2shows extractions of the nuclear2arcsec of the spectra of the galaxies.This corresponds to844to2020pc for the radiogalaxies,and98pc for the normal elliptical galaxy.3.LINE AND CONTINUUMMEASUREMENTS3.1.CaT indexThe CaT was detected in all of the objects,although in three cases(3C285,3C382and4C73.08)it was totally or par-tially aﬀected by residuals left by the atmospheric band cor-rections and the measurement of its strength was thus pre-cluded.For the remaining cases,the strength was measured in the rest-frame of the galaxies against a pseudo-continuum, following the deﬁnition of the CaT index of D´ıaz,Ter-levich&Terlevich(1989).In Hydra A,3C285and3C382, the red continuum band is seriously aﬀected by residuals left from the atmospheric absorption removal.We deﬁned two alternative continuum bands,8575˚A<λ<8585˚A and 8730˚A<λ<8740˚A,that substitute the red-most contin-uum window of the CaT index.We checked this new deﬁni-tion against the original one in the elliptical galaxy,which doesn’t have residuals in its continuum bands,and the agree-ment between the two systems was good within5per cent.Velocity dispersions were measured by cross-correlating the galaxy spectra with the stellar spectra obtained with the same setup.The errors in the velocity dispersions calculated in this way were less than10per cent.A high velocity dispersion tends to decrease the mea-sured values of indices based on EW measurements.The CaT index has to be corrected from broadening of the ab-sorption lines by the corresponding velocity dispersion.In order to calculate the correction we convolved stellar pro-ﬁles with gaussian functions of increasing width,and mea-sured the CaT index in them.A good description of the cor-rection found for our data is given by the functional form ∆EW(˚A)=(σ(km s−1)−100)/200.The corrections were applied to the values measured in the galaxies,and con-verted into unbroadened indices.The values of velocity dispersions(σ),uncorrected EW (CaT u)and corrected EW(CaT),are listed in Table2. 3.2.λ4000˚A and Balmer Break indicesStellar populations can be dated through the measurement of theλ4000˚A or Balmer breaks.In intermediate to old pop-ulations the discontinuity atλ4000˚A results from a combi-nation of the accumulation of the Balmer lines towards the limit of the Balmer absorption continuum atλ3646˚A(the Balmer break)and the increase in stellar opacity caused by metal lines shortwards ofλ4000˚A.Table3lists the values of theλ4000˚A break index,∆4000˚A,measured in the spectra of the6narrow-line ra-diogalaxies and the elliptical galaxy in our sample.This ex-cludes3C382,which has a spectrum dominated by a strong blue continuum and broad-emission lines,and shows very weak stellar atmospheric features and no break.We adopted the deﬁnition given by Hamilton(1985),which quantiﬁes the ratio of the averageﬂux-level of two broad bands,one cov-ering the break(3750-3950)and one bluewards of the break (4050-4250).Both bands contain strong metallic and Balmer absorption lines in the case of normal galaxies.In active galaxies,the measurement can be contaminated by emission of[Ne III]λ3869˚A,which in our case is weak.The contami-nation by high-order Balmer lines in emission is negligible. The net eﬀect of emission contamination is to decrease the Balmer break index.In the radio-galaxies,we have estimated this eﬀect by interpolating the continuum levels below the [Ne III]emission,and we estimate that the ratio can be af-fected by6per cent at worst,in the case of3C192,and by less than3per cent for the rest of the objects.Table3lists emission-devoid indices.Hydra A and3C285have spectra which are much bluer than those of normal elliptical galaxies.In order to quantify better the strength of the break and the ages of the popula-tions derived,we have performed a bulge subtraction using as template the spectrum of NGC4374,scaled to eliminate the G-band absorption of the radiogalaxies.Since the ve-locity dispersion of the stars in NGC4374and in the ra-diogalaxies are comparable inside the spectral resolution of our data,no further corrections were needed.The G-band absorption is prominent in stars of spectral types later thanc 0000RAS,MNRAS000,000–0004I.Aretxaga,E.Terlevich,R.Terlevich,G.Cotter,A.I.D´ıazF5and it is especially strong in types K.NGC4374is a normal elliptical galaxy,with a spectral shape which com-pares well with those of other normal ellipticals in the spec-trophotometric atlas of galaxies of Kennicutt(1992).Thus, by removing a scaled template of NGC4374,we are isolat-ing the most massive stars(M>∼1M⊙)in the composite stellar population of the radiogalaxies.Figure3shows the bulge subtractions obtained on these two radiogalaxies.We measured on the bulge-subtracted spectra∆4000˚A and also the Balmer break index as deﬁned by the classi-cal Dλ1method of stellar classiﬁcation designed by Barbier and Chalonge(Barbier1955,Chalonge1956,see Str¨o mgren 1963).The latter quantiﬁes the Balmer discontinuity in terms of the logarithmic diﬀerence of the continuum levels (D)and the eﬀective position of the break(λ1).The method places a pseudo-continuum on top of the higher order terms of the Balmer series in order to measure the eﬀective posi-tion of the discontinuity.Figure4shows the placement of continua,pseudo-continua and the measurements of D and λ1for an A2I star from the stellar library of Jacoby,Hunter &Christian(1984).The functional dependences on the ef-fective temperature and gravity of the stars are suﬃciently diﬀerent for D andλ1to satisfy a two-dimensional classiﬁ-cation.The Dλ1method could only be reliably applied in the cases of Hydra A and3C285.For the other radiogalaxies, the bulge-subtractions led to results that did not allow the identiﬁcation of the absorption features and/or the break in an unambiguous way due to the resulting poor S/N.Fig-ure5shows the Dλ1measurements performed on the bulge-subtracted spectra of Hydra A and3C285.We have placed diﬀerent continuum levels to estimate the maximum range of acceptable parameters of the stellar populations that are involved.Table3lists theλ4000˚A and Balmer break indices mea-sured in both the bulge-subtracted and the original spectra of the radiogalaxies.3.3.Lick indicesThe presence of prominent Balmer absorption lines,from Hγup to H12λ3750˚A,is one of the most remarkable features of the blue spectra of two of the seven radiogalaxies,while Hβand Hαareﬁlled up by conspicuous emission lines.A clear exception to the presence of the Balmer series in absorption is the broad-line radiogalaxy3C382.In order to estimate the Balmer strength,crucial to date any young stellar population involved,we use the EW of the H10λ3798˚A line,which appears only weakly contaminated by emission in the radiogalaxies.H10is chosen as a compro-mise of an easily detectable Balmer line that shows both a minimum of emission contamination and clear wings to mea-sure the adjacent continuum.The Balmer lines from Hβto H9λ3836˚A are contaminated by prominent emission,which in Case B recombination comes in decreasing emission ra-tios to Hβof1,0.458,0.251,0.154,0.102,0.0709(Osterbrock 1989);H10has an emission contamination of0.0515×Hβ. At the same time,the absorption strengths are quite similar from Hβto H10,although the EW(H10)is actually sys-tematically smaller than EW(Hβ)in young to intermediate-age populations.Gonz´a lez-Delgado,Leitherer&Heckman (2000)obtain,in their population synthesis models,ratios of EW(Hβ)/EW(H10)between1.3and1.6for bursts with ages0to1Gyr and constant or coeval star formation histo-ries.Lines of order higher than10have decreasing emission contamination,but they also increasingly merge towards the Balmer continuum limit.A caveat in the use of H10as an age calibrator comes from the fact that this line might be contaminated by metal-lic lines in old populations.Although our measurements of H10in NGC4374are around1.5˚A,an inspection of the spec-tra of three elliptical galaxies(NGC584,NC720,NGC821) observed in the same wavelegth range(but with lower S/N) and archived in the Isaac Newton Group database,indicates that a wide range of EW(H10),from2to4˚A,could char-acterize elliptical galaxies,while their Hβindices are in the 1to2˚A regime.If conﬁrmed by better data,these results could indicate that although the upper Balmer series is de-tected in elliptical galaxies,it could indeed be contaminated by the absorptions of other species.Clearly,more work needs to be done in the near-UV spectra of elliptical galaxies be-fore conclusive evidence can be derived for the behaviour of EW(H10)in old stellar systems,and its contamination by metallic lines.In all the radio-galaxies observed in this work,the H10 proﬁle is narrow and reproduces the shape of the wings of the lower-order Balmer absorption lines.Hydra A and3C285 clearly provide the bestﬁttings.As an illustration,Figure6 shows the estimated absorption line proﬁles for the Hβ,Hγand Hδlines,assuming a constant ratio between their EWs and that of H10,and also a scaled(×1.4)H10proﬁle for the case of Hβin Hydra A.We also measured indices that are mostly sensitive to the metal content of the stellar populations involved. The Lick indices of Mg and Fe(e.g.Worthey et al.1994) serve this purpose.In order to avoid the contribution of [O III]λ4959˚A to the continuum measurement for the molec-ular index Mg2,we have displaced the lower continuum band of this index to4895.125˚A<λ<4935.625˚A.This redeﬁni-tion does not alter the value of the index in the elliptical galaxy,which shows no[O III]emission.Table4compiles the EW of H10,and the metallic in-dices Mg b,Fe5270,Fe5335,[MgFe],Mg2of the Lick system, measured in the rest-frame of the galaxies in our sample.The atomic indices are aﬀected by broadening,like the CaT in-dex,while Mg2is only aﬀected by lamp contributions in the original IDS Lick system(Worthey et al.1994,Longhetti et al.1998).We have calculated broadening corrections as in section3.1for the atomic lines,and adopted the correc-tions calculated by Longhetti(1998)for the molecular lines. The uncorrected values of these indices are denoted with a subindex u in Table4.The errors of the individual line and molecular indices were estimated adopting continua shifted from the bestﬁt continua by±1σ.This lead to average er-rors between an8and a10per cent for individual line and molecular indices,and∼6per cent for[MgFe].The agreement between our measurements of Lick in-dices and those carried out by other authors(Gonz´a lez1993, Davies et al.1987,Trager et al.2000a)on our galaxy in com-mon,NGC4374,is better than10per cent.4.DISCUSSIONc 0000RAS,MNRAS000,000–000Stellar populations in the nuclear regions of nearby radiogalaxies5parison with elliptical galaxies andpopulation synthesis modelsThe analysis of the spectral energy distributions and colours of elliptical galaxies suﬀers from a well known age-metallicity degeneracy(Aaronson et al.1978).However,this is broken down when the strengths of suitable stellar absorption lines are taken into account(e.g.Heckman1980).The plane com-posed by the[Hβ]and[MgFe]indices,in this sense,can dis-criminate the ages and metallicities of stellar systems.It is on the basis of this plot,that a large spread of ages in ellip-tical galaxies has been suggested(Gonz´a lez1993).Bressan, Chiosi&Tantalo(1996)claim,however,that when the UV emission and velocity dispersion of the galaxies are taken into account,the data are only compatible with basically old systems that have experienced diﬀerent star formation histories(see also Trager et al.2000a,2000b).A recent burst of star formation that involves only a tiny fraction of the whole elliptical mass in stars,would rise the[Hβ]index to values characteristic of single stellar populations which are 1to2Gyr old(Bressan et al.1996).Most likely,the stellar populations of radiogalaxies are also the combination of diﬀerent generations.Direct sup-port for this interpretation in the case of Hydra A comes from the fact that the stellar populations responsible for the strong Balmer lines are dynamically decoupled from those responsible for the metallic lines(Melnick,Gopal-Krishna& Terlevich1997).This interpretation is also consistent with the modest ∆4000˚A measurements we have obtained.Figure7shows a comparison of the values found in radiogalaxies,with those of normal elliptical,spiral and irregular galaxies,including starbursts,from the atlas of Kennicutt(1992).The radio-galaxies3C98,3C192,4C73.08and DA240have indices of the order of1.9to2.3,which overlap with those of normal E galaxies,∆4000˚A=2.08±0.23.These values correspond to populations dominated by stars of ages1to10Gyr old, if one assumes the coeval population synthesis models of Longhetti et al.(1999).However,Hydra A and3C285have indices in the range1.4to1.6,typical of coeval populations which are200to500Myr old.Once the bulge population is subtracted,the∆4000˚A indices of Hydra A and3C285 decrease to1.2and1.0respectively,which are typical of systems younger than about60Myr.Hamilton(1985)measured the∆4000˚A index in a sam-ple of stars covering a wide range of spectral types and lu-minosity classes.He found a sequence of increasing∆4000˚A from B0to M5stars,with values from1to4mag respec-tively.A comparison with the sequence he found leads us to conclude that the break in the bulge subtracted spectrum of Hydra A is dominated by B or earlier type stars while that of3C285is dominated by A type stars.The index ∆4000˚A does not clearly discriminate luminosity classes for stars with spectral types earlier than G0.The equivalent width of the H10absorption line in these two radiogalaxies give further support to the inter-pretation of the Balmer break as produced by a young stel-lar population.In Hydra A weﬁnd after bulge subtraction EW(H10)≈3.9˚A,which,according to the synthesis models of Gonz´a lez-Delgado et al.(2000)gives ages of7to15Myr for an instantaneous burst of star formation,and40to 60Myr for a continuous star formation mode,in solar metal-licity environments.In the case of3C285,EW(H10)≈6˚A would imply an age older than about25Myr for a single-population burst of solar metallicity.The metallic indices of normal elliptical galaxies range between the values0.56<∼log[MgFe]<∼0.66(Gonz´a lez 1993),which characterizes oversolar metallicites for ages larger than about5Gyr.This is also the typical range of our radiogalaxies,although3C285shows a clear departure with log[MgFe]≈0.4.However,[MgFe]tends to be smaller for populations younger than a few Gyr and similar overso-lar metallic content.Since3C285has a clear burst of recent star-formation,we conclude that its overall abundance is also most probably solar or oversolar.4.2.The blue stellar contentA better estimate of the spectral type and luminosity class of the stars that dominate the break in Hydra A and3C285 comes from the two-dimensional classiﬁcation of Barbier and Chalonge.In Figure8the solid squares connected by lines represent the maximum range of possible Dλ1values mea-sured in these radiogalaxies.The Balmer break index is sensitive to the position-ing of the pseudo-continuum on top of the higher order Balmer series lines,which in turn is sensitive to the merging of the wings of the lines,enhanced at large velocity dis-persions.In order to assign spectral types and luminosity classes to the stars that dominate the break,therefore,it is not suﬃcient to compare the values we have obtained with those measured in stellar catalogues.The values mea-sured for the radiogalaxies can be corrected for their in-trinsic velocity dispersions;we have chosen instead to recal-ibrate the index using template stars of diﬀerent spectral types and luminosity classes convolved with gaussian func-tions,until they reproduce the width of the Balmer lines observed in the radiogalaxies(FWHM≈12.5˚A).We used the B0to A7stars from the stellar library of Jacoby et al. (1984),which were observed with4.5˚A resolution.The val-ues of the Dλ1indices measured in these broadened stars are represented in Figure8by their respective classiﬁcation. By comparison we also plot the grid traced by the locus of unbroadened stars,as published by Str¨o mgren(1963).The broadening of the lines shifts the original locus of supergiant stars from theλ1<∼3720˚A range(Chalonge1956)to the 3720<∼λ1<∼3740˚A range,occupied by giant stars in the original(unbroadened)classiﬁcation.Giant stars,in turn, shift to positionsﬁrst occupied by dwarfs.Most dwarfs have Balmer line widths comparable to those of the radiogalaxies, and thus their locus in the diagram is mostly unchanged.The value of the D index indicates that the recent burst in Hydra A is dominated by B3to B5stars,and the eﬀective position of the Balmer break(λ1)indicates that these are gi-ant or supergiant stars,respectively.These stars have masses of7and20M⊙(Schmidt-Kaler1982).From the stellar evo-lutionary tracks of massive stars with standard mass-loss rate at Z⊙or2Z⊙(Schaller et al.1992,Schaerer et al.1993, Meynet et al.1994)we infer that these stars must have ages between7to8Myr(B3I)and40Myr(B5III).Note that the B4V stars in Figure8,near the location of Hydra A, cannot originate the break and at the same time follow the kinematics of the nucleus(see section5.3).Any dwarf star located in the stellar disk of Hydra A would show absorptionc 0000RAS,MNRAS000,000–000。