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l 答案及题⽬解析Key:1.B2.A3.C4.C5.A6.B7.D8.D9.B10.C 11.D 12.B 13-14.125题⽬解析：1. In paragraph 1, what does the author say about the presence of a blowhole in cetaceans?（Factual Information Question）A.It clearly indicates that cetaceans are mammals.B.It cannot conceal the fact that cetaceans are mammals.C.It is the main difference between cetaceans and land-dwelling mammals.D.It cannot yield clues about the origins of cetaceans.相关原句：Their streamlined bodies, the absence of hind legs, and the presence ofa fluke and blowhole cannot disguise their affinities with land dwelling mammals.（Paragraph 1）本题解析：cannot disguise…意为“不能掩盖…”，因此 “the presence of blowhole cannot disguise their affinities with land dwelling mammals”意思是“具有blowhol（出⽓孔）并不能掩盖这⼀事实：鲸类动物和陆栖哺乳动物有姻亲关系（affinities）”；B选项中cannot concea（不能隐藏）l恰好与 cannot disguise相吻合，并指出鲸类动物是哺乳动物的事实，因此选择B。
英语第一部分听力（共两节，满分30分）做题时，先将答案标在试卷上。
录音内容结束后，你将有两分钟的时间将试卷上的答案转涂到答题卡上。
第一节（共5小题；每小题1.5分，满分7.5分）听下面5段对话。
每段对话后有一个小题，从题中所给的A、B、C三个选项中选出最佳选项。
听完每段对话后，你都有10秒钟的时间来回答有关小题和阅读下一小题。
每段对话仅读一遍。
例: How much is the shirt?A.£19.15.B.£9.18.C.£9.15.答案是C。
1. How much money do the speakers have in all?A. $18.B. $28.C. $38.2. What is the probable relationship between the speakers?A. Librarian and borrower.B. Customer and salesman.C. Father and daughter.3. When will the speakers get there for the concert?A. At 6: 00 pm.B. At 7: 00 pm.C. At 7: 30 pm.4. What was the matter with the man that day?A. He drove through heavy traffic.B. His car broke down on his way home.C. He drove carelessly and got a traffic ticket.5. What does the woman want to know?A. How long the man will be away.B. Why she can't go with the man.C. When the man is going to Chicago.第二节（共15小题；每小题1.5分，满分22.5分）听下面5段对话或独白。
2024届高三“天使计划”第二轮英语注意事项：1.答卷前，考生务必将自己的姓名、准考证号填写在答题卡上。
2.回答选择题时，选出每小题答案后，用铅笔把答题卡上对应题目的答案标号涂黑。
如准考证号：如需改动，用橡皮擦干净后，再选涂其他答案标号。
回答非选择题时，将答案写在答题卡上。
写在本试卷上无效。
3.考试结束后，将本试卷和答题卡一并交回。
第一部分听力(共两节，满分30分)第一节(共5小题;每小题1.5分,满分7.5分)听下面5段对话。
每段对话后有一个小题，从题中所给的A、B、C三个选项中选出最佳选项。
听完每段对话后，你都有10秒钟的时间来回答有关小题和阅读下一小题。
每段对话仅读一遍。
例:How much is the shirt?A.£19.15.B.£9.18.C.£9.15.答案是C。
1.What will the man do?A.Change the plan.B.Wait for a phone call.C.Sort things out.2.What does the woman want to do?A.See a film with the man.B.Offer the man some help.C.Listen to some great music.3.Which place are the speakers trying to find?A.A hotel.B.A bank.C.A restaurant.4.What does the man like about the play?A.The story.B.The ending.C.The actor.5.At what time will the two speakers meet?A.5:20.B.5:10.C.4:40.第二节(共15小题;每小题1.5分,满分22.5分)听下面5段对话或独白。
去北京故宫旅游的英语作文六年级全文共3篇示例，供读者参考篇1A Trip to the Majestic Forbidden CityWow, what an incredible experience it was to visit the Forbidden City in Beijing! I had been looking forward to this school trip for months. Learning about the history and culture of ancient China is one of my favorite subjects. But being able to actually walk through the sprawling palace complex that was home to 24 emperors over the course of nearly 500 years was truly mind-blowing.We arrived at the Forbidden City nice and early after about an hour's drive from our hotel. Our teacher Mrs. Wang made sure we were all wearing hats and sunscreen since it was a scorching summer day in the capital. As we approached the infamous golden-glazed tiled roof buildings, I could hardly contain my excitement.The Forbidden City is absolutely massive, way bigger than I had imagined. With an area of over 180 acres, it has 9,999 rooms in total! That's like a whole neighbourhood packed into onepalace. No wonder it was once the exclusive domain of the imperial family and their attendants. Ordinary people like you and me weren't even allowed to step foot inside until the 20th century.We entered through the main Meridian Gate, an imposing entrance guarded by two huge stone lion sculptures. Just passing under that gate made me feel like I was traveling back in time to dynastic China. Everything from the traditional sloping glazed-tile roofs to the intricate painted decorations transported me to another world and era.Our first stop was the Outer Court, which contains some of the largest halls and palaces used for important ceremonies and events. The three main structures - the Gate of Supreme Harmony, the Memorial Gate and the Gate of Celestial Purity - were just breathtaking with their yellow glazed roof tiles and rich decorations like dragons and phoenixes. Apparently the yellow color was reserved only for the emperor since it was considered the most noble hue.We spent over an hour exploring the expansive Outer Court, taking a million pictures of every little architectural detail. My favorite was definitely the Hall of Supreme Harmony with its mind-boggling 11 wing-rooms branching off the sides. I couldhardly fathom how extravagant and luxurious it must have felt for the emperor's living quarters and palace to be so enormous and ornate.Next up was the Inner Court, which contained more intimate and personal palaces and pavilions reserved for the imperial family's living quarters. Although the individual buildings were more modestly sized compared to the grander Outer Court halls, it was still an absolute rabbit warren of intricately designed residences, gardens, and temples.I was particularly fascinated by the Palace of Heavenly Purity where the emperors actually lived and slept. Can you imagine how pampered and privileged their daily lives must have been, surrounded by a literal city of lush courtyards, libraries, and theaters? The level of luxury and comforts they had access to boggles my middle-class kid mind.Another part that really stuck out was the Imperial Garden, carefully landscaped and manicured with winding pathways, decorative rockeries, and elaborate pavilions. It was like an oasis of green right in the heart of the palace grounds. I loved examining all the little scenes and motifs sculpted into the rockeries that told traditional stories and folktales. The level of craftsmanship and artistry was just phenomenal.We finished our visit by climbing up onto the tall corner watching terrace that gave us a panoramic view of the entire Forbidden City layout sprawled out below us. Just soaking in the sheer scale and grandeur of it all was absolutely awe-inspiring to my young eyes. It really hit me just how powerful and wealthy the imperial Chinese dynasties must have been to construct a place of such epic proportions and beauty.As we wandered through the narrow palace alleyways making our way back to the exit, I spotted several cats lazily sunbathing and scampering about the ancient stone pathways. Apparently there are hundreds of stray cats that make the majestic Forbidden City their home these days! Seeing those little furry creatures roaming the same courtyards that dynasties of imperial rulers once strode was such a whimsical contrast. It really helped bring the historic sites down to earth and sparked my imagination about what daily life here must have really been like for the common servants and attendants.By the time we finished touring the Forbidden City, my mind was absolutely blown by the excess of it all. Just the sheer manpower, skill, resources and riches required to construct and maintain such a sprawling palace city for China's elite rulers over the centuries is almost unfathomable. While it was undoubtedlybuilt upon the toil and labor of countless common people, there is no denying the incredible architectural beauty and artistry that makes it one of the crowning glories of China's ancient civilization.I'll never forget the first moment those yellow-tiled roofs came into view, nor the last glimpse I had walking back through the gates surrounded by the red palace walls. Visiting the Forbidden City was like being transported through a real-life time machine to the grandest era of imperial China. It ignited my imagination and sparked my curiosity about history and culture like never before. I'm so grateful I had the chance to experience this bucket-list wonder of the world with my own eyes while still just a kid. It's a memory I'll forever cherish and an adventure that will hopefully inspire me to keep learning about our world's rich heritage.篇2A Trip to the Forbidden CityWow, what an incredible experience visiting the Forbidden City in Beijing was! I had been looking forward to this school trip for months. Learning about ancient Chinese history and culture is one of my favorite subjects. Finally getting to see the famouspalace where emperors lived and ruled for centuries was a dream come true!The bus ride from our school to the Forbidden City seemed to take forever. My best friend Lily and I could hardly contain our excitement as the huge palace complex came into view. The bright red walls and golden roofs shimmered in the sunlight. I had seen pictures, but it was even more magnificent in person!Our tour guide told us the Forbidden City is the largest ancient palatial structure in the world. It was home to 24 emperors during the Ming and Qing dynasties over nearly 500 years. Can you imagine living in a place like that? With a total area of 180 acres, it's as big as 90 soccer fields! The walls are 30 feet high and the whole thing is surrounded by a moat to protect the royal family. No wonder it was forbidden for regular people to enter back then.As we walked through the entrance called the Meridian Gate, it felt like being transported back in time. The heavy wooden doors were decorated with intricate designs of dragons and phoenixes, symbols of imperial power. Our guide explained that in ancient China, only the emperor was allowed to walk or be carried through the central walkway. Commoners had to walk on the sides.The first courtyard we entered was huge, with a giant stone plaza for holding ceremonies and greeting ambassadors from far away lands. Bronze cauldrons for fire were placed along the sides. At the far end stood the Gate of Supreme Harmony where the emperor gave speeches on special occasions.Next we passed through the Gate of Heavenly Purity into the inner court where the royal family lived and conducted daily affairs. The first building was the Palace of Heavenly Purity where important ceremonies like the emperor's wedding took place. We couldn't go inside, but our guide said it has five spectacular halls joined together in a straight line. It must have been breathtaking!My favorite part was the exquisite gardens behind the living quarters. They were designed according to the principles of Feng Shui with winding pathways, decorative rocks, pools, and lush landscaping. I can picture the emperor and his family strolling the grounds and enjoying the serenity.In the Imperial Garden, a large elaborate theater is used for performing the ancient Kunqu Opera, with its beautiful singing, dancing and costumes. Our guide told us stories of how the royal operas were enacted on this very stage centuries ago for theroyal court's entertainment. How amazing it must have been to watch such a lavish spectacle up close!We also toured several museums within the palace complex, filled with priceless treasures and artifacts from China's dynastic eras. Displays held intricate jade and porcelain artworks, silk robes and tapestries, jewelry, ceramics, weapons and more. I marveled at the creativity and craftsmanship that went into each exquisite piece. Artisans of the past were truly masters of their trades.From richly decorated throne rooms draped in yellow silk to the huge kitchens that prepared feasts for thousands, every area gave us a glimpse into the daily lives of emperors, empresses and their servants. Our guide brought the ancient traditions and customs to life with his engaging stories. For a few hours, I was transported back to imperial times.After hours of exploring, my feet were sore but my mind was bursting with all the history I had learned. The Forbidden City is definitely one of the grandest and most fascinating places I've ever visited. I have such an appreciation now for China's rich imperial legacy and just how advanced its civilization was compared to others during that era.Walking out through the north gate, I looked back one last time at the majestic architecture and imagined the lavish ceremonies, grand receiving of subjects, festivals and celebrations that took place there over the centuries. So much history dwelled within those walls! What an honor to experience this priceless cultural treasure.China should be extremely proud of its imperial heritage and the meticulous efforts taken to preserve the Forbidden City for future generations like me. It's a true masterpiece of traditional art and architecture on an epic scale. Yet despite its brilliance and splendor, it also struck me as an isolated, closed-off world where normal people were forbidden to enter or experience the grandeur. The common folks who built and served the palace were shut out from its inner circles of privilege and power.In that way, the modern, open society we live in today seems much more enlightened and fair, at least in my 11-year-old opinion! I'm grateful to have the opportunities for education, travel and experiencing other cultures that my ancestors could only dream about.Still, visiting this incredible UNESCO World Heritage site was an unforgettable journey into the past that sparked my curiosityto learn even more. I'm already looking forward to our next field trip!篇3A School Trip to the Forbidden CityWow, what an incredible experience visiting the Forbidden City in Beijing was! Our 6th grade class took a field trip there last month, and I'm still daydreaming about all the amazing things I saw and learned.The Forbidden City used to be the imperial palace for 24 different emperors during the Ming and Qing dynasties. It's called the "Forbidden City" because regular people weren't allowed to go inside for a really long time. Only the emperor, his family, and servants could live and work within the city walls. Can you imagine how lonely and isolated that must have felt, even though the palaces and gardens were absolutely breathtaking?Our class rode a tour bus for almost an hour to get from our school to the Forbidden City, which is located right in the heart of Beijing. As we drove through the crazy Beijing traffic, our tour guide Ms. Wang told us all kinds of fascinating stories and facts to get us excited for our visit.Finally, we arrived at the Meridian Gate, which is the massive front entrance to the Forbidden City grounds. I couldn't believe how beautifully decorated it was, with incredible stone carvings of dragons and phoenixes. Ms. Wang explained that dragon images symbolize the emperor's power, while phoenixes represent the empress.Once we walked through the Meridian Gate, it was like stepping back in time to ancient China. The views were unbelievable - one stunning traditional building after another, with bright red walls, yellow glazed tile roofs, and elaborate decorations everywhere you looked. Our guide told us there are 9,999 rooms within the Forbidden City walls! Can you even imagine trying to clean all of those rooms? No thanks!The first major building we visited was the Gate of Supreme Harmony. This gate was reserved only for the emperor - no one else could walk through it except him. We took a class photo in front of the gate, and I loved looking up at the intricate roof decorations with those fierce dragon images. So cool!From there, we wandered into the Outer Court, where many of the ceremonial halls and audience rooms used to be. Everything seemed larger-than-life, from the towering woodenpoles and stone ramps to the giant bronze vats for boiling water. Ms. Wang told us the vats were kept boiling 24/7 in case of fire.My favorite place in the Outer Court was the magnificent Hall of Supreme Harmony. This was the location where emperors would sit on the dragon throne to meet with the masses on holidays and special occasions. I dream of being an emperor someday so I could actually sit on that iconic red lacquered wood throne! Although it would be kind of awkward with all those visitors watching me...After the Outer Court, we entered the Inner Court, which was where the emperor, his wives, and their children actually lived, worked, and studied. We strolled through lush imperial gardens with beautiful pavilions, rock sculptures, and even a small hill with a locked gate at the top. Apparently only the emperor was allowed to walk on that hill! What a rip-off.One of the neatest places was the Palace of Heavenly Purity, where young emperor princes received their education. I loved seeing the ancient classroom with its carved throne for the emperor and smaller chairs for the teachers and students. Our guide showed us examples of the complex lessons the princes had to master, like obscure literary texts and Confucianphilosophy. I'm suddenly feeling very grateful for math and science class at my normal school!After touring the palace buildings and gardens, our class had some free time to explore the Forbidden City's museums and exhibits. My friends and I wandered through halls filled with mind-blowing treasures, from jade sculptures and calligraphy masterpieces to imperial silk robes and ancient weapons. I've never seen so many dazzling jewels, intricate woodcarvings, and cultural relics all in one place.We also visited the Palace Museum's clock exhibition, which had the craziest collection of old timekeepers I've ever witnessed. There were strings that pulled weights, candle clocks that measured time by how much wax melted, and automatons that rang bells and twirled to mark the hours. My favorite was this massive metal orb with a spiral groove carved into the exterior. It used a stream of water running along the groove to power the clock's movement. I already find measuring time to be a hassle with normal clocks and apps - can you imagine having to deal with instruments like those every day?By the end of our visit, we had been exploring the Forbidden City for over 5 hours. My legs were exhausted from all the walking, but my brain was overflowing with newfoundknowledge and appreciation for ancient Chinese history and culture. As we reunited outside the palace gates, everyone was buzzing with excitement about all the incredible things we had seen and experienced together that day.On the bus ride back to school, my classmates and I swapped our favorite memories and compared which imperial treasures or buildings we liked best. Some kids were fascinated by the extravagant palaces and artwork belonging to the emperors, while others were more intrigued by the humble areas for servants and guards to live and work. A few of my friends couldn't stop talking about the entertaining performances we saw, like acrobats spinning plates or dancers dressed in colorful traditional costumes.As for me, I think my favorite part was simply wandering through the quiet, secluded gardens and walkways, trying to imagine what life must have been like living within those walls centuries ago. To be surrounded by such immense beauty, but also feel so isolated from the outside world? It's a stark contrast that I'm still trying to wrap my 11-year-old brain around.Whether you're interested in ancient architecture, stunning arts and antiquities, fascinating histories, or just a glimpse into the extraordinary lives of Chinese royalty, the Forbidden City hassomething to spark your curiosity. Our class field trip made me realize what a magical, mysterious place it truly is. I already can't wait to go back and explore more someday, now that I've had just a small taste of the wonders contained inside the Forbidden City walls.。
An island,once a tranquil haven,was suddenly shaken by the tremors of a volcanic eruption.The serene landscape,with its lush greenery and pristine beaches,was transformed into a scene of chaos and destruction as the earth roared to life.The following narrative captures the essence of such an event.On a typical day,the islands inhabitants were going about their daily routines,unaware of the impending disaster.Children played by the shore,fishermen prepared their boats for the days catch,and tourists marveled at the beauty of the surroundings.The tranquility was abruptly shattered by a deep,rumbling sound that echoed across the island.The ground beneath their feet trembled,and a plume of smoke rose ominously from the heart of the island.As the volcano began to erupt,the sky turned dark with ash,casting a shadow over the island.The once clear blue waters were now obscured by the falling debris,and the air was filled with the acrid smell of sulfur.The eruption was a spectacle of natures power, with molten lava spewing from the earths core and cascading down the slopes of the volcano.The islands inhabitants,realizing the gravity of the situation,scrambled to evacuate. Emergency sirens wailed,and the local authorities coordinated a mass exodus to safer areas.Families clutched their belongings,leaving behind their homes and livelihoods in a desperate bid to escape the wrath of the volcano.The lava flows,relentless and unstoppable,consumed everything in their path.Trees, homes,and crops were engulfed in a fiery inferno,turning the once lush landscape into a barren wasteland.The heat was unbearable,and the ground cracked under the intense pressure.As the eruption continued,the island was enveloped in a blanket of ash and smoke. Visibility was reduced to mere meters,and the air was thick with the fine particles of volcanic debris.The ash rained down on the island,coating everything in a layer of gray. The once vibrant colors of the island were now muted,and the beauty that had once drawn people to the island was marred by the scars of the eruption.The aftermath of the eruption was devastating.The island,once a paradise,was now a desolate and lifeless place.The volcano had left its mark,a stark reminder of the power and unpredictability of nature.The recovery process was slow and arduous,with the islands inhabitants working tirelessly to rebuild their lives and restore their home.The eruption served as a stark reminder of the delicate balance between man and nature.While the island may have been transformed by the eruption,the resilience and determination of its people shone through.As they worked to rebuild their lives,they did so with the knowledge that the island,like the volcano,would one day awaken again,and they would be prepared for the next time natures fury was unleashed.。
Climate change is one of the most pressing issues facing the world today,with farreaching impacts on the environment,economy,and society.As a global community, it is imperative that we take collective action to mitigate the effects of climate change and adapt to the changes that are already underway.The Science Behind Climate ChangeThe primary cause of climate change is the increase in greenhouse gases,such as carbon dioxide CO2,methane CH4,and nitrous oxide N2O,in the Earths atmosphere.These gases trap heat from the sun,leading to a rise in global temperatures.Human activities, such as the burning of fossil fuels,deforestation,and industrial processes,have significantly contributed to this increase.Consequences of Climate ChangeThe effects of climate change are widespread and include rising sea levels,more frequent and severe weather events,changes in precipitation patterns,and the loss of biodiversity. These changes pose a threat to food security,water resources,and human health. Additionally,they can exacerbate existing social and economic inequalities,particularly in vulnerable communities.Mitigation EffortsTo reduce the impact of climate change,it is crucial to implement mitigation strategies that aim to reduce greenhouse gas emissions.This can be achieved through a combination of policy measures,technological advancements,and changes in individual behavior. Key mitigation efforts include:1.Transitioning to Renewable Energy:Shifting from fossil fuels to renewable energy sources,such as solar,wind,and hydroelectric power,can significantly reduce CO2 emissions.2.Improving Energy Efficiency:Enhancing the efficiency of buildings,transportation, and industrial processes can lead to a reduction in energy consumption and associated emissions.3.Reforestation and Afforestation:Planting trees and preserving existing forests can absorb CO2from the atmosphere,acting as a natural carbon sink.4.Adopting Sustainable Agricultural Practices:Implementing practices that reduce therelease of methane and nitrous oxide from agriculture,such as improved manure management and precision farming,can help mitigate climate change.Adaptation MeasuresWhile mitigation is essential,it is also important to prepare for the changes that are already occurring.Adaptation measures can help communities and ecosystems better cope with the impacts of climate change.These measures include:1.Building Resilient Infrastructure:Designing and constructing infrastructure that can withstand extreme weather events,such as floods and storms,is crucial for reducing the damage caused by these events.2.Developing Early Warning Systems:Implementing systems that provide timely information about impending weather events can help communities prepare and respond more effectively.3.Promoting Diversification in Agriculture:Encouraging farmers to grow a variety of crops can reduce the risk of crop failure due to changing climate conditions.4.Protecting and Restoring Ecosystems:Preserving natural habitats and restoring degraded ecosystems can enhance their ability to provide services such as water purification,flood control,and carbon sequestration.Role of Individuals and CommunitiesIndividuals and communities can play a significant role in addressing climate change by adopting sustainable practices in their daily lives.This includes reducing energy consumption,choosing sustainable products,and supporting policies that promote environmental protection.ConclusionClimate change is a complex and multifaceted issue that requires a comprehensive and coordinated response.By understanding the science behind climate change,recognizing its consequences,and implementing both mitigation and adaptation measures,we can work towards a more sustainable and resilient future.It is the collective responsibility of all nations and individuals to take action and ensure that we leave a habitable planet for future generations.。
小学上册英语第一单元测验试卷英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.The boiling point of a substance is the temperature at which it turns to _______.2.In the zoo, we saw a _______ (大象) spraying water with its trunk.3.He is a _____ (评论员) offering insights into current events.4.What is the capital of Ethiopia?A. Addis AbabaB. NairobiC. AsmaraD. DjiboutiA5.The puppy is very ________.6.Which part of your body helps you to see?A. EarsB. EyesC. NoseD. MouthB7.The ________ (博物馆) showcases local history.8.What is the main function of the heart?A. To digest foodB. To pump bloodC. To filter wasteD. To breatheB To pump blood9.Vinegar is an example of an ______ solution.10.ts can ______ (通过分枝) to spread. Some pla11.It is _____ outside today. (cold)12.What do we call the famous wall in China?A. Great WallB. Berlin WallC. Hadrian's WallD. Wall of JerichoA13.What do we call a person who travels to space?A. AstronautB. PilotC. ScientistD. EngineerA14.The rabbit is ________ carrots.15.What do we call a baby rabbit?A. KittenB. BunnyC. PupD. Calf16.The Aztecs were known for their _______ civilization.17.The ancient Greeks engaged in ________ to settle disputes.18.of Mexico is located ________ (在美国南部). The Gupt19.What is the main purpose of a map?A. To show roadsB. To provide directionsC. To represent areasD. All of the above20.What do you call a group of wolves?A. PackB. FlockC. GaggleD. MurderA21.I love to sing ______ songs.22.I like to draw ______ (漫画人物) and create my own ______ (故事).23.I made a scrapbook of my favorite ________ (回忆) from summer camp. It’s full of ________ (照片).24.Space is a vacuum, meaning it has very little _______.25. A cat's agility helps it catch ______ (猎物).26.The _______ of a wave can be determined by its wavelength.27.Which of these is an ocean?A. AmazonB. NileC. AtlanticD. Mississippi28.The __________ can be quite chilly during autumn. (天气)29.What is the soft drink made from cola?A. TeaB. CoffeeC. ColaD. Juice30.我的朋友喜欢 _______ (活动). 她觉得这很 _______ (形容词)31.Sarah is a ______. She enjoys writing poems.32.What is the term for a person who travels to different countries?A. TouristB. ExplorerC. TravelerD. AdventurerA33. A ______ is a geological feature that attracts scientists and researchers.34. A _______ is a type of reaction that breaks down compounds into simpler substances.35.My ________ (玩具名称) is a fun friend.36.The ________ (根茎植物) can be edible.37. A __________ is a geological feature formed by the action of waves on rock.38.What is the name of the famous ship that sank in 1912?A. Queen MaryB. TitanicC. BritannicD. LusitaniaB39.What is the name of the fairy tale character who lost her glass slipper?A. Snow WhiteB. CinderellaC. ArielD. BelleB40.The city of Dublin is the capital of _______.41. A garden can provide a ______ (美丽的) view in your backyard.42.What do you call a large, round fruit that is typically orange?A. AppleB. PeachC. OrangeD. Banana43.The chemical formula for glucose is _____ (CHO).44.The __________ in summer is often very hot and dry. (天气)45.Plants need _______ to grow.46.The chemical symbol for molybdenum is _____.47.What is the name of the largest land animal?A. RhinoB. ElephantC. HippoD. GiraffeB48.In 1492, Columbus sailed the ocean blue to find a new __________. (航道)49.What is the capital city of Moldova?A. ChișinăuB. BălțiC. TiraspolD. Bender50.The jackal is a clever ______ (动物).51.What do you call a person who plays chess?A. PlayerB. Chess masterC. StrategistD. Chess player52.What do we call the part of the plant that produces seeds?A. FlowerB. LeafC. StemD. RootA53.The ice cream is ___. (cold)54.My friend is a ______. He enjoys hiking.55.What is the name of the famous ship that sank on its maiden voyage?A. TitanicB. LusitaniaC. BritannicD. Andrea DoriaA56.I have a red ______ (气球) for my birthday party. It floats high in the ______ (天空).57.I see a _____ (rainbow) in the sky.58. A chemical reaction can be classified based on the ______.59. A process that separates a substance from a mixture is called ______.60.How many wheels does a bicycle have?A. 2B. 4C. 3D. 161.The capital city of Saint Kitts and Nevis is __________.62.I sometimes make up stories about my ________ (玩具名). They have their own ________ (名词) and adventures.63.My mom enjoys __________ (旅行) to new places.64.What do we call the person who repairs shoes?A. TailorB. CobblerC. BakerD. Mechanic65.My sister is my best _______ who is always ready to help.66.The parrot has a bright ______ (羽毛).67.I enjoy participating in ______ (学校项目) that focus on community service. It’s rewarding to give back.68.For breakfast, I usually eat ______ (面包) and drink ______ (牛奶). It gives me energy for the day.69.Mount Everest is located in the __________ mountains.70.The _______ (小蜗牛) carries its house on its back.71.My favorite animal is a ______ (猴子) because they are funny.72.Did you see a _______ (小老鼠) in your house?73. A _______ can be a fun project for children.74.What is the color of a typical sunflower?A. YellowB. GreenC. BlueD. RedA75.The dolphin makes clicking ______ (声音).76.They play _____ (hide-and-seek).77.Which color is a ripe strawberry?A. BlueB. GreenC. RedD. YellowC Red78.We will _______ (celebrate) my birthday tomorrow.79.What is the capital city of Bangladesh?A. DhakaB. ChittagongC. KhulnaD. Sylhet80.The lizard basks in the _________. (阳光)81.What do we call the study of ancient civilizations?A. ArchaeologyB. AnthropologyC. SociologyD. HistoryA82.The __________ is a major geographical region in Africa. (撒哈拉沙漠)83.Many _______ need sunlight to grow.84.The ________ (气候适应) is essential.85.The __________ (历史的启迪) guides our path.86.The chemical symbol for rhenium is ____.87.What is the term for a baby goat?A. CalfB. KidC. FoalD. LambB88.The man has a funny ________.89.The dog is _______ (wagging) its tail.90.The garden is ________ (五颜六色).91.The __________ is a region known for its deserts and dry climate.92.I like to listen to ________ (古典音乐) while studying.93.My favorite activity in the spring is ______ (种花).94.Which of these is a vegetable?A. AppleB. CarrotC. BananaD. Grape95. A mixture of solids is called a ______ blend.96.What is 20 7?A. 12B. 13C. 14D. 1597.We like to go to the ___ (beach/mountains).98.The __________ (历史研究) helps us understand the past.99.The main component of the Earth's atmosphere is __________. 100.The __________ (历史的纪录片) provide visual representations.。
A simple way to break a bad habit一个简单的方式戒掉坏习惯When I was first learning to meditate, the instruction was to simply pay attention to my breath, and when my mind wandered, to bring it back.当我第一次学习冥想的时候，得到的指示就是，简单地注意自己的呼吸，而当我的心思开始游走了，就把它拉回来。
Sounded simple enough. Yet I'd sit on these silent retreats, sweating through T-shirts in the middle of winter. I'd take naps every chance I got because it was really hard work. Actually, it was exhausting. The instruction was simple enough but I was missing something really important.听起来很简单。
但当我在静坐冥想时，即使在冬天也会让我汗流浃背。
我抓到机会就会小睡片刻，因为真的很辛苦。
实际上，是精疲力竭了。
指示是很简单，但我错过了很多重要之地方。
So why is it so hard to pay attention? Well, studies show that even when we're really trying to pay attention to something -- like maybe this talk -- at some point, about half of us will drift off into a daydream, or have this urge to check our Twitter feed. 那为什么专注会这么困难呢？根据研究指出，就算是我们尝试着专注于一些事情--就好像这个演讲--到某个时间点，我们当中会有一半的人，都会恍惚进入神游状态，或是会有一股冲动，想去查看一下推特的内容。
浙江强基联盟2024-2025学年高三8月联考英语试题一、听力选择题1．How is the weather in the mountains?A．Rainy.B．Sunny.C．Snowy.2．What are the speakers mainly talking about?A．Cooking.B．Traditions.C．Experiments.3．Why does the man read in weak light?A．He is afraid of light.B．His brother is sleeping.C．He doesn’t care about his eyes.4．How much money did the man lend the woman?A．$70.B．$80.C．$100.5．What is the man's opinion about sleep?A．Four hours is plenty for him.B．Six hours is the least for most people.C．Eight hours is too much for the woman.听下面一段较长对话，回答以下小题。
6．Where does the conversation take place?A．In the kitchen.B．In the bedroom.C．In the study.7．When does the man plan to go to sleep?A．At 11:00 p. m.B．At 3:00 a. m.C．At 4:00 a. m.听下面一段较长对话，回答以下小题。
8．How long did Marco Polo travel?A．For 13 years.B．For 17 years.C．For 24 years.9．What does the man suggest the woman do at the end?A．Read a storybook about Xuanzang.B．Write an essay about Marco Polo.C．Imagine the life in ancient times.听下面一段较长对话，回答以下小题。
a r X i v :a s t r o -p h /0703667v 1 26 M a r 2007February 3,2008.To be submitted to ApJ.Preprint typeset using L A T E X style emulateapj v.10/09/06QUASAR PROXIMITY ZONES AND PATCHY REIONIZATIONAdam Lidz 1,Matthew McQuinn 1,Matias Zaldarriaga 1,2,Lars Hernquist 1,&Suvendra Dutta 1February 3,2008.To be submitted to ApJ.ABSTRACTLyman-alpha (Ly α)forest absorption spectra towards quasars at z ∼6show regions of enhanced transmission close to their source.Several authors have argued that the apparently small sizes of these regions indicate that quasar ionization fronts at z 6expand into a largely or partly neutral intergalactic medium (IGM).Assuming that the typical region in the IGM is reionized by z ≤6,as is suggested by Ly αforest observations,we argue that at least 50%of the volume of the IGM was reionized before the highest redshift quasars turned on.Further,even if the IGM is as much as ∼50%neutral at quasar turn-on,the quasars are likely born into large galaxy-generated HII regions.The HII regions during reionization are themselves clustered,and using radiative transfer simulations,we find that long skewers through the IGM towards quasar progenitor halos pass entirely through ionized bubbles,even when the IGM is half neutral.These effects have been neglected in most previous analyses of quasar proximity zones,which assumed a spatially uniform neutral fraction.We model the subsequent ionization from a quasar,and construct mock Ly αforest spectra.Our mock absorption spectra are more sensitive to the level of small-scale structure in the IGM than to the volume-averaged neutral fraction,and suggest that existing proximity-zone size measurements are compatible with a fully ionized IGM.However,we mention several improvements in our modeling that are necessary to make more definitive conclusions.Subject headings:cosmology:theory –reionization –intergalactic medium –large scale structure ofuniverse1.INTRODUCTIONThe Epoch of Reionization (EoR),when HII regions grow around galaxies and/or quasars,eventually overlap and fill the entire IGM,is fundamental to our under-standing of cosmological structure formation.Detailed observations of the EoR will characterize the nature of the first luminous sources in the Universe,describe their impact on the surrounding IGM,and fill in a significant gap in our knowledge of the history of the Universe.Prospects for observational advances are bright:21cm observations (e.g.Madau et al.1997,Zaldarriaga et al.2004,for a review see Furlanetto et al.2006a),improved measurements of polarization of the cosmic microwave background (CMB)(Zaldarriaga 1997,Kaplinghat et al.2003),small-scale CMB fluctuations (e.g.Zahn et al.2005,McQuinn et al.2005),increasingly deep narrow-band Ly-αsurveys (e.g.Haiman &Spaans 1999,Barton et al.2004,Furlanetto 2006b,McQuinn et al.in prep.),optical afterglow spectra of gamma ray bursts (GRBs)(Barkana &Loeb 2004),quasar absorption spectra (Fan et al.2006),and other probes,promise a wealth of new data in the near future.What have we learned from existing observations?This is,of course,an intrinsically interesting question,but it is also an important one for directing the design of future surveys and experiments.For example,cur-rent constraints can provide important guidance regard-ing the optimal target redshift range for future reioniza-tion surveys.Inferences about the duration of the EoR come fromElectronic address:alidz@1Harvard-Smithsonian Center for Astrophysics,60Garden Street,Cambridge,MA 02138,USA2Jefferson Laboratory of Physics;Harvard University;Cam-bridge,MA 02138,USAmeasurements of anisotropies in the polarization of the CMB (Page et al.2006),narrow-band surveys for Ly-αemitters (e.g.Malhotra &Rhoads 2005),the optical afterglow of a z =6.3GRB (Totani et al.2006),and par-ticularly the absorption spectra of high redshift quasars (e.g.Fan et al.2006).While valuable and exciting,these observations have yielded constraints on the EoR that are generally weak and subtle to interpret.The cen-tral value for the electron scattering optical depth from WMAP favors reionization activity at z 11,but,at the 1−σlevel,current limits are consistent with rapid reion-ization ending near z ∼6(Page et al.2006).Indeed,at ∼3−σthe data are consistent with no reionization whatsoever (Page et al.2006).Malhotra &Rhoads (2005)have constrained the ion-ized volume fraction in the IGM by counting the abun-dance of Ly-αemitters at z =6.5,and requiring a mini-mum ionized volume around the sources to avoid atten-uating their Ly-αphotons.In this manner,Malhotra &Rhoads quote an upper limit on the neutral volume frac-tion of the IGM,X HI 0.5.In reality,the sources likely reside in much larger ionized regions (e.g.Furlanetto et al.2006b),and this constraint should be tightened with more detailed modeling.For example,Dijkstra at al.(2006)find that these observations imply X HI 0.2.The optical afterglow spectra of z 6GRBs can,in principle,be used to search for damping-wing absorption redward of Ly-α,a signature of a largely neutral IGM (Miralda-Escud´e 1998,Barkana &Loeb 2004).However,most GRB optical afterglows show evidence for damped Lyman-alpha absorbers associated with their host galax-ies,which are problematic to distinguish from a largely neutral IGM (Totani et al.2006).Even so,Totani et al.(2006)use the Ly-βregion of a z =6.3afterglow to argue against a largely neutral IGM,giving a constraint2of X HI 0.6.Presently,the most detailed information on the state of the IGM at z∼6comes from Ly-αforest absorption towards high redshift quasars(e.g.Fan et al.2006).It is difficult to derive constraints on the ionization state of the IGM from the spectra of these quasars,owing to the large absorption cross section for Ly-αphotons.Indeed, a highly ionized IGM(X HI 10−4)results in complete absorption in a z∼6Ly-αforest spectrum(Gunn& Peterson1965).In spite of this intrinsic complication,a little ingenuity has led to substantial progress.For example,one can measure absorption in the Ly-βand Ly-γtroughs of a quasar spectrum.Owing to their weaker absorption cross sections,when these transitions are saturated they imply tighter constraints on the ion-ization state of the IGM than the optical depth in Ly-α. This approach has been used to suggest that the IGM is evolving rapidly near z∼6(e.g.Fan et al.2002,Cen& McDonald2002,Lidz et al.2002,Fan et al.2006).The constraints are,however,consistent with a mostly ion-ized IGM.Furthermore,the transmission is influenced strongly by rare underdense regions at high redshift,and so the conclusions depend sensitively on the probability distribution of gas in the IGM(Oh&Furlanetto2005, Becker et al.2006a).Another interesting statistic is to consider how much the absorption,averaged over large stretches of spectra,varies from sightline-to-sightline. Close to and during reionization,the ultraviolet radi-ation background shouldfluctuate strongly(e.g.Zuo 1992,Wyithe&Loeb2005a)and potentially increase the sightline-to-sightline scatter in the mean absorption. However,current measurements are broadly consistent with densityfluctuations alone(Lidz et al.2006a,Liu et al.2006).It is also possible to use a metal line tracer of the ioniza-tion state of the IGM,such as OI,which conveniently lies redward of Ly-αand has an ionization potential similar to that of hydrogen(Oh2002).In fact,high-resolution Keck spectra of the z∼6SDSS quasars do reveal some OI lines,with4out of6detected systems lying towards the highest redshift quasar known(Becker et al.2006b). Interestingly,some of the OI systems are nearby regions that show transmission in the Lyαand Lyβforests of this quasar(Becker et al.2006b).The interpretation of these observations is unclear:the OI systems might re-flect dense clumps of neutral gas in a highly ionized IGM, or instead could indicate inhomogeneous metal pollution in a more neutral IGM.Finally,the tightest constraints claimed on the ioniza-tion state of the z∼6IGM come from measurements of the proximity regions around z∼6quasars.Several au-thors,starting with Wyithe&Loeb(2004),have argued that these regions are small,indicating that quasar ion-ization fronts are expanding into a largely neutral IGM. Mesinger&Haiman(2004)claim to detect the edge of a quasar ionization front around one of the SDSS z∼6 quasars,using additional leverage from the Lyβregion, and suggest that the neutral fraction is X HI 0.25at z∼6.These authors’constraint comes not so much from the apparent size of the proximity zone,but from the detailed radial dependence of the transmission and from a stretch of spectrum with Lyβtransmission and no corresponding Lyαtransmission.They attribute this to damping wing absorption,a signature of a partly neu-tral IGM.Oh&Furlanetto(2005),however,argue that such stretches occur at high redshift even when the IGM is highly ionized and may not indicate damping wing absorption.Fan et al.(2006)emphasize caution in in-terpreting proximity region measurements,but observe rapid evolution in the sizes of quasar proximity regions from z=5.7−6.3,arguing for a correspondingly rapid evolution in the neutral fraction.Theirfinal constraint is based on only the evolution of the proximity region size,which they argue reflects the change in the neu-tral fraction.Consequently,they quote a significantly more conservative limit than previous authors,requiring a volume-weighted neutral fraction of only X HI 10−3. Recently,more detailed proximity zone calculations have been performed.Bolton&Haehnelt(2006)studied quasar transmission carefully using1D radiative transfer calculations,and determined that it is difficult in general to distinguish highly ionized and mostly neutral models with proximity zone measurements.Maselli et al.(2006) came to a similar conclusion.Mesinger&Haiman(2006), however,argue that detailedfitting of the transmission pdf in the quasar proximity zones favors a partly neutral IGM,with a lower limit of x HI 0.033,and a consider-ably larger preferred value.In each of these studies,the authors have assumed that quasar ionization fronts expand into a uniformly ionized surrounding IGM,with some low level,yet homogeneous background ionization.If the z 6quasar spectra truly probe the pre-reionization epoch,then this is likely a very poor approximation.The pre-reionization IGM should resemble swiss cheese(Loeb2006),with large HII re-gions forming around clustered galaxies embedded in a surrounding neutral IGM(e.g.Sokasian et al.2003, Ciardi et al.2003,Furlanetto et al.2004a,c,Iliev et al.2006,Zahn et al.2006,McQuinn et al.2006a,Trac &Cen2006).Naively,this complicates distinguishing partly neutral and highly ionized models on the basis of quasar proximity zones.Quasar ionization fronts may extend further along sightlines that traverse several ion-ized HII regions,and be more limited along other direc-tions that traverse several neutral patches.Moreover, transmission at the edge of the proximity zone may be related to background galaxies rather than the quasar itself,making it still harder to locate the‘edge’of the proximity zone using absorption spectra(see also Wyithe &Loeb2006a).Our present paper extends these earlier works,and focuses on how‘patchy reionization’impacts quasar proximity zones.The outline of our paper and our basic line of argu-ment is as follows.Given that most of the volume of the IGM appears to be highly ionized by z 6,we argue that reionization is unlikely rapid enough for the IGM to be mostly neutral when the highest redshift quasars observed turned on(§2).From these considerations,we suggest that the IGM is at least50%ionized at quasar turn-on.In§3,we describe3D radiative transfer calcu-lations for plausible partly neutral models,detailing the initial ionization state of the IGM.Here we argue that the highest redshift quasars are born into large HII regions, even if as much as50%of the volume of the IGM is neu-tral.Yu&Lu(2005)previously argued that overdense z∼6quasar environments should reionize before typical regions,but here we examine the consequences of this in more detail,and arrive ultimately at somewhat differ-3ent conclusions.Even in this maximally(50%)neutral scenario,wefind long skewers towards quasar progeni-tor halos which pass entirely,or predominantly,through ionized bubbles.Using the initial ionizationfield from our3D calcula-tions as input,we perform(more detailed)1D radiative transfer calculations,describing the subsequent propaga-tion of quasar ionization fronts(§4).Here wefind that quasar front extents,in our patchy reionization mod-els,depend sensitively on the long ionized pathways cre-ated by surrounding galaxies before the quasar is born. Since quasars are typically born into large HII regions, the fronts tend to extend further in patchy reionization models than in models with a uniform IGM of the same neutral fraction,although with significant sightline-to-sightline variation.In§5we construct mock absorption spectra and show,in agreement with previous authors (Bolton&Haehnelt2006,Maselli et al.2006),that it is difficult to accurately recover the position of a quasar front,and that estimates of the front position from ab-sorption spectra are generally underestimates.This is a consequence of the typically high Lyαopacity at the edge of a z∼6quasar front.We suggest,however,an alterna-tive algorithm forfinding front positions from absorption spectra.We then estimate the importance of unresolved (in our numerical simulations)small scale structure on our mock absorption spectra.Our results are more sen-sitive to the highly uncertain level of small scale struc-ture in the IGM than to the volume-weighted ionization fraction.Finally,in§6we conclude and discuss possible future research directions.Throughout,we assume aflat,ΛCDM cosmology pa-rameterized by:Ωm=0.3,ΩΛ=0.7,Ωb=0.04, H0=100h km/s/Mpc with h=0.7,and a scale-invariant primordial power spectrum with n=1,normalized to σ8(z=0)=0.9.Our adopted value forσ8is a lit-tle larger than the central value favored by3-year con-straints from WMAP(Spergel et al.2006),but uncer-tainties in the cosmological model are sub-dominant to other aspects of our theoretical modeling,which we de-tail subsequently.2.INITIAL CONDITIONSNow,we consider the question:what should we expect for the ionization state of the gas around z 6quasars when they turn on?In this section,we address this issue using a rough analytic model;in the subsequent section we describe3D radiative transfer calculations which ex-amine this in more detail.First,recall that constraints from z 6quasar spectra suggest that the IGM is highly ionized by z 6(e.g. Fan et al.2002),although see§6for a critical discussion. To date,the highest redshift quasar observed is at z q= 6.42.If this source turns on over roughly a Salpeter time(t s∼4×107yrs)before being observed,then its turn-on redshift is z on∼6.66.For the surrounding IGM to be mostly neutral at quasar birth,yet highly ionized by z 6,reionization must proceed extremely rapidly, occurring over a short redshift span of∆z 0.7or∼108yrs.Obviously,reionization must occur even more rapidly if measurements truly indicate that other quasars at slightly lower redshifts with z q 6,(e.g.the z q= 6.28quasar SDSS J1030+0524),are also born when the surrounding IGM is highly neutral.Second,the z 6quasars are thought to reside in very rare and massive host halos(Fan et al.2001,Li et al. 2006),and hence the surrounding IGM is likely overdense out to large scales around the quasars(Loeb&Eisenstein 1995,Barkana2004,Faucher-Gigu`e re et al.2007).These regions should reionize before typical ones since halo col-lapse and galaxy formation are expected to occur earlier in large-scale overdensities(e.g.Barkana&Loeb2004). The abundance of ionizing sources in an overdense re-gion at a given time should resemble the abundance of sources in a typical region at a later time.Even if typi-cal regions in the IGM are neutral when quasars turn on, the same may not be true of the overdense environments where quasars reside(see also Yu&Lu2005).Recom-binations are also more efficient in overdense regions–this could offset the tendency for overdensities to reion-izefirst,but this is unlikely to remove the trend.This is because galaxy formation is more sensitive to large scale overdensity than the recombination rate–indeed, the abundance of high mass halos is exponentially sensi-tive to the large scale overdensity(Barkana&Loeb2004, Furlanetto&Oh2005,Wyithe&Loeb2006b).2.1.Globally-Averaged Ionization FractionsIn this section we illustrate these effects using simple analytic estimates.We perform these calculations with the analytic model of Furlanetto et al.(2004).In its simplest incarnation,this method assumes that a galaxy of mass M gal can ionize a surrounding mass in the IGM of M ion=ζM gal.With this assumption,one can show that the average ionization fraction of gas in a region of radius R and linear overdensityδR is given by:x i|δR,R =ζ(f coll|M min,R,δR).(1) Here,(f coll|M min,R,δR)is the fraction of mass in halos with mass larger than M min in a region of over-densityδR and radius R.In this equation,the impact of recombi-nations is absorbed into the parameterζ.For our simple estimates below we hence ignore the recombination-rate enhancement in over-dense regions.The collapse fraction in an overdense region is given by:(f coll|M min,δc,M,δM)=erfc δc−δM2σ2min−2σ2M,(2)whereδc is the critical overdensity for collapse,M indi-cates the mass scale corresponding to the spatial scale R, andσ2min andσ2M are the variance of the linear density field smoothed on mass scales M min and M,respectively. The global collapse fraction follows from this expression in the limitδM→0,σ2M→0.This equation implies that overdense regions will have larger collapse fractions than regions at the mean density and,under the assumptions of Equation(1),will be ionized earlier.Consder,first,the redshift evolution of the globally-averaged ionization fraction,by takingδR→0and R→∞in Equation(1).The precise duration of reionization depends sensitively on the nature of the sources produc-ing ionizing photons at early times,which is highly uncer-tain.If rare,yet efficient sources reionize the IGM then the collapse and ionization fractions should grow rapidly with redshift(Equations1and2).Furthermore,the du-ration of the reionization epoch depends on the efficiency4of thermal feedback,the time taken to photo-evaporate mini-halos in the IGM,as well as the detailed proper-ties and evolution of the cosmic star-formation rate and the fraction of ionizing photons escaping into the IGM.In our model,all of these details are subsumedinto a single,redshift-independent parameter,ζ(Equation 1).Nevertheless,we can roughly gauge the range of possi-bilities by varying the parameter M min in Equation (2),in each case normalizing ζto match x i =1at z =6.Of course,reionization may finish at significantly higher redshift (i.e., x i =1is reached at z >6);our goal here is to find the maximally neutral case at quasar turn-on provided that the entire volume is reionized by z 6.We consider a wide range of values for M min .On the low end,we adopt M min =M cool ,and on the high end we take M min =103M cool .Here,M cool denotes the dark matter halo mass corresponding to a virial temperature of 104K (M cool =1.3×108M ⊙at z ∼7,e.g.Barkana &Loeb 2001),above which atomic line cooling is effi-cient,allowing gas to cool,condense to form stars,and produce ionizing photons.The higher minimum source mass scenarios approximate models where photo-heating has limited the efficiency of star-formation in small mass halos (Thoul &Weinberg 1996,Navarro &Steinmetz 1997,Dijkstra et al.2004),and models in which su-pernova winds suppress star-formation in low mass halos (e.g.Springel &Hernquist 2003).We note that our high-est minimum source mass model (M min =103M cool )is rather extreme,considerably larger than the suppression masses suggested by the above studies.We consider this case anyway to illustrate a plausible upper limit.We show the redshift evolution of the globally-averaged ionization fraction in the bottom panel of Figure 1.The first qualitative feature apparent from the Figure is that reionization is generally quite extended:for example,in the cooling mass model the average ionization fraction is x i =0.1,0.5,0.7,1.at z ∼12,8,7,and z ∼6re-spectively.If the highest redshift quasar observed thus far,at z q =6.42,turns on at z on ∼6.66as motivated above,the average ionization fraction in this model is ∼80%at turn-on.The process is less extended if the sources are rare,yet very efficient.For example,in the M min =102M cool case,the IGM is only ∼60%ionized at quasar turn on.However,even in the very extreme M min =103M cool scenario,50%of the IGM is ionized by quasar turn on at z ∼6.66.The ionization fraction evolves more rapidly in the high minimum mass models,since here the host halos are still on the exponential tail of the mass-function near z ∼6,and hence their abun-dance evolves quickly with redshift.Note that we likely over-estimate this effect,since we use Press-Schechter (1974)theory for simplicity:the halo abundance from recent high redshift numerical simulations more closely matches the Sheth-Tormen (1999)mass function,indi-cating that Press-Schechter theory underestimates the abundance of rare halos (Reed et al.2003,Heitmann et al.2006,Zahn et al.2006,Lukic et al.2007,although see Trac &Cen 2006).Note that although our constraint argues that the IGM is at least somewhat ionized when the highest redshift quasar observed turns on,it is not in conflict with the formal limits from Wyithe &Loeb (2004),Wyithe et al.(2005a),and Mesinger &Haiman (2004,2006).Furthermore,we have taken ζas a free parameter,sim-Fig. 1.—Redshift evolution of the volume-averaged ionization fraction.Top panel:The black solid line shows the redshift evo-lution of the ionization fraction in a typical region of the IGM,with the ionizing source efficiency calibrated so that x i =1at z =6,assuming all halos down to M cool contribute ionizing pho-tons with equal efficiency.The other curves indicate the average ionization fraction in spherical regions of different size,each with a massive quasar host (here M dm ,host =1013M ⊙)at the center.The vertical blue dotted line indicates a plausible turn-on time for a z =6.42quasar.The curves illustrate that:i)the process of reionization takes a sufficiently long time that,even if it completes at z =6,the typical location in the IGM is already ∼80%ion-ized (in this model)when the z =6.42quasar turned on;and ii)provided that quasar host halos reside in highly overdense regions,their surroundings will reionize earlier than the typical region in the IGM.Bottom panel:The redshift evolution of the volume-averaged ionization fraction for a typical region as a function of the minimum host halo mass.If rare,very efficient,sources pro-duce most of the ionizing photons,reionization occurs more rapidly than if highly abundant yet less efficient sources produce most of the ionizing photons.In each model,more than 50%of the IGM volume is ionized at quasar turn-on.ply fixing it to match x i =1at z ∼6.In fact,if all of the ionizing photons are indeed produced by the very rare halos with M M min =103M cool ,the sources need to be exceedingly efficient to reionize the IGM by z 6.Specifically,we estimate the number of ioniz-ing photons per stellar baryon required in this model to achieve x i =1by z =6,using Equations (82)and (83)of Furlanetto et al.(2006a).We adopt typical values of three recombinations per ionized hydrogen atom,an escape fraction of f esc =0.1,and a star-formation effi-ciency of f ⋆=0.1,and find that N γ∼70,000ionizing photons per baryon are required in this scenario.This ionizing efficiency is substantially larger than the value expected for a Salpeter IMF,N γ∼4,000(e.g.Cohn &Chang 2006),although it is achievable if the IMF is very top-heavy (e.g.Bromm et al.2001)even at z ∼7in spite of apparently wide-spread metal enrichment by z ∼6(Ryan-Weber et al.2006).Finally,in §6we argue that it is conceivable that x i =1is achieved later than z =6–reionization5might end at as low a redshift as z∼5.5.Even if reionization completes only by z=5.5wefind that the neutral fraction at quasar turn-on is less than50%in all of our models except our extreme M min=103M cool model.Moreover,reionization is likely more extended than in our model–for example,thermal feedback and mini-halos can extend the duration of reionization com-pared to our simple predictions(e.g.Haiman&Holder 2003,McQuinn et al.2006a).Therefore,our simple es-timates indicate that the IGM is unlikely as much as 50%neutral at quasar turn-on and is more likely at least 70%ionized by this time.These rough estimates are inevitably model dependent,but are in accord with ob-servational constraints from Lyαemitters(Malhotra& Rhoads2005,Haiman&Cen2005,Dijkstra et al.2006), and GRB optical afterglow spectra(Totani et al.2006) which coincidentally give similar upper limits for the neu-tral fraction near the plausible turn-on redshifts of the z 6quasars.2.2.The Ionization Field Around Quasar ProgenitorHalosThe above calculations apply to typical regions in the IGM.We now extend our analysis to consider the ion-ization of the overdense environments expected around high redshift quasars.We will compute the probability distribution of the ionized fraction spherically-averaged around plausible quasar host halos,prior to quasar turn-on.Here,our calculation is similar to previous work by Yu&Lu(2005),but it differs in detail.Wefirst consider the linear overdensity profiles around the z∼6quasars following Loeb&Eisenstein(1995)and Barkana(2004). We will subsequently insert the linear density profile into Equation(1).Thefirst quantity of interest is the cross-correlation coefficient,ρr,R,between densityfluctuations at a single point when smoothed on two different scales, r and R.The correlation coefficient is related to the co-variance,σ2r,R,and individual variances,σ2r,σ2R,by the relationρr,R=σ2r,R/(2π)3W(kr)W(kR)P(k).(3)If the window functions in this equation are(spheri-cally symmetric)top-hatfilters in k-space then the co-variance is precisely equivalent to the variance on scale R –i.e.,σ2r,R=σ2R,and the correlation coefficient is given byρr,R=σR/σr.The co-variance will be a little differ-ent if the window functions are top-hats in real space.In spite of this,for simplicity we adopt the sharp k-space correlation coefficient even though we calculateσR and σr using a real-space top hat,as is frequently done in Press-Schechter type calculations.In what follows,we ignore the‘cloud-in-cloud’problem of Press-Schechter theory,and do not include an‘ab-sorbing barrier’in our calculation(Bond et al.1991). Barkana(2004)constructs a more elaborate model for the initial overdensity profile around massive halos that is consistent with extended Press-Schechter theory.How-ever,in the limit of the very rare halos we consider here, Barkana(2004)shows that this more elaborate model reduces to our present one,which we hence adopt for simplicity.Assuming the densityfield is a bi-variate Gaussian with the correlation coefficient given above,we can immediately write down an expression for the de-sired conditional probability distribution:we would like to know the differential probability that a point is at a linear overdensity˜δR when smoothed on a scale R at some redshift z1,given that it is at linear overdensity˜δc when smoothed on a smaller scale,r at redshift z2.This expression is:P(δR,σR,z1|δc,σr,z2)=12πσr2σ2r−2σ2Rexp δ2c2σ2R .(4)In this equation,all quantities are linearly-extrapolated to the present day.Hence,if˜δR is the linear overdensity at smoothing scale R and redshift z1,and D(z1)is the linear growth factor normalized to unity today,thenδR=˜δR/D(z1)is the linear overden-sity on scale R today.If we takeδc to be the critical overdensity for collapse scaled to the present day,then this equation tells us the conditional probability that a region will have a large scale overdensity,δR on scale R, given that the region contains a massive halo–i.e.,the region reaches the collapse threshold,δc,at a smaller smoothing scale,bining Equations(1),(2),and (4),and computing the Jacobian|dx i/dδR|−1,we can determine the desired ionization probability distribution. The ionization fraction,averaged over an ensemble of quasar host halos,just follows from determining the mean of the resulting ionization probability distribution. Note that in this calculation we neglect sources outside the region of interest,but since we are interested in rather large smoothing scales,this is probably not too poor an approximation.Our results for the average ionization in regions that will host a massive quasar host halo at z2=z q=6.42 are shown in the top panel of Figure1.Here we as-sume that z 6quasars reside in1013M⊙halos as in Haiman&Loeb(2001)and Li et al.(2006),although this choice is uncertain.Our simulation results in sub-sequent sections assume quasars reside in slightly less massive and more common halos.For the present calcu-lation we adopt our model in which all host halos down to M cool contain ionizing sources and contribute to reion-ization.Figure1demonstrates that a significant volume of gas around quasar progenitors is generally reionized well before reionization completes in a typical region of the IGM.Indeed,∼100%of the volume in a sphere of ra-dius15co-moving Mpc centered on the z=6.42quasar progenitor is ionized in our model,on average,by z 7,∼50Myr before the quasar turns on.As one averages over progressively larger volumes,the mean interior over-density approaches the cosmic average,and the ioniza-tion approaches its cosmic mean value.Indeed,thefigure indicates that spheres of radius35−45Mpc/h appear to be essentially representative samples.Given that the purported proximity zone size is∼40co-moving Mpc/h (e.g.Wyithe et al.2005a),one might conclude that this bias is hence unimportant for interpreting quasar prox-imity zone measurements.In fact,we will show in the next section that1D skewers through the IGM towards。
