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[ESO1138]天文学家首度建立宇宙再游离时期的变化时程表

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叶程婉 发表于 2011-10-15 21:16 | 显示全部楼层 |阅读模式 来自: 中国–广东–佛山–顺德区 电信

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英文原贴地址 :http://www.eso.org/public/news/eso1138/
中文翻译引自 :http://pansci.tw/archives/8244


                               
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Scientists have used ESO’s Very Large Telescope to probe the early Universe at several different times as it was becoming transparent to ultraviolet light. This brief but dramatic phase in cosmic history — known as reionisation — occurred around 13 billion years ago. By carefully studying some of the most distant galaxies ever detected, the team has been able to establish a timeline for reionisation for the first time. They have also demonstrated that this phase must have happened quicker than astronomers previously thought.
An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang [1].
The new observations have allowed astronomers to establish a timeline for what is known as the age of reionisation [2] for the first time. During this phase the fog of hydrogen gas in the early Universe was clearing, allowing ultraviolet light to pass unhindered for the first time. The new results, which will appear in the Astrophysical Journal, build on a long and systematic search for distant galaxies that the team has carried out with the VLT over the last three years.
“Archaeologists can reconstruct a timeline of the past from the artifacts they find in different layers of soil. Astronomers can go one better: we can look directly into the remote past and observe the faint light from different galaxies at different stages in cosmic evolution,” explains Adriano Fontana, of INAF Rome Astronomical Observatory who led this project. “The differences between the galaxies tell us about the changing conditions in the Universe over this important period, and how quickly these changes were occurring.”
Different chemical elements glow brightly at characteristic colours. These spikes in brightness are known as emission lines. One of the strongest ultraviolet emission lines is the Lyman-alpha line, which comes from hydrogen gas [3]. It is bright and recognisable enough to be seen even in observations of very faint and faraway galaxies.
Spotting the Lyman-alpha line for five very distant galaxies [4] allowed the team to do two key things: first, by observing how far the line had been shifted toward the red end of the spectrum, they were able to determine the galaxies’ distances, and hence how soon after the Big Bang they could see them [5]. This let them place them in order, creating a timeline which shows how the galaxies’ light evolved over time. Secondly, they were able to see the extent to which the Lyman-alpha emission — which comes from glowing hydrogen within the galaxies — was reabsorbed by the neutral hydrogen fog in intergalactic space at different points in time.
“We see a dramatic difference in the amount of ultraviolet light that was blocked between the earliest and latest galaxies in our sample,” says lead author Laura Pentericci of INAF Rome Astronomical Observatory. “When the Universe was only 780 million years old this neutral hydrogen was quite abundant, filling from 10 to 50% of the Universe’ volume. But only 200 million years later the amount of neutral hydrogen had dropped to a very low level, similar to what we see today. It seems that reionisation must have happened quicker than astronomers previously thought.”
As well as probing the rate at which the primordial fog cleared, the team’s observations also hint at the likely source of the ultraviolet light which provided the energy necessary for reionisation to occur. There are several competing theories for where this light came from — two leading candidates are the Universe’s first generation of stars [6], and the intense radiation emitted by matter as it falls towards black holes.
"The detailed analysis of the faint light from two of the most distant galaxies we found suggests that the very first generation of stars may have contributed to the energy output observed," says Eros Vanzella of the INAF Trieste Observatory, a member of the research team. "These would have been very young and massive stars, about five thousand times younger and one hundred times more massive than the Sun, and they may have been able to dissolve the primordial fog and make it transparent."
The highly accurate measurements required to confirm or disprove this hypothesis, and show that the stars can produce the required energy, require observations from space, or from ESO’s planned European Extremely Large Telescope, which will be the world’s largest eye on the sky once completed early next decade.
Studying this early period in cosmic history is technically challenging because accurate observations of extremely distant and faint galaxies are needed, a task which can only be attempted with the most powerful telescopes. For this study, the team used the great light-gathering power of the 8.2-metre VLT to carry out spectroscopic observations, targetting galaxies first identified by the NASA/ESA Hubble Space Telescope and in deep images from the VLT.
Notes
[1] The most distant galaxy that has been reported with a distance measured by spectroscopy is at a redshift of 8.6, placing it 600 million years after the Big Bang (eso1041). There is a candidate galaxy thought to be at a redshift of about 10 (480 million years after the Big Bang) identified by the Hubble Space Telescope, but this is awaiting confirmation. The most distant galaxy in this study is at a redshift of 7.1, placing it 780 million years after the Big Bang. The Universe today is 13.7 billion years old. The new sample of five confirmed galaxies with Lyman-alpha detections (out of 20 candidates) includes half of all galaxies known at z>7.
[2] At the time the first stars and galaxies formed, the Universe was filled with electrically neutral hydrogen gas, which absorbs ultraviolet light. As the ultraviolet radiation from these early galaxies excited the gas, making it electrically charged (ionised), it gradually became transparent to ultraviolet light. This process is technically known as reionisation, as there is thought to have been a brief period within the first 100 000 years after the Big Bang in which the hydrogen was also ionised.
[3] The team measured the effects of the hydrogen fog using spectroscopy, a technique which involves splitting and spreading out the light from the galaxy into its component colours, much like a prism splits sunlight into a rainbow.
[4] The team used the VLT to study the spectra of 20 candidate galaxies at redshifts close to 7. These come from deep imaging studies of three separate fields. Of these 20 targets five were found to have clearly detected Lyman-alpha emission. This is currently the only set of spectroscopically confirmed galaxies around z=7.
[5] Because the Universe is expanding, the wavelength of light from objects gets stretched as it passes through space. The further light has to travel, the more its wavelength is stretched. As red is the longest wavelength visible to our eyes, the characteristic red colour this gives to extremely distant objects has become known as ‘redshift’. Although it is technically a measure of how the colour of an object’s light has been affected, it is also by extension a measure both of the object’s distance, and of how long after the Big Bang we see it.
[6] Astronomers classify stars into three categories, known as Population I, Population II and Population III. Population I stars, like our Sun, are rich in heavier elements synthesised in the hearts of older stars and in supernova explosions: as they are made up from the wreckage of previous generations of stars, they only came into existence later in the Universe. Population II stars have fewer heavy elements in them and are predominantly made up of the hydrogen, helium and lithium created during the Big Bang. These are older stars, though there are still many of them in existence in the Universe today. Population III stars have never been directly observed, though they are thought to have existed in the early years of the Universe. As these contained only the material created during the Big Bang, they contained no heavier elements at all. Because of the role of heavier elements in the formation of stars, only very large stars with very short lifespans were able to form at this stage, and so all the Population III stars quickly ended their lives in supernovae in the early years of the Universe. Up to now, no solid evidence of Population III stars has been confirmed even in observations of very distant galaxies.
More information
This research was presented in a paper “Spectroscopic Confirmation of z∼7 LBGs: Probing the Earliest Galaxies and the Epoch of Reionization”, to appear in the Astrophysical Journal.
The team is composed of L.Pentericci (INAF Osservatorio Astronomico di Roma, Rome, Italy [INAF-OAR]),  A. Fontana (INAF-OAR), E. Vanzella (INAF Osservatorio Astronomico di Trieste, Trieste, Italy [INAF-OAT]), M. Castellano (INAF-OAR), A. Grazian (INAF-OAR), M. Dijkstra (Max-Planck-Institut für Astrophysik, Garching, Germany), K. Boutsia (INAF-OAR), S. Cristiani (INAF-OAT), M. Dickinson (National Optical Astronomy Observatory, Tucson, USA), E. Giallongo (INAF-OAR), M. Giavalisco (University of Massachusetts, Amherst, USA), R. Maiolino (INAF-OAR), A. Moorwood (ESO, Garching), P. Santini (INAF-OAR).
ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive astronomical observatory. It is supported by 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning a 40-metre-class European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.
Links
Research paper
Photos of the VLT
Contacts
Dr. Laura Pentericci
INAF Rome Astronomical Observatory
Rome, Italy
Tel: +39 06 94 286 450
Email: laura.pentericci@oa-roma.inaf.it
Dr. Adriano Fontana
INAF Rome Astronomical Observatory
Rome, Italy
Tel: +39 06 94 286 456
Email: adriano.fontana@oa-roma.inaf.it
Richard Hook
ESO, La Silla, Paranal, E-ELT and Survey Telescopes Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Email: rhook@eso.org



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 楼主| 叶程婉 发表于 2011-10-15 21:22 | 显示全部楼层 来自: 中国–广东–佛山–顺德区 电信
中文翻译: http://pansci.tw/archives/8244
•        INAF罗马天文台(INAF Rome Astronomical Observatory)天文学家Adriano Fontana等人,利用欧南天文台(ESO)超大望远镜(Very Large Telescope)探索早期宇宙中,对紫外光而言变透明的阶段,即所谓的「再游离时期(reionisation)」。再游离时期大约发生于130亿年前;藉由小心测量一些最遥远星系的状况,这些天文学家首度能建立再游离过程的时程表,且证明这个阶段必定发生得比天文学家原本设想的还快。相关论文发表在天文物理期刊(Astrophysical Journal)中。
这些天文学家耗费3年时间所测量的这些迄今已知最遥远星系,大都位在大霹雳后约7亿8000万年到10亿年之间的时期中。距今130亿年前的再游离时期,原本电中性的氢雾被星系或恒星所发出的辐射游离而变成离子,让紫外光可以穿透而抵达地球,地球上的观察者才能透过望远镜观察了解该时期发生了什么事件。
Fontana表示:考古学家从不同土层中发现的人造物品,可以重建某个历史时期的概貌。天文学家的工作也是类似,但更好,因为可以藉由直接观察不同宇宙时期中不同星系所发出的微光而直接看到遥远的过去。星系之间的差异性可以告知我们宇宙在各个重要时期的变化状况及变化速度。
由于不同化学元素所发出的光谱特征不同,从各元素固定的发射谱线可以反过来鉴定该天体含有哪种化学元素。其中一条最强、最亮的紫外发射谱线就是氢原子的莱曼-Alpha谱线(Lyman-alpha line),因此即使在遥远又昏暗的星系光谱上,也通常可以鉴别出该谱线的存在。
Fontana等人研究20个红移值大于7的遥远星系后,在其中5个最遥远的星系谱线中确认出莱曼-Alpha谱线特征,让他们可以从事两个关键工作:第一,从莱曼-Alpha谱线往红光部分偏移的大小(即所谓的红位移),可以估算出这些星系的距离,再以之估算大霹雳之后多久可以见到这些星系;从这些资料,天文学家就可以建立一个星系发出的光线如何随时间演化的时程表。
第二,不同时间点的星系内氢离子的莱曼-Alpha谱线,都有被星系际空间中的中性氢气吸收的状况,在Fontana等人所研究的星系中,最早的星系(红移值7.1)和最晚的星系,紫外光被遮蔽的比例差异颇大。例如:大霹雳之后仅7亿8000万年的最早星系周围,中性氢气非常丰富,占了当时宇宙整体体积的10~50%左右;但在2亿年之后,中性氢的比例降到非常低的程度,与今日差不多。所以看来似乎再游离发生个过程必定比天文学家之前想的还要快很多。
除了探索氢雾被清除的速率外,Fontana等人的观测也显示紫外光应是使再游离发生的主要能量来源。目前有多种讨论使再游离发生的光线来源的理论,其中最可能的来源是宇宙的第一代恒星(星族III,Population III),另一种较可能的来源是物质落入黑洞过程中所发出的强烈辐射。(注:星族III恒星指几乎不含氢与氦以外重元素的恒星,基本上只有在宇宙非常早期时才存在,不过迄今不曾观测到星族III的恒星过。)
而仔细研究其中两个迄今已知最远星系所发出的昏暗光线后,Fontana等人认为宇宙第一代恒星应该是再游离能量来源的首选。因为第一代恒星应该都是非常年轻的大质量恒星,比我们的太阳还年轻5000倍以上,质量更可能高达太阳的100倍以上,所以它们所发出的辐射绝对强烈到足以使氢雾再游离,让宇宙变透明。
这些天文学家计划在ESO下一代的新望远镜—欧洲极大望远镜(European Extremely Large Telescope,E-ELT)完成后,利用E-ELT继续确认他们的这个理论假说是否正确。
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 楼主| 叶程婉 发表于 2011-10-15 21:23 | 显示全部楼层 来自: 中国–广东–佛山–顺德区 电信
本帖最后由 叶程婉 于 2011-10-15 21:24 编辑


                               
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gohomeman1 发表于 2011-10-15 22:40 | 显示全部楼层 来自: 中国–浙江–宁波 电信
这周ESO更新的内容实在多,我貌似欠账越来越多了

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靠一个人的力量哪里能忙得过来啊。  详情 回复 发表于 2011-10-16 12:29
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 楼主| 叶程婉 发表于 2011-10-16 12:29 | 显示全部楼层 来自: 中国–广东–佛山–顺德区 电信
gohomeman1 发表于 2011-10-15 22:40
这周ESO更新的内容实在多,我貌似欠账越来越多了

靠一个人的力量哪里能忙得过来啊。

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