中智联合博士后揭开宇宙黑暗时代神秘面纱

中科院南美天文中心(中智中心)第一期中智联合博士后、现上海天文台研究员、中科院“率先行动”百人计划青年俊才候选人郑振亚博士参加的超高红移窄带巡天项目:“宇宙再电离时期的莱曼阿尔法星系”(简称LAGER)取得突破性进展。此项目得到了中智中心第一期联合研究基金为期两年的支持。郑振亚博士在智利从事中智联合博士后工作期间,作为中智天文合作的一个重要项目,申请到CTIO 4米望远镜时间10晚,麦哲伦时间4晚,获取了大量高质量的观测数据,为此项目的开展和成功奠定了坚实的基础。

该项目由科大王俊贤老师提出,郑振亚博士作为共同组织者和主要参与人,负责申请望远镜时间、观测数据处理和分析等具体工作,通过中国,智利,美国的合作,正在借助智利的CTIO 4米口径光学望远镜和自己定制的窄带滤光片(中心波长大约9640Å,宽度约95Å)对四个天区进行窄带巡天,寻找宇宙年龄8亿年左右(红移7左右)的莱曼阿尔法发射线星系(Lyman alpha emitting galaxies)。目前已经完成的第一个天区的观测结果即给出了目前世界上最大的红移7 Lyman alpha星系样本,并由此发现在该宇宙年龄处,宇宙星系际弥散介质中氢的电离比例为约50%。这一结果已经发表在国际一流天文杂志ApJL上,美国国家光学天文台以“遥远的星系揭开宇宙黑暗时代末期的面纱”为题专门撰文报道了此项研究突破。研究同时被美国天文学会AAS Nova Journals Digest栏目推荐介绍。


图1:宇宙演化示意图,时间是从左到右演化。左边的亮点是大爆炸,也就是现有宇宙学理论认为的宇宙开端。中间蓝色的区域是本文介绍的黑暗时期,此时宇宙中大量的中性氢会吸收大量可见光使得我们很难看到这个时期的星系。直到大部分的中性氢被来自恒星或星系的紫外辐射电离成质子和电子后,宇宙才显得“透明”起来。图中黄色方框区域体现的是宇宙的再电离的完成时间。从这个示意图我们可以看到再电离是一个逐步完成的结果,并且星系的紫外辐射会电离出一个个的空泡,而不是一个均匀的电离过程。郑振亚的结果支持了这一理论猜测。图片来源:https://www.noao.edu/news/2017/img/distant-universe.jpg

我们的宇宙在大爆炸后约三十万年时进入了一个黑暗时代(图1中的蓝色区域)。此时的宇宙充满了大量的中性氢。在引力的作用下,一部分中性氢会形成恒星甚至星系。可是中性氢大量吸收了恒星和星系发出的可见光,导致我们几乎探测不到这一时期任何天体,显得十分“黑暗”。这团中性氢像雾霾一样阻止了天文学家看清楚宇宙黑暗时代的星系。直到恒星形成释放的紫外光子把此时宇宙里的中性氢电离为质子和电子后,宇宙才终于对可见光“透明”了起来,这一过程叫做宇宙的再电离。

图2:图中白框指示的是郑振亚团队发现的23个Lyman alpha星系候选体位置。右边是最亮的两个源放大后的图像。图片来源:https://www.noao.edu/news/2017/img/LAGER-full-composite.jpg

宇宙何时从黑暗时代进入再电离时代,不但是宇宙学的基本的问题,也是星系形成演化,恒星形成等领域的重要前沿问题。关于再电离的一个可行的研究方法是寻找高红移的恒星形成星系,尤其是莱曼阿尔法发射线星系。越古老的莱曼阿尔法发射线星系,越能帮助我们理解宇宙再电离的细节。

红移7左右的发射线星系非常稀有。尽管窄波段巡天的方法可以有效的从图像上分辨出发射线星系候选体,目前天文学家已经在红移6.6处发现了近百个发射线星系,但是在红移7左右的相关研究一直进展缓慢,在2006年到2012年间,天文学家发现了20多个红移7的星系候选体,只有3个星系被确认处于这一红移时期。此巡天项目的第一篇文章就给出了23个红移7的Lyman alpha 星系候选体,成为这一领域最大的样本。后续光谱观测表明样本中至少70%的星系被确认处于这一时期(Hu et al. 2017)。进一步的分析表明,在宇宙年龄8亿年到10亿年的过程中,亮的Lyman alpha星系数量变化不大,而暗的星系变化显著(图3)。这一方面表明宇宙再电离在红移7左右并未结束,也表明电离过程可能不是均匀发生在宇宙各个区域,正在被电离的宇宙也许充满了一个个被电离的泡泡(参见图1的黄色方框区域的示意)。

图3:高红移宇宙Lyman alpha星系光度函数。郑振亚博士的工作是图中的红色原点。可以看到红移7左右Lyman alpha星系光度函数是分段的。相对红移6.6的时期,红移7左右Lyman alpha星系在暗端有强烈演化,而亮端演化不大。图片来自这一巡天项目的文章,第一作者是中智中心博后郑振亚。

科大王俊贤老师发起的这个高红移巡天项目开始于2012年。这些年来,这一项目团队系统的开展了设计定制滤光片,申请观测,光谱认证等,这一项目是世界上该红移处唯一的大天区巡天。这个项目以极高的效率和成功率前所未有的提高了在红移7左右的Lyman alpha星系样本数量。考虑到这个样本还只是LAGER巡天项目四个天区中的一个天区的结果,可以预见这一项目还会向我们展示更多早期宇宙的故事。

图4: Materion公司定制的窄带滤光片,正要被装到暗能量相机上。这个滤光片的中心波长是9640Å,宽度大约95Å。这一设计是为了找到红移7的发射线星系的同时避开大气的OH线污染。图片来源:http://astro.ustc.edu.cn/news/201603/W020160314319626554087.jpg

该论文第一作者为上海天文台郑振亚研究员(中国科大2012届博士毕业生,中智中心第一期博后),同时也是LAGER项目的共同组织者。这一项目得到了中智中心第一期联合研究基金项目的支持。

相关文章:
http://iopscience.iop.org/article/10.3847/2041-8213/aa794f
https://arxiv.org/pdf/1703.02985.pdf
https://arxiv.org/pdf/1706.03586.pdf

参考文献:
https://www.noao.edu/news/2017/pr1703.php
http://astro.ustc.edu.cn/news/201603/t20160314_238496.html
http://www.ctio.noao.edu/noao/content/Properties-N964-filter
http://news.ustc.edu.cn/xwbl/201707/t20170712_280146.html

中智联合博士后Claudio Ricci发现并合星系中的黑洞被“雾霾”笼罩

This illustration compares growing supermassive black holes in two different kinds of galaxies. A growing supermassive black hole in a normal galaxy would have a donut-shaped structure of gas and dust around it (left). In a merging galaxy, a sphere of material obscures the black hole (right).

Credits: National Astronomical Observatory of Japan

Black holes get a bad rap in popular culture for swallowing everything in their environments. In reality, stars, gas and dust can orbit black holes for long periods of time, until a major disruption pushes the material in.

A merger of two galaxies is one such disruption. As the galaxies combine and their central black holes approach each other, gas and dust in the vicinity are pushed onto their respective black holes. An enormous amount of high-energy radiation is released as material spirals rapidly toward the hungry black hole, which becomes what astronomers call an active galactic nucleus (AGN).

A study using NASA’s NuSTAR telescope shows that in the late stages of galaxy mergers, so much gas and dust falls toward a black hole that the extremely bright AGN is enshrouded. The combined effect of the gravity of the two galaxies slows the rotational speeds of gas and dust that would otherwise be orbiting freely. This loss of energy makes the material fall onto the black hole.

“The further along the merger is, the more enshrouded the AGN will be,” said Claudio Ricci, lead author of the study published in the Monthly Notices Royal Astronomical Society. “Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust.”

Ricci and colleagues observed the penetrating high-energy X-ray emission from 52 galaxies. About half of them were in the later stages of merging. Because NuSTAR is very sensitive to detecting the highest-energy X-rays, it was critical in establishing how much light escapes the sphere of gas and dust covering an AGN.

The study was published in the Monthly Notices of the Royal Astronomical Society. Researchers compared NuSTAR observations of the galaxies with data from NASA’s Swift and Chandra and ESA’s XMM-Newton observatories, which look at lower energy components of the X-ray spectrum. If high-energy X-rays are detected from a galaxy, but low-energy X-rays are not, that is a sign that an AGN is heavily obscured.

The study helps confirm the longstanding idea that an AGN’s black hole does most of its eating while enshrouded during the late stages of a merger.

“A supermassive black hole grows rapidly during these mergers,” Ricci said. “The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy.”

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory for NASA’s Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR’s mission operations center is at UC Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror archive. JPL is managed by Caltech for NASA.

For more information on NuSTAR, visit:

http://www.nasa.gov/nustar

http://www.nustar.caltech.edu

转载自https://www.nasa.gov/feature/jpl/merging-galaxies-have-enshrouded-black-holes

国家天文台“极端”自适应光学仪器在智利完成客座观测

7月,中科院国家天文台南京天光所(简称天光所)研制的“极端”自适应光学系统(Extreme Adaptive Optics,简称:Ex-AO)作为客座仪器,成功对接欧洲南方天文台(ESO)3.6米新技术望远镜(NTT),圆满完成了测试观测。该套Ex-AO系统在近红外H波段获得了衍射极限成像,表明具备了用于系外行星科学成像观测的能力。 Continue reading