‘快闪’星系 Galaxies in a Flash

星系，像我们的银河系，是由恒星、气体、尘埃和暗物质在引力作用下聚集在一起的巨大的宇宙岛屿。

Galaxies, like our Milky Way, are vast cosmic islands of stars, gas, dust, and dark matter held together by gravity. 

一个星系中平均有几千亿颗恒星，那就已经大得让我们头晕了，但在我们的宇宙中还有2万亿个这样的星系。而且这只是现在可以观测到的，随着技术的发展，会有越来越多的星系出现。

With an average of a few hundred billion stars in a galaxy, they are huge enough to get ours heads around, but out there are 2 trillion more of them in our universe, and that’s just the ones estimated observable, as technology develops, more and more will show up.

图为哈勃望远镜拍到的约46亿光年外的星系们
Image of clusters of galaxies ~4.6 billion light years away taken by Hubble

在这么多的星系中，有许多像我们的星系，但也有许多大不相同的。对大多数星系来说，在他们形成后，至少需要几十亿年的时间，才能在结构上发生明显的、可测量的变化。根据标准宇宙模式(ΛCDM)，通常大规模的星系都是由较小部分组成的，这也被称为层次发展。

With such large number of galaxies, there are many like ours but a range of others vastly different. For the majority of galaxies, there will be at least several billion years after their formation before having dramatic, measurable changes to their population and morphology (galaxy’s structural property). According to the standard cosmological paradigm (ΛCDM), the most massive structures are normally form as a result of smaller constituents building up, also known as hierarchical growth.

然而，最近的研究发现，一些星系出奇得没有遵循这个规律。

However recently, studies have some galaxies that surprising didn’t follow the paradigm.

在大爆炸后的最初20亿年里，这些星系居然能够积累到难以置信质量（超过太阳质量的1000亿倍），同时在那期间耗尽了它们所有的气体储存，然后就此沉寂。也就是说，星系中不再会有恒星形成。

These ones were able accumulate incredible stellar masses (more than a hundred billion times Solar Mass) within the first 2 billion years after the big bang, while also using up all their gas reservoirs and becoming quiescent. This means, stars are no longer forming in those galaxies. 

相比较，银河系现在已经大约136亿岁了，并还在以相当于每年一个太阳速度产生恒星。所以，这些形成并进化得如此迅速，还耗尽气体供应了星系必定曾经拥有一段“疯狂”的生命周期。

Bearing in mind that the Milky Way is now ~13.6 billion years old and continues to from the equivalence of one Sun every year. So, these massive galaxies must have had a ‘crazy’ lifetime forming, evolving and exhausting the gas supplies so quickly. 

这里，我们将探索这些暂时沉睡的猛兽的神秘世界，了解它们是如何被创造出来的。

Here, we dive into the mysterious world of these enormous quiescent creatures, to learn how and why they were created.

图为NASA用哈勃望远镜收集8对星系合并
8 galaxy mergers collected by NASA from Hubble's data

星系复杂性

Galaxy Complexity

尽管星系的种类繁多，但只要对单个星系中少数具有代表性的成员进行详细分析，我们就有足够的信息来了解整个星系了。对于类似大质量静止星系（MQG）这样的稀有星系群来说，对一个性质的研究就足够产生许多深刻的发现了，从而激发更多这方面的研究。

Although there is a huge diversity of galaxies, only detailed analysis on a few representative members of a single galaxies would be enough to learn about an entire galactic population. With the rare groups, such as the massive quiescent galaxies (MQG), study on a single property would be enough to have many insightful results, sparking for more research. 

因此，为了更多地探索这些奇异星系的基本性质，Paolo Saracco和他在米兰的团队，决定深入的研究一下之前发现的，名为C1-23152的，MQG的主要性质。

So, in order to explore more on the general properties of these exotic ones, Paolo Saracco and his team, based in Milano, decided to study the key properties of the previously discovered MQG named C1-23152.

图为美国国家射电天文台的MQG的模拟图像
Illustration of a MQG from the National Radio Astronomy Observatory of United States

这个星系它只有19亿岁的时候在z = 3.35，这里z是代表着红向移动的大小。之前，不同研究所对这个星系也曾研究过，并使用了各种成像方法，其中包括用哈勃太空望远镜来测量结构属性的数据，以及用光谱学来确认红移和其他基本特征。

This galaxy is located at z = 3.35 when it was only 1.9 billion years old, where z is the magnitude of redshift. Previous study of this galaxy used to number imaging methods, including data from the Hubble Space Telescope to measure structural properties, and spectroscopy to confirm the redshift and other basic features.

但在Saracco和他的团队的新研究中，他们使用了从亚利桑那州的大型双目望远镜(LBT) 获得的详细近红外光谱。目标是利用这一光谱确定C1-23152的星系年龄、金属丰度和速度弥散度，同时从这些信息中获得星系中恒星的形成历史。

In this recent study, Saracco and the team used the Large Binocular Telescope (LBT) in Arizona to obtain a detailed near-infrared spectrum. The goal was to use this spectrum to conclusively establish C1-23152’s stellar age, metallicity and velocity dispersion, while also obtaining the star-formation history from the information.

图表为从大型双目望远镜获得的C1-23152光谱
The LBT spectrum of galaxy C1-23151

测量结果

The Measurements

几乎所有的星系物理特性都被编码在它们产生的光波中。因此，研究的关键就是从光度学和光谱学的结果往回推算。利用LBT的高质量光谱，Saracco和他的团队同时提取了吸收线拟合(ALF)和全光谱拟合(FSF)。他们将这两种数据与一系列已知的星系物理属性（如恒星年龄、金属丰度和质量）的合成模型进行了比较。

Almost all physical properties of galaxies are encoded in the light they emit. Therefore, the key to this investigation is to work backwards from photometry and spectroscopy results obtained. Using the high quality Large Binocular Telescope spectrum, the team extracted both absorption line fitting (ALF) and full spectrum fitting (FSF).  In both cases, they compared the data to a range of synthesis models with known physical properties like the stellar age, metallicity and mass. 

团队还利用了UltraVISTA的光度法数据进行了标准光谱能量分布(SED)的建模，揭示了星系中恒星的形成以及恒星和气体的布局。
The team also performed the standard spectral energy distribution (SED) modelling with the photometry from the UltraVISTA survey, revealing the star formation and geometrical arrangement of stars and gases in the galaxy.

图为欧洲南方天文台（ESO）的VISTA望远镜外观
The outside of European Southern Observatory's (ESO) VISTA telescope 

经过研究，C1-23152确定是一个早期类型的静止星系，其质量是太阳的2千亿倍，还有着一个活跃星系核。它的形态形成于观测前的6亿年，z ~ 4.6的时候。恒星群形成是在观测前的1.5到6亿年之间，在1.5亿年前恒星形成逐渐到达了尾声。在高峰时期，每年出现的恒星总质量是太阳质量的400多倍。

Indeed, C1-23152 is found to be an early-type quiescent galaxy hosting an active galactic nucleus and assembled a mass of 2 hundred billion times the Sun. Its morphology was shaped within ~600 million years prior the observation, since z ~ 4.6. The stellar population is formed between ~600 to ~150 million years before the observed epoch, with the latter being the time since star formation coming to an end. During the peak time, more than 400 times the Solar Mass worth of stars appeared every year.

归纳推理的魅力

The Magic of Inductive Reasoning

那么这些发现是如何影响我们对大量静止星系的认识的呢?
So how do these findings affect our knowledge of the massive quiescent galaxies in general?

ALF和FSF的数据显示的快速形成，以及形态学上看到的高表面质量密度，暗示了一种新的星系形成周期过程。虽然从一开始就耗散了气体能力储存，但它的形成免去了小星系之间的合并，因为合并来的大星系会有着比这里低很多的表面质量密度。

The rapid formation suggested by the ALF and FSF together with the high surface mass density seen from its morphology, have suggested a new cycle of galaxy buildings. Although dissipative from the start, it did not require mergers between smaller ones, as this will result in much lower surface mass density than seen here.

图为NASA哈勃望远镜拍摄的两个融合中的星系

Two galaxies merging taken by NASA's Hubble

而且，比太阳高的的金属丰度也促成了恒星的耗散速度的增长，以及星系的活跃星系核也影响了星系的命运，尽管活跃星系核是如何影响星系快速形成的原因还有待研究。

The greater-than-solar metallicity also contribute to fast, dissipative growth of stars, as well as the active galactic nucleus, though it is not entirely clear yet on how it affects the quick formation yet.

但不管怎么样，正如Saracco和他的团队在他们的文献中解释的那样，”这项研究表明，它们在早期宇宙中巨大星系的快速熄灭过程中发挥了作用。”而它们肯定是在早期宇宙形成的原因是：它们的高红移。因为是影响质量密度的最重要的因素之一，密度最高的那些星系只有可能产生于早期宇宙中。

However, as Saracco and his team explained in their article: ‘this study suggests that they can play a role in the very fast quenching process of massive galaxies in the early Universe’. The reason for them must the earlier ones because their high magnitudes of redshift, which would be the most important factor affecting the mass density, hence, the densest ones are only expected in the early universe.

虽然宇宙中依然有很多的未知性，但这次Saracco和他的团队对C1-23152的研究还是提供了，许多对早期宇宙中星系形成和成长，的新认识。通过对光谱的仔细分析，研究小组不仅证实了之前观测到的许多特点，他们还推断出了可能的形成过程。这些过程既符合已知的星系演化的理论，也诠释了这种极端星系产生的起源。

There's still a lot of uncertainty in the universe, but the research on C1-23152 this time have provided a new understanding of galaxy formation and grow in the early universe. With careful analysis of the spectrum, the team confirmed many of the previously observed properties of it and deducted possible formation processes that both fit into ready know theories of galaxy evolutions over time and explains origin of this extreme case.

慢慢地，

每一次对独特星系的深入研究，

比如C1-23152，会让我们更接近

星系形成和演化的完整真相，

包括所有错综复杂的信息。

Slowly, 

each in-depth study of unique galaxies, 

like C1-23152, brings us closer to 

a full image of galaxy formation and evolution, 

with all of the intricacies included.

图为被称为“极深场”(eXtreme Deep Field, XDF)的照片，是NASA用哈勃望远镜经过10年拍摄的后，由一片片天空碎片照片组合而成的

Known as the eXtreme Deep Field (XDF), is a combination of pieces of sky images taken by NASA's Hubble Space Telescope over the course of 10 years. 

图片来自 Sci-News, KSU, UltraVISTA, NASA官网

部分信息参考来自的由 来自意大利国家天体物理研究所的 Paolo Saracoo 和他的团队的 “The Rapid Build-up of Massive Early-type Galaxies, Supersolar Metallicity, High Velocity Dispersion and Young Age for an ETG at z=3.35" 论文

Pictures from official website of Sci-News, KSU, UltraVISTA, NASA.

*Parts are from “The Rapid Build-up of Massive Early-type Galaxies, Supersolar Metallicity, High Velocity Dispersion and Young Age for an ETG at z=3.35" by the scientist team lead by Paolo Saracoo from the Italy's National Institute for Astrophysics