“Little red dot” in early Universe is a naked supermassive black hole

早期宇宙中的“小红点”是一个裸露的超大质量黑洞

The James Webb Space Telescope (JWST) was designed to give us the ability to look at one of the earliest periods in the evolution of the Universe, a time when some of the earliest stars were putting out enough light to ionize the hydrogen that accounted for almost all of the normal matter present at the time. There were lots of ideas about what we might see, but the Universe is full of surprises. One of the first surprises was the existence of what picked up the moniker “little red dots,” which are exactly what their name suggests. After some initial arguments, it became clear that these were early versions of the supermassive black holes that presently sit at the center of almost every galaxy. Now, gravitational lensing has allowed astronomers to confirm that a little red dot is little more than a supermassive black hole without much in the way of a galaxy around it.

詹姆斯·韦伯太空望远镜（JWST）的设计初衷，是让我们能够观测宇宙演化最早期的阶段。那时，一些最早期的恒星发出的光足以电离当时几乎所有的普通物质——氢。关于我们会看到什么，科学界曾有许多猜想，但宇宙总是充满惊喜。最初的惊喜之一是发现了被称为“小红点”（little red dots）的天体，正如其名，它们看起来确实就是一个个小红点。经过最初的争论，人们逐渐明确，这些天体其实是超大质量黑洞的早期形态，而这类黑洞目前位于几乎每个星系的中心。现在，通过引力透镜效应，天文学家证实，一个“小红点”其实就是一个几乎没有星系环绕的裸露超大质量黑洞。

Making a little red dot bigger

让“小红点”变大

The little red dot in question is called Abell 2744−QSO1, and gravitational lensing has both magnified it and caused it to appear three times in the vicinity of the galaxy cluster that did the lensing. Based on details in its spectrum, we’re looking at the object as it appeared just 700 million years after the Big Bang. We’ve already known about QSO1 for a couple of years, and it has been the subject of intense study. One paper noted that the three lensed images of the object differ in some of their details. Since the light from each of those took different paths to Earth, and thus different amounts of time, this suggests there have been variations in QSO1’s emissions—consistent with a black hole feeding on different amounts of material over time. And, based on the luminosity of the object, people had estimated that the black hole itself was quite large for that early in the Universe’s history, at above 10 million times the mass of the Sun. Other work revealed that most of the material around it was gas that had formed relatively few stars. And, just last month, a detailed look at the spectrum of QSO1 showed that there is very little other than hydrogen present, consistent with the object having produced very few stars by this point in its history.

这个被称为 Abell 2744−QSO1 的“小红点”，在引力透镜的作用下被放大，并在透镜星系团附近出现了三个影像。根据其光谱细节，我们观测到的是它在大爆炸后仅 7 亿年时的状态。我们对 QSO1 的了解已有几年，它一直是深入研究的对象。一篇论文指出，该天体的三个透镜影像在某些细节上存在差异。由于来自每个影像的光到达地球的路径不同，耗时也不同，这表明 QSO1 的辐射存在变化——这与黑洞随时间推移吞噬不同数量物质的情况相吻合。根据其光度，人们曾估计该黑洞在宇宙早期历史中相当巨大，质量超过太阳的 1000 万倍。其他研究表明，它周围的大部分物质是气体，且形成的恒星相对较少。就在上个月，对 QSO1 光谱的详细分析显示，除了氢之外几乎没有其他物质，这与该天体在其历史的这一阶段仅产生了极少数恒星的结论相一致。

The big uncertainty in all of this is the relationship between the luminosity of the object and the mass of the black hole. We derive that relationship from the recent Universe, where supermassive black holes are embedded in mature galaxies that provide some structure to the material that the black hole is feeding on. There’s no guarantee that this same relationship would hold this early in the Universe’s history. Fortunately, thanks to the magnification of the gravitational lensing, QSO1 provides us a fantastic opportunity to find out how far back this relationship holds.

这一切中最大的不确定性在于天体的光度与黑洞质量之间的关系。我们推导这一关系所依据的是近期的宇宙，在那里的超大质量黑洞嵌入在成熟的星系中，这些星系为黑洞吞噬的物质提供了一定的结构。但无法保证这种关系在宇宙历史的早期阶段依然成立。幸运的是，得益于引力透镜的放大作用，QSO1 为我们提供了一个绝佳的机会，去探究这种关系究竟能追溯到多早。

A “galaxy” with very few stars

一个几乎没有恒星的“星系”

To get a more detailed picture of what’s at the center of QSO1, a large international team constructed a detailed picture of the environment around it. These included the amount of light emitted by different areas, as well as how fast the material in those areas was moving relative to the Earth, as determined by the red- and blue-shifting of hydrogen emissions. (The data is nicely consistent, with one side of the object showing red shifting, and the opposite side blue.) They also looked at the velocity dispersion, which registers how much variation there is relative to the mean velocity. With this data in hand, they built models to test which system best explained it. In every case, the best fit was a system with a massive point source at its center, and the rest of the material rotating around it. Attempts to build versions with a star cluster around the black hole similar to that seen in the Milky Way led to a much less accurate match to the real-world data. These models placed the black hole’s mass at about 50 million times that of the Sun, which is in line with previous estimates. That suggests the rules governing black hole luminosity haven’t changed in at least 13 billion years.

为了更详细地了解 QSO1 中心的情况，一个大型国际团队构建了其周围环境的详细图像。这些数据包括不同区域发出的光量，以及通过氢发射光谱的红移和蓝移确定的物质相对于地球的运动速度。（数据非常一致，天体的一侧显示红移，另一侧显示蓝移。）他们还研究了速度弥散，即相对于平均速度的变化程度。掌握这些数据后，他们建立了模型来测试哪种系统最能解释观测结果。在所有情况下，最吻合的模型都是一个中心拥有巨大点源、其余物质围绕其旋转的系统。尝试构建类似于银河系中那种黑洞周围存在星团的模型，与真实观测数据的匹配度要低得多。这些模型将黑洞的质量定为太阳的 5000 万倍左右，这与之前的估计一致。这表明，支配黑洞光度的规律在至少 130 亿年间没有发生变化。

Attempting to estimate the mass of any stars surrounding the black hole suggested there were very few. “The Keplerian rotation curve leaves little room for any stellar component,” the researchers conclude. Attempts to estimate the total stellar mass in the “galaxy” that the black hole sits in came up with an upper limit of 20 million solar masses—less than half of the mass of the black hole itself. In other words, over two-thirds of the mass of QSO1 resides in the black hole, with the stars accounting for less than one-third. Which explains why the word ‘galaxy’ is in quotes above. “To our knowledge, this upper limit makes QSO1 the most ‘naked’ massive BH ever found,” the team concludes.

尝试估算黑洞周围恒星的质量，结果显示恒星数量极少。研究人员总结道：“开普勒旋转曲线几乎没有给任何恒星成分留下空间。”对黑洞所在“星系”中总恒星质量的估算上限为 2000 万个太阳质量——不到黑洞自身质量的一半。换句话说，QSO1 超过三分之二的质量集中在黑洞中，而恒星占比不到三分之一。这就是为什么上文中“星系”一词要加引号的原因。研究团队总结称：“据我们所知，这一上限使 QSO1 成为迄今为止发现的最‘裸露’的大质量黑洞。”

Making supermassives

超大质量黑洞的形成

A lot of the paper is dedicated to the consideration of how this particular black hole got so big so early in the Universe’s history. There are three leading ideas for it: primordial black holes formed in the immediate aftermath of the Big Bang; direct collapse of massive gas clouds that skip the formation of stars entirely; or runaway mergers of black holes formed in early, dense star clusters. Here, the researchers argue that having a supermassive black hole with so few stars around suggests we can ignore option three. If there are no dense stellar clusters, you can’t form enough black holes to merge. This leaves two mechanisms that are entirely theoretical at this point. That said, the discussion seems to suggest that many of the direct collapse models that currently work require a major source of ultraviolet radiation, and more mass around than we see in QSO1. That would seemingly favor a primordial black hole as the source, although that would likely require it to have grown by a factor of 10 in the 700 million years of its existence. That, in turn, would suggest there were mergers among this population early in the Universe.

论文的大部分篇幅致力于探讨这个特殊的黑洞为何能在宇宙历史如此早期就变得如此巨大。目前有三种主流观点：大爆炸后立即形成的原始黑洞；跳过恒星形成阶段、直接坍缩而成的巨大气体云；或者在早期致密星团中形成的黑洞发生失控合并。研究人员认为，拥有一个周围几乎没有恒星的超大质量黑洞，意味着我们可以排除第三种选项。如果没有致密星团，就无法形成足够的黑洞来合并。这留下了两种目前完全处于理论阶段的机制。话虽如此，讨论似乎表明，许多目前可行的直接坍缩模型需要强大的紫外线辐射源，且周围的物质质量要比我们在 QSO1 中看到的更多。这似乎更倾向于原始黑洞是其来源，尽管这可能要求它在存在的 7 亿年里增长了 10 倍。反过来，这也暗示在宇宙早期，这类天体之间曾发生过合并。