Supermassive black hole growth - a small-scale solution to a large-scale problem

在几乎每个星系的中心都有一个黑暗的物体，其质量是太阳的一百万倍以上。尽管这些天体质量巨大，但它们被自身的引力压缩到了极高的密度，密度之大，以至于光线无法从它们的表面逃逸；这些是超大质量黑洞（SMBHs）。这些奇异的物体是如何形成的，是天文学家面临的一个突出的谜团，也是我觉得特别有趣的谜团之一。通过观察遥远的距离，我们能够瞥见宇宙生命最初几十亿年发生的事情。尽管在这样的距离上进行研究非常困难，但可以对一些SMBHs进行“称重”。令人惊讶的是，这表明它们在宇宙只有大约10亿年的时候就已经完全成熟了。尽管科学家们认为黑洞的增长需要很长时间，但这一发现表明，黑洞的增长速度非常快，因为物质以极快的速度落入黑洞——比辐射和引力平衡所允许的速度要快，也就是所谓的爱丁顿极限。吸积是如何以这样的速度运行的尚不清楚，但这是天体物理学中一个紧迫的问题——我的目标是解决这个问题。虽然我们不能直接研究SMBHs的增长，但我们可以观察到物质是如何落在附近星系的SMBHs上的，以及落在我们银河系内更小的黑洞（在双星系统中绕轨道运行的恒星：bhb）上的。因此，我们知道物质不会直接落在黑洞上，而是形成一个吸积盘，发出大量辐射。在非常高的速率下，并不是所有的物质都落入黑洞；相反，一部分以“风”或“喷流”的形式被排出。风以相当“慢”的速度（约光速的10%）从圆盘表面携带物质，而喷流则以接近光速的速度从黑洞附近喷出更强大的物质。类似的外流将伴随着SMBH的增长，这是合乎逻辑的，物质从吸积流中被带走，并重新分配到周围环境——一种“反馈”形式。了解爱丁顿吸积的本质和相关的外流对于理解SMBHs的成长和反馈对宿主星系的影响是必要的。在实践中，这需要观察当吸积流达到爱丁顿极限并与流出流耦合时如何变化，即它们如何相互作用。在实践中，这已被证明是极其困难的：吸积流发射到附近星系的SMBHs和银河bhb的辐射被中间的物质所掩盖，阻止了对耦合的观察。我的建议以一种新的方式来解决这个问题：通过观察在附近星系中以高速率吸积的bhb，那里的介入性物质的数量要少得多，从而可以研究流入的发射。这些星系外的bhb有两种“口味”：一种是在银河系中常见的，显示出强大的喷流，另一种更亮，被认为是爱丁顿吸积的一种更极端的形式，具有强大的风。我正在领导对两个附近星系中具有强大喷流的新bhb的首次重大搜索。通过不同波长范围内的几种仪器，包括世界上最先进的射电望远镜VLA和美国宇航局的x射线卫星Swift，我将观察圆盘和喷流是如何一起变化的，从而限制流入的性质和喷流是如何发射的。为了了解最明亮的风源，我将结合新颖的分析技术和理论来揭示流入和流出的本质。通过研究本地宇宙中SMBHs的吸积，我将把我的发现外推到更大的黑洞质量，在那里流入和流出的耦合无法研究。最后，通过使用高红移SMBH生长的模拟，我将能够探索反馈对宿主星系的影响，否则就会隐藏在一个时代。

In the centre of almost every galaxy sits a dark object, with a mass over a million times that of the Sun. Although massive, these objects have been compressed by their own gravity to extremely high densities, so dense in-fact that light cannot escape their surface; these are the supermassive black holes (SMBHs). How these exotic objects formed is one of the outstanding mysteries facing astronomers and one I find particularly intriguing.By looking to great distances we are able to glimpse what happened in the first few billion years of the Universe's life. Although very difficult to perform studies at these distances, a few SMBHs can be 'weighed'. Surprisingly this has shown that they were fully-grown when the Universe was only ~1 billion years old. Although scientists thought that growth should take a long time, this discovery demands that they instead grew incredibly quickly by material falling onto the black hole at extreme rates - faster than should be allowed by the balance of radiation and gravity, the so-called Eddington limit. How accretion operates at such rates is unclear but is a pressing issue in astrophysics - an issue I aim to address.Although we cannot study the SMBHs growing directly, we can observe how material falls onto SMBHs in nearby galaxies and onto much smaller black holes (with orbiting stars in binary systems: BHBs) within our own Galaxy. As a result, we know that material doesn't fall directly onto the black hole but forms an accretion disc, which emits large amounts of radiation. At very high rates, not all of the material falls onto the black hole; instead some fraction is expelled in 'winds' or 'jets'. Winds carry material from the surface of the disc at fairly 'slow' speeds (~10% of the speed of light) whereas jets are much more powerful ejections of matter from close to the black hole at almost the speed of light. It is logical that similar outflows will have accompanied the SMBH growth, with matter taken from the accretion flow and redistributed to the surroundings - a form of 'feedback'.Understanding the nature of Eddington accretion and the associated outflows is necessary for understanding the growth of SMBHs and the impact feedback must have had on the host galaxy. In practice this requires observing how the accretion flow changes as it reaches the Eddington limit and couples to the outflow, i.e. how they interact. In practice this has proven to be extremely difficult: emission from the accretion flow onto both SMBHs in nearby galaxies and onto Galactic BHBs is obscured by intervening material, preventing a view of the coupling. My proposal approaches this problem in a new way: by looking at BHBs accreting at high rates in nearby galaxies where the amount of intervening material is much lower, allowing the emission from the inflow to be studied. These extragalactic BHBs come in two 'flavours': those which are commonly seen in the Milky Way and show powerful jets, and those which are even brighter and thought to be a more extreme form of Eddington accretion with powerful winds. I am leading the first major search for new BHBs with powerful jets in two nearby galaxies. By observing with several instruments across a range of wavelengths, including the world's foremost radio telescope, the VLA, and NASA's X-ray satellite, Swift, I will observe how the disc and jets change together, thereby constraining both the nature of the inflow and how the jets are launched. In order to understand the brightest sources with powerful winds, I will combine novel analysis techniques with theory to reveal the nature of both inflow and outflow. By studying accretion onto SMBHs in the local Universe, I will extrapolate my findings to larger black hole masses where the coupling of inflow and outflow cannot be studied. Finally, by using simulations of high redshift SMBH growth I will be able to explore the impact of feedback on the host galaxies, in an epoch otherwise hidden from view.