Ionised outflows: the missing link in AGN-host galaxy evolution?

在大多数星系的中心都有一个巨大的黑洞，其质量高达10亿个太阳。在一些附近的星系中，黑洞的引力吸引了周围旋转的气体和尘埃盘中的物质；在这种物质被永远吞噬之前，它会通过发射包括可见光、紫外线和x射线在内的辐射来损失能量。这个所谓的活动星系核的总能量输出可能大于它所在的整个星系的能量输出。气体云以每秒几百到几千公里的速度离开原子核。气体云是高度电离的（大部分电子已经从原子中剥离），起源于被强辐射场从黑洞环境中炸出的物质。正是这种辐射导致了气体的电离。我们在附近星系中看到的这些外流物可能只是一个幽灵般的提醒，提醒我们在银河系演化的更早、更剧烈的时代发生了一个更为戏剧性的现象。现在越来越多的注意力集中在电离流出物如何将黑洞的演化与周围星系的演化联系起来。当黑洞成长时，它被气体和尘埃所笼罩，从视线中消失。一旦黑洞足够大，吸积产生的辐射会吹走这些物质，也会吹走周围星系中正在形成恒星的物质。这种巨大的外流结束了星系中恒星形成的过程。随着时间的推移，随着越来越多的物质要么被黑洞消耗，要么被流出物赶走，吸积燃料的供应就会耗尽，直到黑洞进入休眠状态。因此，从活动星系核流出的电离物质对星系及其核心的巨大黑洞的发展至关重要，因此对整个宇宙的演化也至关重要。我们通过它们对活动核发出的辐射的影响来研究它们；特定能量的辐射被流出的气体吸收掉了，通过用光谱仪观察这些吸收特征的模式，我们可以知道气体移动的速度，我们和黑洞之间有多少气体，以及它的电离程度有多高。来自太空的x射线和紫外线辐射大多无法到达地球表面，因为对我们来说幸运的是，它们被大气吸收了。因此，我们必须利用环绕地球轨道的卫星上的望远镜来研究来自深空物体的高能辐射，而不是大气层的屏蔽效应。我的研究包括通过太空望远镜的x射线光谱学，研究在附近和遥远的活动星系中电离流出物的物理特性（因为光到达我们需要时间，更遥远的物体探测宇宙历史上更早的时间）。最终目的是提供事实信息，以检验流出物如何将黑洞和宿主星系的演化联系起来的理论。其中的关键因素是确定质量从核心流出并流入周围星系的速率；我将使用从x射线和紫外线的吸收特征测量的物理性质来估计这一点。我的目标是从距离我们很远的活动星系核的光谱中估计质量流出率，以便观察质量流出率如何随着宇宙时间的推移而变化。然后，我将把这个结果与恒星形成的历史和黑洞的生命周期进行比较，以了解电离流出物是如何将两者的演化联系起来的。

At the heart of most galaxies lies a gigantic black hole with a mass of up to a thousand million Suns. In some nearby galaxies, the gravitational pull of the black hole draws in matter from a rotating disc of gas and dust that surrounds it; before this material is swallowed forever, it loses energy by emitting radiation including visible light, ultraviolet and X-rays. The total energy output of this so-called active galactic nucleus can be greater than that of the whole galaxy that it resides in. Clouds of gas stream away from the nucleus at speeds of several hundred to several thousand kilometres per second. The gas clouds are highly ionised (most of the electrons have been stripped from the atoms) and originate in material blasted out of the black holes's environment by the intense radiation field. It is this radiation which causes the ionisation of the gas. These outflows we see in nearby galaxies could be just the ghostly reminder of a far more dramatic phenomenon at an earlier, violent era in galactic evolution. Increasing attention is now being focused on how ionised outflows connect the evolution of the black hole to that of the surrounding galaxy. While the black hole grows, it is shrouded in gas and dust, obscured from sight. Once the black hole is big enough, the radiation output from accretion can blow away this material, and also matter in the surrounding galaxy that is forming stars. This massive outflow thus ends the process of star formation in the galaxy. Over time, as more and more matter is either consumed by the black hole or driven away in the outflow, the supply of accretion fuel is depleted until the black hole becomes dormant. Ionised outflows from active galactic nuclei are thus central to the development of galaxies and the giant black holes at their cores, and therefore to cosmic evolution as a whole. We study them through the effect they have on the radiation emitted from the active nucleus; radiation at certain energies is absorbed away by the outflowing gas, and by observing the patterns of these absorption features with spectrometers, we can tell how fast the gas is moving, how much gas lies between us and the black hole, and how highly ionised it is. X-ray and ultraviolet radiation from space is mostly prevented from reaching us on the Earth's surface since, fortunately for us, it is absorbed by the atmosphere. We therefore have to study the high energy radiation from deep space objects using telescopes on board satellites in orbit around the Earth, above the screening effects of the atmosphere. My research involves studying, through X-ray spectroscopy with space telescopes, the physical properties of ionised outflows both in nearby and in distant active galaxies (because of the time light takes to reach us, more distant objects probe earlier times in the history of the universe). The ultimate aim is to provide factual information with which to test theories of how the outflow connects the evolution of the black hole and the host galaxy. The key factor in this is determining the rate at which mass is flowing out of the nucleus and into the surrounding galaxy; I will estimate this using physical properties measured from absorption features in X-ray and ultraviolet light. I aim to estimate the mass outflow rates from spectra of active galactic nuclei at a wide range of distances from us, in order to see how the mass outflow rates change over cosmic time. I will then compare the results of this with the history of star formation and the lifecycles of black holes, in order to understand how the ionised outflows connect the evolution of the two.