The cycle of life, death and rebirth in massive early-type galaxies; star formation, black-holes and feedback

我的研究重点是了解我们周围宇宙中存在的最大星系。这些天体很有趣，因为它们被认为是星系演化的终点。它们由数十亿颗古老的恒星组成，具有红色的光学颜色，通常被认为没有冷气体——新恒星的燃料。因此，他们经常被描述为“又红又死”。天文学家仍然不完全清楚是什么导致了这些星系的消亡，或者它们是否能再次复活。我正在研究的正是这些过程。天文学家认为，这些大质量星系变得“红而死”的原因之一是它们核心处的超大质量黑洞。这些神秘的物体显然与星系的形成有关，因为它们似乎与它们的星系宿主同步增长。为了了解黑洞在星系形成过程中的作用，我开发了一种新技术，通过追踪围绕黑洞旋转的分子气体云的运动来测量它们的质量。这项技术令人兴奋，因为它开启了更精确地测量黑洞质量的可能性，而且比以往任何时候都要多。作为卢瑟福奖学金的一部分，我的目标是用这种技术来揭示潜伏在附近星系中心的黑暗怪物。当黑洞吞噬物质时，它们会发射出大量的高能光，并能加速大量粒子的喷射。这两个过程都能影响气体云，将它们抛出星系。这些外流是一种机制，可以帮助形成我所研究的缺乏气体的大质量星系。几年前，我帮助确定了一个最近的黑洞以这种方式排出大量气体的例子。在我的卢瑟福研究期间，我将利用这种流出物中的气体的化学性质来确定它是来自黑洞的极强的光，还是导致我们看到的流出物的粒子喷射。这将给我们提供更多的谜团，帮助解释为什么大多数拥有大黑洞的大质量星系都缺乏气体。像我研究的那些大质量星系，并不一定要保持“红色和死亡”。如果垂死恒星的物质冷却并成为新一代恒星的燃料，那么它们就可以复活。它们也可以与其他小星系合并，并窃取它们的燃料。今天我们周围的巨大的“红色和死亡”星系中，大约有四分之一正在重生。了解是哪个过程导致了这种情况，以及周围的星系有什么影响，将使我们确定这些星系是否注定要失败，回到“红色和死亡”，或者它们是否最终可以完全恢复生命。我发现这些获得了恒星形成燃料的天体，似乎并没有很有效地利用它。与银河系这样的星系相比，它们形成恒星的效率非常低。我的目标是找出为什么会发生这种情况，以及这能告诉我们什么关于宇宙中控制恒星形成的物理原理。

My research focuses on understanding the biggest galaxies that exist around us in the local universe. These objects are interesting because they are thought to be the end point of galaxy evolution. They are made up of billions of old stars, have red optical colours, and are generally thought to be free of cold gas - the fuel for new stars. Thus they are often described as "red and dead". Astronomers still don't fully know what caused these galaxies to die off, or if they can come back to life again. It is these processes that I am investigating.One of the ways astronomers think these massive galaxies become "red and dead" is due to the supermassive black-holes that lie at their cores. These enigmatic objects are clearly linked to galaxy formation, as they seem to grow in step with their galaxy host. In order to understand the role of black holes in the formation of galaxies, I developed a new technique to measure their masses, by tracing the motions of molecular gas clouds swirling around them. This technique is exciting, because it opens up the possibility of measuring black hole masses more accurately, and in more galaxies than ever before. As part of my Rutherford fellowship I aim to use this technique to reveal the dark monsters lurking at the hearts of nearby galaxies. When black holes swallow matter they emit large amounts of high energy light, and can accelerate large jets of particles. Both of these processes can affect gas clouds, throwing them out of galaxies. These outflows are one mechanism that can help form the gas-poor massive galaxies I study. A few years ago I helped identify one of the nearest examples of a black hole expelling large amounts of gas in this way. During my Rutherford fellowship I will use the chemistry of the gas in this outflow to determine whether it is the extremely strong light from the black hole, or the jet of particles that is causing the outflow we see. This will give us one more piece of the puzzle, helping to explain why most massive galaxies with big black holes are gas poor. Massive galaxies, like the ones I study, don't have to stay "red and dead". The can come back to life if material from dying stars can cool and become fuel for a new generation of stars. They can also merge with other small galaxies and steal their fuel. Around 1/4 of the massive "red and dead" galaxies around us today are currently in the process of being reborn. Understanding which process causes this, and what effect surrounding galaxies have, will allow us to determine whether these galaxies are destined to fail and go back to being "red and dead", or if they can eventually come back to life fully. I have discovered that these objects that have obtained fuel for star-formation, don't seem to be using it very effectively. They are very inefficient at forming stars when compared to galaxies like our own Milky Way. I aim to find out why this is happening, and what this can tell us about the physics controlling star formation in the universe.