Understanding obscured star formation through cosmic time

对附近星系的早期观测让天文学家有了一个惊人的发现——大多数本地星系很容易被分为两类。它们要么是螺旋形的，正在形成恒星，包含许多年轻的蓝色恒星（就像我们自己的银河系），要么是球状的（椭圆的）形状，主要由古老的红色恒星组成。这个相对简单的观察导致了一些重要的问题：不同的星系是如何形成和演化的，以及为什么它们现在主要占据两个不同的类别。解开不同影响的一种方法是研究更远的星系，由于光速有限，这也意味着我们在宇宙更年轻的时候看到它们，星系在它们自己的进化早期。然而，研究遥远宇宙中年轻星系的一个关键困难是，它们可能含有大量的尘埃——一种像烟雾一样的小颗粒，挡住了大部分星系的光学光线。这些尘埃使星系变得暗淡，很难（有时是不可能）用光学和近红外望远镜观察到，所以在不知道尘埃隐藏了多少光的情况下，我们无法准确地测量遥远的星系。更糟糕的是，尘埃最多的（光学上最难探测的）星系是最活跃的——它们形成新恒星的速度比我们的银河系快1000倍——它们推动了我们对星系演化的理解的极限。事实上，自从20年前发现大量遥远尘埃星系以来，对它们的观测一再与模型不符，并引发了对星系形成和演化方式的新启示。为了追踪星系中的尘埃，并精确测量在宇宙历史的不同时期有多少恒星正在形成，我使用远红外线。远红外数据显示，在本地宇宙中非常多尘和活跃的星系是罕见的，但在年轻的遥远宇宙中有很多，因此它们被认为是星系演化的关键阶段。然而，即使是最新的星系形成和演化模型和模拟也难以准确地再现这些遥远活动星系的数量和性质。这个问题在最早的时期（大爆炸后大约10亿年内）尤其普遍，因为模拟很少能产生足够多的这些充满尘埃的“星暴”星系。不同的模拟在如何产生最活跃的尘埃星系上也存在分歧——一些需要两个大质量星系的碰撞，但另一些只需要一个星系就能产生恒星爆发。在这个奖学金期间，我将使用来自智利的一个新的，非常强大的红外天文台-阿塔卡马大型毫米/亚毫米阵列（ALMA）以及其他国际望远镜的数据来研究遥远星系中的尘埃，并通过观察确定不同星系演化模型中的哪一个（如果有的话）是正确的。首先，我将使用ALMA来确定是否需要星系合并来触发大规模的遥远恒星爆发。然后，我将检查最早的尘埃星系的样本，以测量它们是否真的像第一次观察所表明的那样明亮和巨大（模拟的一个大问题），或者它们是否可能是“引力透镜”，即被前景中另一个星系的质量放大并使其看起来更亮。如果它们中的许多都是引力透镜，那么我们之前对它们的大小和活动水平的测量将是错误的，这将再次影响星系演化模型。最后，我将为下一代的广域巡天做重要的准备工作，利用统计学建立新的方法来精确定位远红外明亮星系，这是对许多单个星系进行详细研究的关键一步。

Early observations of nearby galaxies led astronomers to a startling revelation - most local galaxies are easy to classify into one of two groups. Either they are spiral shaped with ongoing star-formation and contain many young, blue stars (like our own Milky Way), or they have spheroidal (elliptical) shapes and are mostly composed of old, red stars. This relatively simple observation leads to the important questions of how the different galaxies formed and evolved, and why they now mostly occupy two distinct classes. One way to untangle the different effects is to study galaxies that are further away, which, due to the finite speed of light, also means that we see them when the Universe was younger and the galaxies were earlier in their own evolution.However, a key difficulty in examining young galaxies in the distant Universe is that they can contain significant quantities of dust - small particles that act like smoke and block out much of galaxies' optical light. This dust makes the galaxies faint and difficult (sometimes impossible) to observe with optical and near-infrared telescopes, so without knowing how much light is hidden by dust we cannot accurately measure distant galaxies. Even worse, the dustiest (and optically hardest to detect) galaxies are the most active - they form new stars about 1000 times quicker than our Milky Way - and they push the limits of our understanding of galaxy evolution. In fact, since the discovery 20 years ago of a large population of distant dusty galaxies, observations of them have repeatedly disagreed with models and triggered new revelations in understanding how galaxies form and evolve.To trace dust in galaxies and accurately measure how many stars are being formed at different times in the history of the Universe I use far-infrared light. Far-infrared data have shown that very dusty and active galaxies are rare in the local Universe, but there are many of them in the young, distant Universe, so they are thought to be a key stage in the evolution of galaxies. However, even the latest models and simulations of galaxy formation and evolution struggle to accurately reproduce the number and properties these distant active galaxies. The problem is particularly prevalent at the earliest times (within about a billion years of the Big Bang) when simulations rarely generate enough of these very dusty "starburst" galaxies. Different simulations also disagree about how to generate the most active dusty galaxies - some require collisions of two massive galaxies, but others can create starbursts from just a single galaxy.During this fellowship I will use data from a new, exceptionally powerful, infrared observatory in Chile - the Atacama Large Millimeter/Submillimetre Array (ALMA), as well as other international telescopes to study the dust in distant galaxies and determine through observations which (if any) of the different galaxy evolution models is correct. Firstly, I will use ALMA to determine whether galaxy mergers are needed to trigger massive distant starbursts. I will then examine samples of the earliest dusty galaxies to measure whether they are really as bright and massive as the first observations suggested (a big problem for simulations), or whether they may be "gravitationally lensed" i.e. magnified and made to appear brighter by the mass of another galaxy in the foreground acting as a lens. If many of them are gravitationally lensed then our previous measurements of their sizes and activity levels would have been wrong, and that will again affect the galaxy evolution models. Finally, I will perform crucial preparation for the next generation of wide-field surveys, using statistics to establish new methods to accurately pinpoint far-infrared bright galaxies, which is a crucial step in being to study many individual galaxies in detail.