Neutral Hydrogen intensity mapping with MeerKAT

在过去的二十年里，我们进入了宇宙学的“黄金时代”。通过卫星和地面望远镜，我们收集了来自早期宇宙的高质量数据，主要是来自大爆炸后发出的光，以及来自晚期宇宙的，通过恒星和星系发出的光。然而，我们宇宙的历史和体积的很大一部分仍未被探索。应对这一挑战的一种方法是观察中性氢（HI）发出的光，这些氢在大爆炸之后和第一个星系形成之前很长一段时间内充满了宇宙。在那之后，HI就存在于星系中，所以我们也可以把它作为一种研究晚期宇宙的新方法。这是我的主要研究领域；这是令人兴奋的，因为它打开了一扇观察宇宙的新窗口，可以突破我们对天体物理学和宇宙学的理解。在接下来的几年里，将使用射电望远镜进行高灵敏度的HI调查，并且部分拟议的研究正在制定新的观测方法和技术，以便最大限度地提高它们的科学产出。我开创了一种新的观测方法，它不需要对单个星系进行困难和昂贵的探测，而是将来自许多星系的整个HI通量绘制成大的3D像素（横跨天空和时间）。我的目标是利用MeerKAT阵列的高强度映射数据，使用这种方法提供宇宙的3D地图。MeerKAT是一个位于南非卡鲁的射电望远镜，它是平方公里阵列的探路者，它将成为世界上最大的射电望远镜。通过使用这些数据，我想回答一些关于宇宙的基本问题，比如暗能量的本质是什么——暗能量是导致宇宙加速膨胀的神秘物质——以及星系是如何演化的。但在此之前，需要做很多工作来提高数据的质量，并找出如何解决强度测绘面临的许多挑战。其中一个挑战与来自我们银河系的巨大辐射（称为“银河同步加速器”）有关，它严重污染了我们的HI地图。换句话说，当MeerKAT观测夜空时，它绘制了HI宇宙学信号，但它也绘制了发射的同步辐射，它比信号大一千倍！因此，我们想要测量的宇宙信号被“埋”在一个更大的天体物理信号之下。我们可以利用前景是频率的平滑函数这一事实来解决这个问题，而HI信号具有尖状结构。因此，原则上，前景可以在不丢失任何信号信息的情况下完全移除。然而，在实践中这是非常具有挑战性的。我将使用可用的MeerKAT数据集来测试各种前景清洗和校准技术的性能。在对数据进行处理和清理之后，我将继续为HI强度映射信号的宇宙学分析构建一个管道。该管道还将通过包括相互关联数据分析工具，说明HI与传统光学星系调查之间强大协同作用的可能性。这对于获得没有系统污染的测量是有用的，这些污染经常困扰着单个调查（但在结合它们时就会消失），因此更可靠。我们开发的工具将用于分析MeerKAT获取的科学验证数据，即覆盖更大部分天空并观察更长时间的数据，因此它们可以用于进行精确的宇宙学测量。然后，我们希望使用强度映射技术在无线电波长中进行HI和宇宙学参数的首次测量，利用多波长协同作用，并彻底改变我们对星系演化和暗能量的理解。

During the last two decades we have entered a "golden era" of cosmology. Using satellites and ground based telescopes we have gathered high quality data from the very early Universe, essentially from light emitted right after the Big Bang explosion, as well as from the late Universe, through the light emitted from stars and galaxies. However, a big part of our Universe's history and volume remains unexplored. A way to attack this challenge is by observing the light emitted from the neutral hydrogen (HI) that filled the Universe for a long time after the Big Bang and before the first galaxies were formed. After that time HI resides within galaxies, so we can also use it as a novel way to study the late Universe. This is my main area of research; it is exciting because it opens a new observational window into the Universe and can push the boundaries of our understanding of astrophysics and cosmology. In the next few years, HI surveys of exquisite sensitivity will be performed using radio telescopes, and part of the proposed research is working out new observational methods and techniques in order to maximise their science output. I have pioneered a new observational method that does not require the -difficult and expensive- detection of individual galaxies but maps the entire HI flux coming from many galaxies together in large 3D pixels (across the sky and along time). I aim to use this method to provide a 3D map of the Universe using HI intensity mapping data from the MeerKAT array. MeerKAT is a radio telescope located in Karoo, South Africa, and it is a pathfinder for the Square Kilometre Array, which is going to be the largest radio telescope in the world. By using these data I want to answer fundamental questions about the Universe, for example what is the nature of dark energy - the mysterious substance responsible for the accelerated expansion of our Universe - and how galaxies evolve. But before doing that, much work is needed to improve the quality of the data and figuring out how to solve the many challenges that intensity mapping faces. One of these challenges has to do with the huge emitted radiation (called "galactic synchrotron") from our own Galaxy, that severely contaminates our HI maps. In other words, when MeerKAT observes the night sky, it maps the HI cosmological signal, but it also maps the emitted synchrotron radiation, which is a thousand times larger than the signal! The cosmological signal we want to measure is therefore "buried" under a much larger astrophysical signal. We can attack this problem by exploiting the fact that foregrounds are smooth functions of frequency, while the HI signal has a spiky structure. So, in principle, foregrounds can be perfectly removed without losing any signal information. However in practice this is very challenging. I will use the available MeerKAT data sets to test the performance of various foreground cleaning and calibration techniques. After the data has been processed and cleaned, I will move on to build a pipeline for the cosmological analysis of the HI intensity mapping signal. This pipeline will also account for the possibility of powerful synergies between HI and traditional optical galaxy surveys, by including cross-correlations data analysis tools. This is useful in order to obtain measurements that are free of systematic contaminations that often plague individual surveys (but drop out when combining them), and therefore more robust. Our developed tools will be used to analyse Science Verification data taken with MeerKAT, i.e. data that cover a larger part of the sky and observe for longer time, so they can be used to make precise cosmological measurements. We then hope to perform the first ever measurements of HI and cosmological parameters in the radio wavelength using the intensity mapping technique, exploit multi-wavelength synergies, and revolutionarise our understanding of galaxy evolution and dark energy.