Cosmic Reionization: Galaxy-IGM Physics in the Early Universe

用哈勃太空望远镜拍摄的星系的戏剧性图像已经成为我们熟悉的银河系以外的东西的象征。然而，这些图片没有显示的是，星系只是充满宇宙的一小部分。在星系之间延伸的是一个巨大的看不见的网络，一个包含宇宙中大部分物质的“宇宙网络”。这个网络是由于引力放大了大爆炸产生的物质中的微小涟漪而形成的，它为星系的形成提供了原材料。现在，在大爆炸后的10亿年内，人们已经观察到了星系，当时宇宙还不到现在年龄的7%。但是第一批星系是如何形成的，又是什么时候形成的呢？由于距离遥远，这个早期的大多数星系都太暗，无法直接观测到。因此，为了了解第一批星系，我们必须通过它们对宇宙网络的影响来研究它们。例如，我们知道，深太空中的气体是高度电离的--电子已经从原子中剥离。我们认为，电离气体的能量来自第一批恒星和星系发出的紫外线。因此，如果我们能确定气体何时被电离，我们就能知道第一个星系是什么时候形成的。这个计划的目标是研究遥远以前的宇宙网，以便了解第一个星系和恒星。为了做到这一点，我将分析网络吸收被称为类星体的非常明亮的物体的光的方式。类星体和地球之间的气体吸收了部分类星体的光，其模式揭示了气体的位置、化学成分、温度和电离状态。这些量反过来反映了气体是如何受到嵌入其中的星系的影响的。我将首先通过测量气体分布的平稳性来确定深太空中的气体何时被电离。当气体被电离时，它也会被加热到10,000多度。由此产生的压力会导致气体膨胀，使网络中最小的凸起变得平滑。更早的加热会产生更多的光滑度，所以通过测量光滑度，我将能够确定电离和加热发生的时间。然后，我将寻找由恒星产生的元素的迹象，如碳和氧。早期宇宙中这些元素的数量将反映恒星在第一批星系中形成的活力，不同元素的相对混合将揭示恒星本身的性质。接下来，我将确定早期星系产生电离光子的效率。通过测量大爆炸后10亿到30亿年间星系对网络中气体的完全电离程度，我将确定在这段时间内所有星系发射了多少紫外线光子。一个关键的问题将是，如果星系确实推动了再电离，星系是否会像所要求的那样，在早期更有效地产生紫外线光子。最后，我将通过分析网络的小尺度结构来研究暗物质的本质。这个项目将决定暗物质的“热”程度，以及快速移动的暗物质粒子是否可能抑制了低质量星系的形成。第一批星系从宇宙网中出现是形成像我们自己的星系过程中的一个关键事件。因此，研究早期宇宙可以让我们在大范围内探索我们的起源。这一领域结合了尖端技术--大型望远镜和先进计算技术--与基础物理相结合，以绘制出一幅位于最遥远前沿的宇宙图景。最终，这项研究将帮助我们了解早期宇宙是如何形成的，以及探索更多的“外面”的东西。

The dramatic images of galaxies taken with the Hubble Space Telescope have become familiar symbols of what it "out there" beyond our own Milky Way. What the pictures do not show, however, is that galaxies are only a small part of what fills the Universe. Stretching between galaxies is a vast and invisible network, a "cosmic web" that contains most of the matter in the Universe. This network formed as gravity amplified tiny ripples in the matter created at the Big Bang, and it provides the raw material out of which galaxies form.Galaxies have now been observed back to within one billion years after the Big Bang, when the Universe was less than 7% of its current age. But how and when did the very first galaxies form? Because of the vast distances, most galaxies from this early time are too faint to be observed directly. To learn about the first galaxies, therefore, we must study them through their impact on the cosmic web. We know, for example, that the gas in deep space is highly ionized -- electrons have been stripped from their atoms. We believe that the energy to ionize the gas came from the ultraviolet light emitted by the first stars and galaxies. If we can determine when the gas became ionized, therefore, we will learn when the first galaxies formed.The goal of this proposal is to study the cosmic web far back in time in order to learn about the first galaxies and stars. To do this, I will analyze the way in which the web absorbs light from very luminous objects known as quasars. The gas between a quasar and the Earth absorbs portions of the quasar light in patterns that reveal where the gas is located, its chemical composition, temperature, and ionization state. These quantities, in turn, reflect how the gas has been affected by the galaxies embedded within it.I will first determine when the gas in deep space was ionized by measuring how smoothly it is distributed. As the gas was ionized it would also have been heated to more than 10,000 degrees. The resulting pressure would have caused the gas to expand, smoothing out the smallest bumps in the web. Earlier heating would have produced more smoothing, and so by measuring the smoothness I will be able to determine when the ionization and heating occurred. I will then search for signs of elements made by stars, such as carbon and oxygen. The quantity of these elements in the early Universe will reflect how vigorously stars formed in the first galaxies, and the relative mix of different elements will reveal the nature of the stars themselves. Next, I will determine how efficiently early galaxies produced ionizing photons. By measuring how completely galaxies ionized the gas in the web from one to three billion years after the Big Bang I will will determine how many ultraviolet photons were emitted by all galaxies during this period. A key question will be whether galaxies produced UV photons more efficiently at earlier times, as required if galaxies did indeed drive reionization. Finally, I will investigate the nature of dark matter by analyzing the small-scale structure of the web. This project will determine how "warm" dark matter is, and whether rapidly moving dark matter particles may have inhibited the formation of low-mass galaxies.The emergence of the first galaxies from the cosmic web was a key event in the process that gave rise to galaxies such as our own. Studying the early Universe, therefore, allows us investigate our origins on a grand scale. This field combines cutting-edge technology -- large telescopes and advanced computing -- with basic physics to assemble a picture of the cosmos at the most distant frontier. Ultimately, this research will help us to understand how the early Universe took shape, as well as to explore more of what is "out there."