Fluctuations in IGM Properties: The Hallmarks of Reionization

宇宙中的绝大多数物质存在于一个巨大的物质网络中，称为星系际介质（IGM）。在宇宙历史的大部分时间里，这个网络中的气体（主要是氢和氦）一直是热的和高度电离的。然而，我们知道，在大爆炸后不久，宇宙冷却到足以让离子和电子重新组合，使气体保持中性。从中性宇宙到电离宇宙的转变发生在宇宙历史的头20亿年。这个过程被称为“再电离”，我们认为是由来自第一批星系和类星体的紫外光子驱动的。这些源是现代宇宙中星系的基石，但其中许多（或大多数）太暗而无法直接观察。通过研究IGM，我们可以确定再电离发生的时间和方式，从而深入了解这些早期的来源以及它们与环境相互作用的方式。对再电离的一个关键预测是，它应该是一个扩展的、不完整的过程。在星系和类星体周围生长的电离气体气泡会引起中性气体比例的大尺度变化。在再电离过程中气体的加热也会产生很大的温度波动。这些电离和温度波动的特征反映了驱动再电离的源的基本性质。例如，如果再电离是由罕见的明亮类星体驱动的，那么气泡会很大，再电离会更不均匀。相比之下，正在形成恒星的星系往往会产生更小、数量更多的气泡。在过去的几年里，在再电离过程中IGM的全局平均性质的研究方面取得了重大进展;然而，关于这一早期阶段的“三维”性质的信息还很少。我们建议对氢和氦再电离期间IGM性质的空间波动进行首次测量，并利用这些测量来确定第一批星系和类星体的性质。我们将把已知最遥远的类星体的联合收割机光谱与先进的宇宙结构模拟相结合，研究在氢再电离结束（大约是大爆炸的10亿年）附近IGM电离的波动。我们将进一步使用类星体光谱来研究氦再电离峰值期间（大爆炸后10亿至20亿年）IGM温度的空间变化。这些项目将利用独特的数据和分析方法，对宇宙再电离的这些关键要素提供第一个观测见解。反过来，一个更全面的再电离图像将有助于指导我们对早期宇宙中星系形成的理解。

The large majority of matter in the Universe resides in a vast filamentary network known as the intergalactic medium (IGM). For most of cosmic history, the gas in this network (mainly hydrogen and helium) has been hot and highly ionized. We know, however, that shortly after the Big Bang the Universe cooled enough to allow ions and electrons to recombine, leaving the gas neutral. The transition from a neutral to an ionized Universe occurred over the first two billion years of cosmic history. Known as "re-ionization", we believe that this process was driven by ultraviolet photons from the first galaxies and quasars. These sources are the building blocks of galaxies in the modern Universe, yet many (or most) of them are too faint to observe directly. By studying the IGM, we can determine when and how reionization occurred, and hence gain insight into these early sources and the ways in which they interacted with their environments.One of the key predictions for reionization is that it should be an extended, patchy process. Bubbles of ionized gas growing around galaxies and quasars introduce large-scale variations in the fraction of neutral gas. Heating of the gas during reionization also creates large fluctuations in temperature. The characteristics of these ionization and temperature fluctuations reflect fundamental properties of the sources driving reionization. If reionization is driven by rare, bright quasars, for example, then the bubbles will be large and reionization more patchy. Star-forming galaxies, in contrast, will tend to produce bubbles that are smaller and more numerous. Over the past several years, significant progress has been made in studying the globally-averaged properties of the IGM during reionization; however, little information yet exists about the "three-dimensional" nature of this early period.We propose to make the fist measurements of spatial fluctuations in the properties of the IGM during hydrogen and helium reionization, and to use these measurements to determine the nature of the first galaxies and quasars. We will combine spectra of the most distant known quasars with advanced simulations of cosmic structure to study fluctuations in the ionization of the IGM near the end of hydrogen reionization (roughly one billion years of the Big Bang). We will further use quasar spectra to study spatial variations in IGM temperature during the peak of helium reionization (one to two billion years after the Big Bang). These projects will leverage unique data and analysis methods to deliver the first observational insights into these key elements of cosmic reionization. A more comprehensive picture of reionization, in turn, will help guide our understanding of galaxy formation in the early Universe.