Mapping Intergalactic Metals at High Redshift

在过去的十年里，宇宙中结构增长的数值模型在理解宇宙中第一批恒星和星系是如何从初始密度扰动中形成方面取得了长足的进步。这些模型的最大成功之一是彻底改变了我们对星系间气体(IGM)的理解，这是在遥远类星体的光谱中看到的莱曼阿尔法吸收谱线和相关金属谱线的“森林”中向我们揭示的。以前，人们根据热的底层光滑的冰河系成分内的压力平衡中的单个云，或在高红移的单个暗物质迷你晕中的吸收来理解日冕的吸收；现在，模型已经成功地表明，莱曼阿尔法森林追踪到了由丝状物和空洞组成的“宇宙网”中密度较低的部分，这是宇宙结构--恒星、星系和星系团--建立的早期阶段。因此，类星体吸收线的研究是对高红移星系和星系团研究的补充，并使我们能够看到星系形成的最早阶段。恒星由He IGM形成，当这些恒星将物质返回到气体中时，它们又被这些恒星中的重元素(“金属”)所富含。来自恒星、星系和类星体的电离辐射也影响着IGM中氢离子和其他元素的平衡。因此，金属在IGM中的分布和离子的分布都是宇宙恒星形成历史的诊断。此外，模型预测，IGM是宇宙中大部分重子存在于高红移的地方，因此，由于这种气体在类星体吸收线中相当容易被观察到，我们幸运地能够解释宇宙中大部分高红移的物质。这个项目将直接涉及理解IGM是如何变得相当均匀地富含金属的，即使红移为3或更高。一种可能性是，在莱曼阿尔法星系森林中观察到的所有金属都直接来自于在最初的结构中形成的恒星，这些恒星后来成为今天的星系，但另一种可能性是，整个IGM在很早的时候就被第一代恒星预先浓缩了：所谓的种群III。由于前星系团形成于IGM中密度较高的区域，来自这些恒星的任何浓缩必然也将局限于这些较高密度的区域。相比之下，人口III恒星将在更广泛的范围内丰富IGM。随着数值模型在将恒星形成和演化的反馈纳入IGM方面变得更加复杂，它们对金属丰度作为距离恒星形成地点的距离的函数的预测将变得更加精确。因此，有一个明确的理由来研究金属密度的分布，这将是这个项目的一个主要重点。该项目的资金由NSF河外天文与宇宙学计划(AST/EXC)提供。*

AST 0098480Cowie, A.Over the past decade, numerical models of the growth of structure in the Universe have made vast strides in understanding how the first stars and galaxies in the Universe formed from initial density perturbations. One of the great successes of these models has been in revolutionizing our understanding of the intergalactic gas (IGM) as revealed to us in the "forest" of Lyman alpha absorption lines and associated metal lines seen in the spectra of distant quasars. Previously, absorption in the IGM was understood in terms of individual clouds in pressure balance within a hot underlying smooth IGM component, or in terms of absorption in individual dark matter mini-haloes at high redshift; now the models have successfully shown that the Lyman alpha forest traces the lower overdensity portions of the "cosmic web" of filaments and voids that is the early phase of the buildup of structure --- stars, galaxies and clusters --- in the Universe. The study of quasar absorption lines is thus complementary to the study of high redshift galaxies and clusters, and allows us to see the earliest stages of galaxy formation. Stars form out of he IGM, and it in turn is enriched with heavy elements ("metals") from these stars as they return materials to the gas. Ionizing radiation from stars, galaxies and quasars also affects the balance of ions of hydrogen and other elements in the IGM. The distribution of metals in the IGM and the distribution of ions are therefore both diagnostics of the star formation history of the Universe. In addition, the models predict that the IGM is where the bulk of the baryons in the Universe exist at high redshift, thus, since this gas is fairly easily observable in quasar absorption lines, we are in the lucky position of being able to account for most of the material in the Universe at high redshift.This project will be directly concerned with understanding how the IGM comes to be rather uniformly enriched with metals, even at redshifts of 3 and higher. One possibility is that all the metals that have been observed in the Lyman alpha forest come directly from stars that are forming in the first structures that will later become present-day galaxies, but another possibility is that the whole IGM was pre-enriched by a first generation of stars at very early times: the so-called population III. Since the pre-galactic clumps form in the regions of higher overdensity in the IGM, any enrichment from these stars would necessarily be confined also to these higher density regions. In contrast, the population III stars would enrich the IGM more broadly. As numerical models become more sophisticated at incorporating feedback into the IGM from star formation and evolution, their predictions of metallicity as a function of distance form the star-forming site, and therefore of density, will become more refined. There is therefore a clear rationale for investigating the distribution of metals with density, and this will be a main focus of this project. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC).***