Tomography of the interstellar medium at small scales

银河系星际介质(ISM)在吸收和发射方面的测量表明，在尺度&lt；10pc范围内存在显著的小尺度结构。然而，人们对ISM的这一小规模组成部分的性质知之甚少，因为鉴于这些结构的体积填充系数很小，系统的调查极其困难。特别是，在银河系晕中发现了一个含有氢分子(H2)的非常小的气团，再加上早先提出的存在非常紧密的冷气团的建议，这表明到目前为止，可能有相当数量的重子物质没有被探测到，这种物质可能占到了迄今为止难以捉摸的暗物质的相当一部分。在这个项目中，我们想要利用银河系球状星团和麦哲伦星云中恒星的光学和(远)紫外吸收光谱，在小尺度上研究银河系圆盘和晕中的ISM。对分布在很小视场内的大量视线的分析(如星团和麦哲伦星云的情况)为在小尺度上系统地研究附近盘和晕ISM的结构和物理性质提供了独特的可能性。关于气体的空间分布和径向速度分布的综合信息将提供小尺度星际介质的断层图像。大小和密度的分布(也许还有ISM的分维)将允许预测有多少非常小和密集的团块存在，以及它们包含多少质量。

Measurements of the Galactic interstellar medium (ISM) in absorption and emission have demonstrated that significant small-scale structure exists at scales <10 pc. However, very little is know about the nature of this smallscale component of the ISM, because systematic investigations are extremely difficult in view of the small volume-filling factor of these structures. In particular, the discovery of a very small clump of gas in the Milky Way halo containing hydrogen molecules (H2) together with earlier suggestions of the existence of very compact cold clumps of gas indicates that perhaps a considerable amount of baryonic matter has thusfar remained undetected and that such material may account for a fair fraction of the hitherto elusive dark matter .In this project we want to study the ISM in the disk and halo of the Milky Way on small scales using optical and (far-)ultraviolet absorption spectra of stars in Galactic Globular Clusters and in the Magellanic Clouds. The analysis of a large number of sightlines distributed over a very small field of view (as in the case of a star cluster and the Magellanic Clouds) offers the unique possibility to systematically investigate the structure and physical properties of the nearby disk and halo ISM on small scales. The combined information about the spatial distribution and the radial-velocity distribution of the gas will provide a tomographic view of the small-scale interstellar medium. The distribution of sizes and densities (and perhaps the fractal dimension of the ISM) will allow to predict how many very small and dense clumps exist and how much mass they contain.