Tomography of the interstellar medium at small scales

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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.