Density structure of molecular clouds as a probe of the ISM physics

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Understanding the physical processes that control the life-cycle of the interstellar medium (ISM) is one of the key themes in the astrophysics of galaxies today. This importance derives from the role of the ISM as the birthplace of new stars, and consequently, as an indivisible constituent of galaxy evolution. Exactly how the conversion of the ISM to stars takes place is intricately linked to the structure of the cold, molecular phase of the ISM. Indeed, we know that the true formation sites of stars are the strong density enhancements in the interstellar molecular clouds. Whatever processes control the evolution of these density enhancements very concretely control the gas-to-stars conversion. However, our current picture of the molecular cloud structure formation has a fundamental caveat: it is based almost entirely on observations of nearby, low-mass molecular clouds that form only low-mass stars. This is because only in nearby clouds, traditional observation techniques can reach an adequately high spatial resolution and sensitivity. Yet, it is the high-mass molecular clouds in which most stars form, and it is the high-mass stars and star-clusters that impact the ISM of galaxies most. Based on the work done during the first ISM-SPP funding period, I present a program that will fill the gap in our understanding of the internal structure of massive molecular clouds. We have developed a new observational technique that provides an unparalleled view of the structure of massive molecular clouds. We also have developed a technique with which such high-fidelity data can be used to study one of the most fundamental measures of the molecular cloud structure, the probability density function of their volume densities. With this program, the full potential of these tools will be put into use. We will derive high-fidelity column density data for a large number of massive molecular clouds, and build an unprecedented census of their internal structure. We will also use the data of recent molecular line surveys to trace the kinematic properties of the clouds. As an integral part of the program, we will connect the obtained observational picture to state-of-the-art, galaxy-scale numerical simulations. This approach allows us to address keyquestions in the field: What physics drive the structure formation in massive molecular clouds? How do high-mass clouds fragment? How does the structure of high-mass clouds gives rise to the star formation within? Are these processes different in low-mass and high-mass molecular clouds? Most crucially, the program allows us to build a view of the molecular cloud structure, and of the processes that control this structure, over the entire mass range of molecular clouds in the ISM.