Scales and thresholds in molecular cloud turbulence

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We propose to exploit the statistical tools developed and investigated in the first application period to systematically compare simulations of the turbulent multi-phase interstellar medium (ISM) with recent observations in order to identify characteristic scales and parameters governing the different physical processes that govern the dynamic evolution of the ISM.We will identify and quantify physical scales and thresholds where the properties of the turbulent cascade are expected to change. These are chemical mixing scales, the typical depth of UV and IR penetration in filamentary clouds, characteristic velocity patterns, and turbulence dissipation scales. Thresholds are given, e.g., by the critical density for the onset of gravitational collapse, for the excitation of particular line cooling processes, and for chemical phase transitions. Combining the scale information with the density, column density and temperature statistics, we will derive the state of the gravitational collapse in molecular cloud filaments and the transition to individual cores. By evaluating parameters such as radiation penetration depths, escape probabilities and the resulting cooling functions, as well as coupling coefficients between different species, in particular neutral and ionized gas particles and dust grains, we will constrain the equation of state of the interstellar medium.The very rich data sets available to our group, provided by extended, high angular resolution line surveys (from the NANTEN2, APEX, IRAM, and MOPRA telescopes) and continuum data (Spitzer, Herschel, Planck imaging), allow us to investigate the ISM from star-forming scales up to the dimension of giant molecular cloud complexes. At the same time, there has been tremendous progress in our capabilities of modeling the multi-phase and multi-scale ISM, with numerical simulations now capable not only to consistently cover subparsec scales up to full galaxies, but also start to include radiative transfer and chemical networks. Combining both lines of research, we will compare high-resolution multi-frequency data with high-precision numerical simulations performed in our group. Our focus will lie on probability distribution functions (PDF) of parameters such as column density, temperature, velocity, and abundances, and on the detection of pronounced scales through wavelet filtering techniques that allow us to trace the imprint of the different physical mechanisms on the turbulent cascade at particular scales. With the PDFs we detect, e.g., column density tresholds, that may correspond to a change of the structure forming processes. Wavelet filtering techniques, like the Delta-variance and our recently developed cross-correlation method measure significant spatial scales. We will further improve this approach in order to assess the significance of filaments for ISM dynamics.