The Role of the Multi-Phase Interstellar Medium in Star Formation: From Clouds to Galaxies

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The collaboration between research groups located at the Purple Mountain Observatory (PMO), the Center for Astronomy at the University of Heidelberg (ZAH) and the Max-Planck Institute for Astronomy (MPIA) in Heidelberg will characterise the physical conditions of the star forming, molecular gas and their relation to the star formation process exploiting new, comprehensive data sets defining the state-of-the-art in the field. The physical conditions of this gas remain poorly constrained in other galaxies, yet they play the key role regulating the efficiency of star formation in galaxies. This joint Chinese-German science program will address this key question via a two-pronged approach: (1) via utilising two new (sub)millimeter imaging surveys of dense molecular gas tracers in a large sample of nearby galaxies: MALATANG (currently underway at the JCMT single dish telescope and led by the Chinese group) and EMPIRE (observations finished and data are science ready) and (2) via a detailed, high-resolution case study of the physical conditions in the molecular gas on scales of individual molecular clouds in the inner part of the nearby star-forming spiral galaxy NGC6946. Our unprecedentedly rich dataset containing more than 20 molecular transitions observed from ~10 to 1mm wavelength by the VLA and PdBI interferometers opens the door for comprehensive modelling of the physical conditions and probing their respective role in regulating star formation on cloud scales. The immediate science goals are (a) to develop a general model of the physical conditions probed by various molecular gas tracers on cloud-scales, (b) calibrate individual (or combination of very few) of these transitions to be used in a wider extragalactic context (c) to constrain the physical conditions of the high-density, imminently star forming material on cloud and galaxy scales and (d) use this information to investigate the dense molecular gas-star formation relation with respect to currently favoured theories of star formation, (e) derive more accurate luminosity to molecular gas mass conversion factors as a function of physical conditions (f) to benchmark chemical tracers for galactic shocks and (g) to build a large database of density sensitive molecular transitions for nearby galaxies. During the 3 years of this grant, we will further jointly develop a series of next generation observing proposals directly building on the results from these studies, with a focus on NOEMA and ALMA, in particular in light of China's future involvement in the former facility. The proposed project is set up with a long-term collaboration in mind, extending significantly beyond the funding period. The requested funding will allow us to merge the strong expertise regarding extragalactic ISM studies in all 3 groups, providing a powerful basis for high-impact results. This grant will jump-start this collaboration and enable frequent and close interaction of all group members in China and Germany.