The Density Structure of Clouds: Large Samples, Star Formation Thresholds, and Theoretical Models Jens Kauffmann Jet Propulsion Laboratory, Pasadena, CA, USA
During the 2010 EPoS conference, I presented a new approach to comprehensively characterize the structure of molecular clouds from their smallest to their largest spatial scales. In that first presentation, I demonstrated that it is possible to constrain the density profiles in clouds from the data generated in this fashion. It turned out that this density --- or mass-size --- information is very valuable, since it permits to identify empirical thresholds for massive star formation (i.e., MSF only occurs in cloud fragments with a mass > 870 M_sun [r / pr]^1.33). In summary, this research provided a first reference scheme to understand how the structure of dense cores depends on the clouds in which they are embedded. But it had the drawback of being based on a very limited sample, and comparison to detailed models was very preliminary. Here, I present an update to this body of work. Maps of dust emission and extinction from Spitzer, 2MASS, and Herschel help to massively increase the sample size. Data at very small radii, critical to test models of high mass star formation, comes from interferometric dust and line observations. Structure analysis processing of synthetic maps of model clouds generated in numerical experiments yield data that permit direct comparison with real observations. This yields a rich observational data set that can be analyzed using appropriate numerical modelling techniques. In some sense, this work permits to predict the small-scale structure in molecular clouds based on cloud properties measured on scales of a few parsec. Such knowledge is becoming increasingly crucial for our understanding of nearby galaxies, where we can resolve clouds of ~10pc size. I illustrate how the properties of Milky Way molecular clouds compare to the properties of extragalactic clouds. This comparison has to be executed with extreme care: in an interferometric study of a very dense galactic center cloud (G0.253+0.016, e.g. Longmore et al., arXiv:1111.3199) I demonstrate that a high average density measured on large spatial scales (~10^5 M_sun within 2.5 pc radius) does not need to lead to significant star formation. In other words, the star formation law relating gas properties and star formation rate may massively depend on environment. If this is true, such dependencies would be extremely important for the understanding of other galaxies.
Collaborators: T. Pillai, Caltech, USA G. Parmentier, University of Bonn, Germany R. Shetty, Zentrum fuer Astronomie Heidelberg, Germany P.C. Myers, Harvard-Smithsonian Center for Astrophysics, USA A.A. Goodman, Harvard-Smithsonian Center for Astrophysics, USA	Key publication