The Hierarchical Structure of Star-Forming Regions in the Milky Way (NSF Proposal). 2011.Abstract.
Intellectual Merit In September of 2011, several of us attended a meeting in Rome called “Milky Way 2011,” organized to showcase the plethora of recent surveys of star-forming material in our Galaxy. We were simultaneously excited and disappointed by the meeting: excited by the wealth of new data; and disappointed to see the lack of coordinated analysis of it. We went home determined to create a collaborative project offering a more synthetic understanding of star formation in the Milky Way. We, and many other astrophysicists, want to know how, where, and when a galaxy like the Milky Way forms stars. Presently, it is easier to answer this question empirically in nearby galaxies, thanks to our “external” vantage point with respect to those systems. But, that vantage point is far away, so our ability make predictive physical models of star formation based on observations of external spiral galaxies is limited, even with new instruments like ALMA. In the Milky Way, we have tremendously detailed studies of star-forming regions within a few hundred pc of the Sun, but none of these nearby regions contains any massive (O) stars characteristic of the star-forming regions observationally detected in external galaxies. Here, we propose to evaluate the star-forming potential of gas under the full variety of conditions observed the Dame et al. (2001) CO Survey of Molecular Gas in the Milky Way. We will not study only nearby clouds, or only massive star-forming regions, but instead every environment our Galaxy has on offer. Our work, using hierarchical “dendrogram” trees to deconstruct molecular gas emission, will be distinguished from previous analyses of star-forming clouds in the Milky Way by how it identifies significant structures within the gas. Essentially all previous work has divided up gas along sharp boundaries, without taking into account the obvious hierarchical nature of the gas. By analogy, this non-hiearchical approach is like dividing up whole metro Minneapolis-St. Paul into just two parts defined by a unique boundary between the cities. This approach takes no account of the fact that these two separate cities are surrounded by a combined “metropolitan area,” and further surrounded by a state called Minnesota that also contains other cities, like Duluth, each of which has its own identity, likely determined in part by its surroundings. We suspect that environment does matter, and that the artificial division of gas into non-overlapping clumps has masked important physical effects. By combining the hierarchical decomposition of star-forming material we will create with catalogs of associated dense star-forming “cores” and young stars, we will evaluate which of several potential physical processes is key to the formation of stars. In particular, we will study whether a feature’s self-gravity, its internal density or pressure distributions, or external pressure–or a combination of those–is most predictive of fecundity. Our results will clarify our understanding of star-formation in the Milky Way, and offer physical insights into empirical trends measured in other galaxies. Broader Impact The PI and several Co-Is are known for their work on data visualization, data sharing, and science education. All the products created in this work, including a 3D visualization of star-forming gas in the Milky Way, will be made available online through the universe3d.org, Astronomy Dataverse, Seamless Astronomy, and WorldWide Telescope sites, all of which the PI has had a hand in creating. The Astronomy Dataverse is presently being deployed as an NSF-data-management-plan-compatible solution for short-term distribution and long-term preservation of astronomical data, and this project will serve as a key demonstrator of the Dataverse’s utility. The new Milky-Way shaped Haus der Astronomie visualization and outreach facility in Heidelberg will play host to a meeting on “Star-Formation in the Milky Way, in 3D,” co-sponsored by the Max-Planck Institute for Astronomy as part of this project.