Oral Presentation
New Views on Turbulence: The Riddle of Gravitational Binding in the Disk and Center of the Milky Way
Presenter: Jens Kauffmann (Haystack Observatory, Massachusetts Institute of Technology)
Surprisingly little is known about turbulence and the gravitational binding in the dense gas of massive clouds at the onset of star formation, such as Infrared Dark Clouds (IRDCs). This is a consequence of the low line intensities in these sources: the study of these targets is still tough today, so that little work has been done so far.
A first deep and thorough study of massive clouds with little star formation resulted in a surprising finding: gas motions in the dense gas of starless clouds in the Galactic Disk are so low that the cloud centers exceed the turbulent Bonnor-Ebert mass by a factor 10 (Pillai et al. 2011; Kauffmann et al. 2013b). These clouds should collapse quickly --- unless there is additional support from strong magnetic fields. Subsequent searches for magnetic fields indeed yield some evidence for support from such fields.
One might wonder whether the situation is similar in the Galactic Center, where clouds are very turbulent on spatial scales of a few parsec. In a series of studies using the SMA (Kauffmann 2013a, 2017a,b) and ALMA we find that clouds are indeed very turbulent and unbound on large spatial scales --- but the densest gas in these clouds has velocity dispersions similar to what is found near the Sun, resulting in very strong gravitational binding of the dense gas. In other words, the linewidth-size relation is unusually steep, resulting in a linear relation that possibly results from Galactic Center dynamics.
A new challenge is emerging today: very little is known about turbulence in filamentary molecular clouds in the Galactic Disk. This is surprising: filaments have recently been studied extensively thanks to Herschel, but the gas kinematics have yet to be probed comprehensively. A first such study, covering more than a hundred filaments (Mattern et al, in prep.), finds that turbulence in these structures is roughly sufficient to provide support against self-gravity. This work also reveals surprising trends in filament density structure that suggests that the idea of a well-defined mass-per-length characterization of filaments is not possible.
In combination, these observations produce critical new insights on turbulence. To give an example, not a single theoretical model has yet explained the the extreme gravitational binding (i.e., factor 10 in mass) this hat has now been observed in many IRDCs.
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