Formation of giant molecular clouds and HI superclouds

Giant molecular clouds and HI superclouds may be formed by large-scale gravitational instabilities in spiral/disk galaxies. Different types of instabilities may occur under varying galactic conditions. In the outer disks of galaxies (and away from spiral arms), shear tends to suppress the growth of the ordinary Jeans instability, but self-gravitational growth can occur via the swing amplifier mechanism (see movies below).

Reduced shear and enhanced surface density within galactic spiral arms make these regions highly favorable for cloud formation and collapse. In arms and in inner disks where the rotation curve is closer to solid-body, Jeans-type instabilites can lead to growth as long as magnetic fields are present to transfer angular momentum out of growing condensations (see movies here). Key observational support for the idea that gravitational instabilities determine the course of events comes from observed thresholds in surface density for active star formation in disks. Other evidence for the importance of self-gravitating instabilities is seen in M51, where prominent spurs jut out from the spiral arms at regular intervals, consistent with the predictions of our dynamical theory .

Using both semianalytic methods and time-dependent numerical simulations, our group is studying the development of self-gravitating clouds in the gaseous component of spiral galaxies. Our work has shown that nonlinear, nonaxisymmetric growth of structure in outer disks is indeed subject to threshold behavior, confirming the longstanding empirical model. We find that the critical values of the Toomre Q parameter for gravitational runaway are similar to observed star formation threshold values, and insensitive to the strength of the large-scale magnetic field. Our work has also shown how gravitational instabilities acting in galactic nuclei -- potentially leading to starbursts -- are qualitiatively quite different from outer-disk instabilities.

 Gravitational instability in shearing
 disk with Q=0.8<Q_c

 Gravitational instability in shearing  
 disk with Q=1.1 ~ Q_c

 Stable development of structure in shearing 
 disk with Q=1.3 >Q_c 

We have also studied the growth and evolution of intermediate-scale ``spur'' structures that result from the dynamical interaction of the gaseous ISM with large-scale stellar spiral arms, as shown below and described in more detail here  . Most recently, we have studied  instabilities in three-dimensional disk models, which allow for comparison of the results of Parker (magnetic-buoyancy) modes with Jeans-type modes. Based on this work, we have concluded that giant molecular clouds likely originate via collective, self-gravitating instability  -- either "swing" or "magneto-Jeans", depending on the local structure of the galactic potential.

   Development of spurs

My collaborators in this work are Woong-Tae Kim (Maryland PhD 2002; presently Harvard University), and Jim Stone (Princeton University). For more information, see:

  • Our publications
  • "Research" and "Movies" links on Woong-Tae Kim's home page

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