Supershells in Spiral Galaxies

Summary

Expanding supershells are perhaps the most prominent manifestation of the violent impact which massive stars have on the gaseous ISM. Commonly thought to be formed as a consequence of mechanical luminosity dumped into the ISM by OB associations, supershells can be viewed as a critical gauge of the energy source which ultimately supports interstellar turbulence. I will review the present understanding of supershell evolution and highlight important issues of ongoing debate, such as the stellar content of expanding bubbles, instabilities leading to secondary star formation in cavity walls, and the degree of mass flux from disk to halo via chimney structures. Much of the discussion will center on emerging methods for closing the loop between theoretical and observational studies.

Despite the availability of sophisticated numerical models describing superbubble structure, virtually no detailed comparison between observational data and model predictions has yet been made. Thilker et al. (1998) developed an automated object recognition method to find, classify, and examine supershells located in spiral galaxies. After compiling a preliminary list of detections via datacube cross-correlation, the technique allows fitting a grid of supershell models to each expanding structure. In this way, we accurately constrain properties such as total kinetic energy, shell mass, and dynamical age within the context of existing models. Such a repeatable, unbiased method is notably superior to purely visual characterization of supershells.

This technique is now being applied to a sample of 21 nearby galaxies, including M31, M33, M81, and M101.