The disk-halo interaction - Superbubbles and the structure of the interstellar medium

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Abstract

The Type II supernovae in our Galaxy are spatially and temporally correlated and the consequences of such correlations are superbubbles and supershells fed by tens or hundreds of supernovae per bubble. These objects evolve and expand rapidly, and they soon break out of the disk of the Galaxy. The collimated structures formed in this process are called chimneys. We assume that the interaction between the disk and the halo is dominated by the upward flow of mass, energy, momentum, and magnetic flux convected in the chimneys. As cooling occurs, the cycle is completed by the downward flow, from the halo to the disk, of gas that has cooled and formed clouds. These clouds rain down onto the disk, returning to it both mass and magnetic flux, and some energy and momentum in the resulting shocks, as the clouds strike the disk. This is similar to the galactic fountain model but with a highly concentrated upward energy flow in chimneys rather than over the entire disk. We make the further simplifying assumption that these superbubbles dominate the energy input into both the disk as well as the halo and examine the consequences of this model for our understanding of the structure of the interstellar medium and the gaseous halo. This admittedly extreme assumption, necessary for our simplified analysis, is motivated by recent observations of the structure of the interstellar medium in our own Galaxy and external galaxies. Our theory indicates a modification in the understanding of the nature of both the interstellar medium and the halo. The essential difference here from the 1977 McKee-Ostriker theory is that, at least currently, for our own Galaxy, the filling factor of the hot gas in the disk is significantly less than unity. We describe the structure of the interstellar medium using as the fundamental parameters the clumping of the Type II supernovae rate and the mean ambient density. We sketch how, for galaxies of various types, the interstellar medium can be three-phase, chimney, or two-phase. The state of the interstellar medium may also vary within a given Hubble type as a function of galactocentric radius. Temporal variations occur when a galaxy changes its star formation rate, for example, if it is triggered into the starburst mode. Some of the additional implications of our model are noted here including aspects of dynamo theory, quasar absorption lines, and starburst galaxies.

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