Bubbles and superbubbles in disk galaxies

Bubbles and Superbubbles are among the largest and most energetic structures in the Interstellar Medium (ISM). It is thought that most bubbles are formed from the combined effects of stellar winds and supernovae, which plow through the ISM, sweeping up matter in to dense atomic hydrogen (HI) shell walls and heating and ionising the gas left behind. Although this explanation works clearly for some of the smaller objects, it is hard to understand how many superbubbles apparently require hundreds to thousands of massive stars to explain their sizes. In some galaxies, like the Large Magellanic Cloud, the H I structure of the disk is dominated by H I shells. Similarly, the ISM of the Milky Way is riddled with tens, perhaps hundreds, of H I shells. In the Milky Way, as well as in the nearest disk galaxies, there is clear evidence of superbubbles that have grown so large as to break out of the disk. It would seem, then, that not only do these objects have a significant impact on the disk, they provide an invaluable source of energy for the halo. In this review I present some of the latest results on bubbles and superbubbles in disk galaxies and their relationship to the halo.