High resolution images of nearby spiral galaxies show large dust and gas bubbles in the spiral arms (Fig. 1; see also Fig. 3 in Lecture 2). Their radii are often larger than the gas scale heights, so these are unlikely to be spheres; they are more like rings in the disk. There are also feathers, comets, and other fine-scale dust structures in optical images - all indicating recent dynamical processes. Sometimes there are small bubbles inside large bubbles, and there are generally more bubbles near the spiral arms than in the interarm regions. The interarm also contains dust streamers, and many of these look like old bubbles left over from more active times in the arms.
Figure 1. Region of the galaxy M51 viewed with the ACS camera of HST, showing bubbles.
Gould's Belt contains the Local Bubble, studied recently by Lallement et al. (2003) using Na I absorption clouds near the Sun. This local bubble is also a source of diffuse x-ray emission from hot gas (Snowden et al. 1998). Presumably the energy came from hot stars in the Sco-Cen association (Breitschwerdt & de Avellez 2006).
Large bubbles in the Milky Way are also evident from IR maps of the sky. Könyves et al. (2007) used IRAS 60 µm and 100 µm images to identify Milky Way bubbles. They reported that the bubble volume filling factor in the inner galaxy is around 30%, and in the outer galaxy it is around 5%. Ehlerová & Palous (2005) catalogued HI shells in the Milky Way using the Leiden-Dwingeloo Survey. They found a volume filling factor of 5%, a mean age of 8.4 Myr, and a ratio of age to filling factor equal to 170 Myr. This latter timescale is the time for the whole interstellar volume (not mass) to be cycled through one or another HI shell. This is 10 times faster than the time for molecular gas to be converted into stars. Since the shells observed by Ehlerová & Palous are atomic, this ISM processing would seem to be independent of the molecular cloud population. The GMCs have a low volume filling factor and the HI shells occupy the space between them. A significant fraction of the HI shell mass can come from GMC disruption in the inner galaxy where the molecular fraction is high.
The LMC is also filled with large shells (Goudis & Meaburn 1978). The largest, LMC4, has no obvious cluster or OB association in the center, although there are A-type stars suggesting a ~ 30 Myr old age (Efremov & Elmegreen 1998). It has pillars at the edge with young star formation, and an arc of young stars without gas in the center, called Constellation III (McKibben Nail & Shapley 1953). Yamaguchi et al. (2001) studied the star formation in this region, pointing out GMCs and young clusters all along the edge of the shell and suggesting these were triggered.
IC 10 is another small galaxy filled with HI holes and shells. Wilcots & Miller (1998) found H at the edges of the shells and discussed these young regions as triggered star formation. Similarly, the small galaxy IC 2574 has a giant shell with an old central cluster and triggered young stars on edge (Walter & Brinks 1999, Connon et al. 2005).