On Holes and Shells in Galaxies

All these observations indicate that the interstellar medium (ISM) of medium to late-type galaxies is dominated by features, which have variously been described as shells, rings, holes, loops, bubbles or cavities (the 'cosmic bubble bath' (Brand & Zealey 1975) or 'violent interstellar medium'). Until recently, most authors concentrated on large spiral systems. However, there are several advantages in using dwarf galaxies to study the structure of the ISM: dwarfs are slow rotators, generally show solid body rotation, and lack density waves. This implies that once features like shells have formed, they won't be deformed by galactic shear and therefore tend to be long lived. Moreover, the overall gravitational potential of a dwarf is much smaller than that of a normal spiral, hence expanding structures can more easily reach large sizes.

Figure 1 (top left) shows an H I-map of HolmbergII, a member of the M81 group (at a distance of about 3.2Mpc) by Puche et al. (1992). This was the first high resolution, high sensitivity H I image of a nearby dwarf galaxy ever obtained and shows that the presence of H I holes clearly dominates its overall appearance (in total, 51 H I holes were catalogued by Puche et al.). This result prompted several questions such as: Is HolmbergII a special case? Do other dwarf galaxies show the same structures in their ISM?

Figure 1 (top right) presents a VLA H I surface brightness map of the dwarf galaxy IC2574, also a member of the M81 group (taken from Walter & Brinks 1998a). As can be seen from this map, the ISM of IC2574 is clearly shaped by H I holes and shells - a total of 48 were catalogued by Walter & Brinks. Figure 1 (bottom left) shows the distribution of neutral hydrogen in DDO47 (Walter & Brinks 1998b) which is a dwarf galaxy situated at an assumed distance of 4Mpc. Again, many holes and shells are visible in this galaxy - in fact, DDO47 resembles HolmbergII quite a bit. So far, about 20 H I holes have been catalogued in this galaxy, most of which are expanding (Walter & Brinks, in prep.).

So do all nearby dwarf galaxies show similar structures? To answer this question, studies of other dwarf galaxies are needed. Here I will briefly discuss NGC3077, another member of the M81 group. The distribution of the neutral hydrogen in this galaxy is presented in Fig. 1 (bottom right). Like all the other maps presented here, this map was obtained with the VLA after combining observations in three different configurations (B-, C- and D-array). An analysis of the data rather surprisingly showed that no expanding hole-like features are visible in this particular galaxy (Walter et al., in prep.). Note however, that NGC3077 is a special case since it is strongly interacting with its high-mass neighbors M81 and M82 -- it may therefore very well be that structures like holes and shells are destroyed prematurely due to the strong tidal forces.

**Figure 1:** *Top left:* H I surface brightness map of HolmbergII (Puche et al. 1992). The greyscale is a linear representation of the H I surface brightness. In total, 51 H I holes were catalogued by Puche et al. *top right:* H I surface brightness map of IC2574 (Walter & Brinks 1998a). The greyscale is a linear representation of the H I surface brightness. We find a total of 48 H I holes in this galaxy, most of which are expanding. *bottom left:* H I surface brightness map of DDO47. The greyscale is a linear representation of the H I surface brightness. A companion galaxy (not visible) has been discovered somewhat south-east of the main galaxy (see Walter & Brinks, 1998b). *bottom right:* H I surface brightness map of NGC3077 (Walter et al., in prep.). The greyscale is a linear representation of the H I surface brightness. The map has not been corrected for primary beam attenuation. The two prominent tidal arms connect this galaxy to M81 and M82 (see Yun, Ho & Lo 1994).
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