So, what created the holes?

Until not too long ago, the origin of these structures was generally thought to lie in the combined effects of stellar winds and supernova explosions produced by young stellar associations. For review articles see Tenorio-Tagle & Bodenheimer (1988), and van der Hulst (1996, and references therein). However, several authors have pointed out that this model is not without its flaws. One of the potential problems with the hypothesis that the H I holes are the result of an evolving OB association in which the most massive stars, through their winds and supernovae, create the observed supershells is the following. Using reasonable assumptions one still expects a substantial population of A and F main sequence stars to be present. However, searches by Radice et al. (1995) and Rhode et al. (1997) in galaxies like HoII have not led to the expected result. A possible alternative explanation has been proposed by Efremov, Elmegreen & Hodge (1998) who suggest that Gamma Ray Bursters, which recently have been conclusively associated with objects at cosmological distances, might provide the required energy, and occur frequently enough, to explain the observed H I supershells.

Another objection to the standard model is that in the case of the largest observed shells, the energy requirements seem to exceed the output of stellar winds and supernovae. To explain those structures an alternative mechanism was proposed: the infall of gas clouds. Tenorio-Tagle et al. (1987) present a numerical simulation and van der Hulst & Sancisi (1988) provide what is probably the best observational evidence for infall, the case of one of the largest holes in M101.

One should also consider the possibility that we are tricked by nature and that the holes that we see to be expanding are actually the result of turbulent motions. A search for their powering sources would then be completely futile. To investigate this possibility I examined turbulence cubes calculated by Mac-Low et al. (1998b) in the very same fashion as was done in our search for H I holes in galaxies (Walter & Brinks 1998a). The result was that only a few percent of the smaller holes may be due to turbulence. However, in the case of the larger holes (>100pc), turbulence is not able to produce coherent features which seem to be expanding. We therefore feel confident that the structures we observe are indeed due to expanding H I shells.

Obviously, to investigate the sources which created the holes, a multi-wavelength approach is needed. In this approach, 21cm observations are needed for the identification of the holes as well as for the determination of their kinematics. Optical observations are indispensable to check the stellar distribution and populations within the shells. Narrow band H observations are important to trace the regions of current star formation. Quite often, H-emission is found to be located close to or on the rim of the holes, as defined by the H I-observations. Finally, X-ray observations are important to check whether the cavities of the H I holes are filled by hot X-ray emitting gas or not. A hot-gas interior is one of the main predictions of theories which state that the holes are created by young OB-associations (see also the discussion below).

So far, only a few shells have been found where such an approach is possible. Examples are the supergiant shell LMC4 (Bomans, Dennerl & KУМrster 1994), the superbubbles N44 (Kim et al. 1998a) and N11 (MacТљLow et al. 1998a), all three situated in the LMC, the supergiant shell SGS2 in NGC4449 (Bomans, Chu & Hopp 1997) and the possible supershell near HolmbergIX (Miller 1995). In the following section, the detection of a supergiant shell in the nearby dwarf galaxy IC2574 is presented which is probably the most prominent supergiant shell known to date. This region has proved to be an ideal laboratory to study the physical nature of supergiant shells in general and is expected to provide conclusive proof as regards to the source lying at its origin.

Т? Copyright Astronomical Society of Australia 1997