The Violent Interstellar Medium of Nearby Dwarf Galaxies

Fabian Walter, PASA, 16 (1), in press.

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Conclusions

H I observations of sufficient angular and velocity resolution as well as sensitivity of dwarf irregular galaxies are now becoming available in the literature. Because dwarfs have several advantages over spiral galaxies (the absence of density waves, the absence of differential rotation and hence shear) one can expect a lot of progress to be made in our understanding of the structure of the ISM. In addition, multi-wavelength studies of the most prominent shells will reveal which physical processes are the cause of the H I holes in general. The prominent supershell in IC2574 is a nice example of what can be done in that respect. In summary:

1.
Dwarf galaxies show a stunning amount of detail in the form of shells and holes in their neutral interstellar medium. These features are similar to those found in large systems like our Galaxy, M31, M33, M101 and NGC6946. Often, the H I shells completely dominate the morphology of a dwarf galaxy, such as in the case of IC2574 or HoII.

2.
The shells can grow to large dimensions because of several conditions which are favourable in dwarf galaxies. The volume density in the plane is low, which facilitates expansion. In the direction perpendicular to the disk, the gravitational pull is smaller than in a massive spiral. Also, because of the thick H I layer, shells are easily contained and unlikely to break out into the halo. Lastly, solid body rotation and a lack of spiral density waves prevent holes from being rapidly destroyed.

3.
A comparison of IC2574 with other galaxies spanning a range of Hubble types and studied in similar detail so far (M31, M33 and HoII) shows that the size distribution of H I holes found in a galaxy is related to its Hubble type in the following way. The size of the largest H I shells is inversely proportional to the global gravitational potential (and hence mass surface density). The energies needed to create these structures, though, are found to be of the same order for all types of galaxies. Hence, whatever physical mechanism lies at their origin, it is not related to the host galaxy, at least to first order.

4.
In order to determine the source(s) which created the observed structures a multi-wavelength approach is needed. So far, a few theories have been invoked to explain the presence of the holes: 1) young OB-associations which drive the expansion of the shell by strong stellar winds and supernova explosions, 2) Gamma Ray Bursters and 3), for the largest holes, the infall of high velocity clouds. The scenario regarding the creation due to strong stellar winds and subsequent supernova explosions still has some problems. Detailed studies of single H I holes are needed to distinguish between the various theories.

5.
The supergiant shell in IC2574 seems to be an ideal target to shed light on this problem. This supergiant shell is clearly defined in H I observations and is surrounded by massive star formation. A pointed ROSAT observation has revealed that the cavity enclosed by the supergiant shell is filled with hot, X-ray emitting gas. A prominent stellar association in the center of this SGS is thought to be the powering source for the formation and expansion of the shell as well as for the heating of the interior X-ray emitting gas. Future investigations, especially regarding the central stellar association, are needed to understand fully what created this fascinating region.


Next Section: Acknowledgements
Title/Abstract Page: The Violent Interstellar Medium
Previous Section: The case of the
Contents Page: Volume 16, Number 1

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