The Violent Interstellar Medium of Nearby Dwarf Galaxies
Fabian Walter, PASA, 16 (1), in press.
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.
© Copyright Astronomical Society of Australia 1997