Proceedings of the
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg
Holes and Shells in IC 2574
1Radioastronomisches Institut, Bonn,
Astronomía, Guanajuato, Mexico
3National Radio Astronomy
Observatory, Socorro, NM, USA
Received 24th May 1998
We present VLA
-synthesis observations of the Violent Interstellar
Medium of the nearby dwarf galaxy IC 2574 (a member of the
M81 group of galaxies) at high spatial and velocity resolution.
The H I
-observations show a stunning amount of detail
in the form of H I
shells and holes in the neutral
interstellar medium of IC 2574, ranging in size from 100 to
The most likely explanation, as has been proposed by previous studies, is
combined effects of stellar winds and supernova-explosions of the most
massive stars, blowing holes and shells into the interstellar medium.
-regions along the edges of larger shells are believed
to correspond to secondary sites of star formation.
Some twenty years ago, Heiles discovered large, shell-like structures in
the 21 cm line of neutral hydrogen (H I) in our Galaxy
(Heiles 1979, ApJ 229, 533).
Since this discovery, a wealth of observations obtained with powerful
Synthesis Radio Telescopes such as the
Very Large Array (VLA)
or the Westerbork Synthesis Radio Telescope (WSRT) revealed similar structures
in our nearest neighbors.
Prominent examples are M 31, M 33, Holmberg II, M 101
& NGC 6946 and DDO 47.
All these observations indicate that the interstellar medium (ISM) of medium
to late-type galaxies is dominated by features, which have been variously
described as shells, rings, holes, loops, bubbles, cavities or the 'Violent
Our aim is to investigate the origin and evolution of
the H I shells and bubbles in the ISM of galaxies.
Intimately linked to this are the questions when fragmentation on the rim of
the shells starts and what drives star formation.
To do so we decided to direct our attention to nearby dwarf galaxies.
Dwarfs are slow rotators, generally show solid body rotation, and lack density
This implies that once features like shells have formed, they won't be
deformed by galactic shear and therefore tend to be long lived.
IC 2574 was observed in the 21 cm line of neutral hydrogen
(H I) with the NRAO
Very Large Array (VLA)
in its B-, C- and D-configuration, leading to a spatial resolution of
70 pc (assuming a distance of 3.2 Mpc) and a velocity resolution of
2.5 km s-1.
In total, 17 hours were spent on source.
The H I-surface brightness map of IC 2574 is shown in
In addition, IC 2574 was observed with the 1.23 m telescope of the
Calar Alto Observatory in March
1997 in Johnson R-, B- and V-bands as well as in
The deep R-image was also used to determine the continuum contribution
in the Halpha-image.
[Click here to see Fig. 1!]
3. Holes and Shells in IC 2574
In total, we found 48 H I-holes in IC 2574 (see
Fig. 1 for an overlay with the total
For each hole, the position, diameter, expansion velocity, and the ambient
H I-density have been catalogued.
From that, the energies needed to produce these features, the ages and
the masses on the rim of the shells were calculated.
Thereafter, the physical properties of the holes were compared with those
found in other galaxies (see Conclusions).
High resolution H I and Halpha imaging using
the VLA and
the Calar Alto observatory of
the dwarf galaxy IC 2574 have been presented.
The main results and conclusions are:
- IC 2574 is a gas-rich dwarf galaxy which shows a stunning amount
of detail in the form of H I shells and holes in
its interstellar medium.
These features are similar to those found in the Galaxy, Ho II,
M 31, M 33, M 101 and NGC 6946.
From the global H I profile we derive a total
H I-mass of
- Circular large scale empty structures are prominent in the high resolution
VLA H I channel maps of IC 2574.
In total, 48 H I holes have been catalogued which
are distributed across the entire galaxy.
The statistics of the H I holes in IC 2574 are
summarized in Brinks & Walter (1998,
- A comparison with other galaxies studied so far (especially M 31,
M 33 and Ho II) confirms the trend that the sizes
of H I holes found in any galaxy is inversely
proportional to their global gravitational potential.
The energies needed to create these structures, however, are found to be
the same for all types of galaxies.
The energy output of a typical starforming region is therefore not related
to its galactic environment and its impact on the interstellar medium is
dependent on the gravitational potential of the galaxy.
For a detailed analysis of these properties see
Brinks & Walter 1998
- The fact that virtually all holes in IC 2574 are circular in shape
is a major difference to other galaxies studied in that detail so far and
can be explained in terms of the absence of galactic shear and spiral
density waves in IC 2574.
Links (back/forward) to:
|First version: ||17th||August,||1998
|Last update: ||30th||September,||1998
Jochen M. Braun &