Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

Holes and Shells in IC 2574

Fabian Walter1, Elias Brinks2,3, and Uli Klein1

1Radioastronomisches Institut, Bonn, Germany
2Departamento de Astronomía, Guanajuato, Mexico
3National Radio Astronomy Observatory, Socorro, NM, USA

Received 24th May 1998

Abstract. We present VLA H I-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 1500 pc. 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. H II-regions along the edges of larger shells are believed to correspond to secondary sites of star formation.

1. Introduction

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 Interstellar Medium'.

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 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.

2. Observations

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 Fig. 1.

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 Halpha. 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 H I). 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).

4. 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:
  1. 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 9.0·108 Msun.
  2. 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, this volume)
  3. 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 (this volume).
  4. 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.
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First version: 17thAugust,1998
Last update: 30thSeptember,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)