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

H I study of the dwarf galaxy DDO 47

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. High resolution VLA-observations of the dwarf irregular galaxy DDO 47 show the presence of many hole-like structures in its neutral interstellar medium. Most of these holes are found to be expanding - their origin is therefore believed to be due to older star forming regions and their aftermaths. At a projected distance of 20 kpc (adopting a distance to DDO 47 of 4 Mpc), a companion galaxy was detected at almost the same systemic velocity (DDO 47 B). This suggests that the projected distance is close to the real one. CGCG 087-033 is believed to be the optical counterpart of the companion. A first dynamical study yields that DDO 47 needs much dark matter (about 80% of its dynamical mass) to explain the observed dynamics. In addition, DDO 47 belongs to the 'thickest' dwarf galaxies studied so far.

1. Introduction and Observations

One of the striking results to emerge from recent studies of dwarf galaxies is that many holes and shell-like structures are visible in their interstellar medium (ISM) - in a much more prominent way as seen in, e.g., spiral galaxies. It is believed that these holes are created by star forming regions and their aftermaths. Since dwarf galaxies are smaller in mass compared to spirals, these events have a more dramatic influence on the shape of the individual galaxy (see van der Hulst 1996, in 'The Minnesota Lectures on Neutral Hydrogen', ASP Conf. Ser. 106, Skillman E. (ed.), for a review). In addition, dwarf galaxies seem to contain more Dark Matter than massive galaxies. Related to this is the observation that the dark matter halos of dwarf irregulars often dominate their dynamics even within the optical radius of the galaxy which is not the case for more luminous spirals.

Although dwarf galaxies are important laboratories for these kind of studies, only a few detailed H I studies of nearby dwarfs have been performed so far. Examples are the Large and the Small Magellanic Clouds (see Kim et al. 1998, this volume), Holmberg II (Puche et al. 1992, AJ 103, 1841) and IC 2574 (Walter & Brinks 1998, AJ, submitted, see also this volume). This is a major motivation for this study of DDO 47. DDO 47 (UGC 3974) was observed in the 21 cm line of neutral hydrogen with the NRAO Very Large Array (VLA) in its D, DnC, C and B-array. In total, 16 hours were spent on source. The velocity and spatial resolution are 2.5 km s-1 and 7" (resulting in a linear resolution of 120 pc at an adopted distance of 4 Mpc). In the course of the data reduction we discovered a companion galaxy at nearly the same systemic velocity.

2. The H I-Halpha connection of DDO 47

In total, we detected 19 H I-holes in DDO 47. An overlay of the positions and sizes of the holes with the H I surface brightness map is given in Fig. 1 (left). Most of the holes were found to be expanding. This suggests that they are produced by star forming regions and their aftermaths. The theory of propagating star formation predicts that secondary sites of star formation form near the rims of expanding H I holes. The idea behind this is that these expanding holes collect the ambient medium on their rims until the mass density gets high enough for star formation to start. To test this scenario in the case of DDO 47, the Halpha-regions detected by Strobel et al. (1991, ApJ 383, 148) are also overlaid with the H I surface density map in Fig. 1 (right). Note that virtually all Halpha emission is situated outside the holes which beautifully fits into this picture.

[Click here to see Fig. 1!]

3. Dynamics of DDO 47 and the thickness of its H I disk

The dynamical mass of DDO 47 was determined to be about 5·109 Msun. Assuming a gaseous mass of 3·108 Msun (H I corrected for the contribution of primordial helium) and a stellar mass of 2·108 Msun (assuming a solar mass to light ratio for the stars) yields that most of DDO 47s mass is present in the form of Dark Matter. Using a simple model which relates the velocity dispersion of the gas to its scale height, we derive the thickness of the H I-layer of DDO 47 to be about 700 pc. DDO 47 is therefore one of the thickest dwarf galaxies studied so far.

4. Results

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First version: 17thAugust,1998
Last update: 08thOctober,1998

Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|