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
- DDO 47 is a gas-rich dwarf irregular galaxy at distance of 4 Mpc.
- A companion has been detected at almost the same systemic velocity.
Its H I mass is about 7% of the one of DDO 47.
The companion has been previously catalogued in the optical as
CGCG 087-033.
The geometry of the pair suggests that the relative orbits are within
the plane of the sky.
The projected distance (20 kpc) is therefore believed to be close
to the true seperation.
- To explain the observed rotation, DDO 47 must have a high Dark
Matter content.
- In total, 19 H I holes were detected, most of
which are expanding.
There is a striking correlation between the positions of the holes and
current star forming regions (as traced by
Halpha-observations).
- The thickness of the H I disk is about 700 pc.
DDO 47 is therefore one of the thickest dwarf galaxies studied so far.
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| First version: | 17th | August, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)