Abstract.
Contrary to theoretical speculations that weak Coriolis forces in slowly
and rigidly rotating dwarf irregulars may inhibit the development
of global magnetic fields, our high resolution
VLA study
of NGC 4449 reveals diffuse, extended synchrotron emission with
a substantial degree of polarization.
However, the regular fields show only some fragments of a spiral pattern.
They forms also polarized fans and ridges suggestive of the magnetic field
structure significantly influenced by large-scale gas flows and compressions
related to star formation.
1. Introduction
Rapidly, differentially rotating disk galaxies are known to possess large-scale
regular magnetic fields (Beck et al. 1996), probably generated by a turbulent
dynamo (e.g. Wielebinski & Krause 1993), driven by Coriolis forces in
the disk.
In very slowly and rigidly rotating dwarf irregulars this mechanism of
building-up global magnetic fields becomes inefficient thus, they may possess
no large-scale regular fields at all.
Until now the observational information on this subject was very poor.
In this work we present our
VLA total power and
polarization data on a dwarf irregular NGC 4449 made with a much higher
resolution and sensitivity than the pioneering study of this object by Klein
et al. (1996).
2. Observational results
Observations were performed using the
VLA in the
D-configuration at 8.4 GHz and 4.8 GHz.
To minimize the loss of extended, smooth structures the total power maps at
8.4 GHz (particularly affected by a missing zero spacing problem) were
combined with a single-dish map by Klein et al. (1996) at 10.55 GHz.
At the highest resolution the high-frequency total power emission shows a
patchy distribution resembling that of the star formation, with strong radio
peaks at the positions of bright star-forming regions.
However, the 8.4 GHz map made with a natural weighting, smoothed to
15" and combined with single dish data reveals also a smooth, diffuse
total power component reaching far beyond the optical disk
(Fig. 1).
The radio emission extends particularly towards north-east and south-east,
penetrating into the huge gaseous halo detected by Bajaja et al. (1994).
Despite the very weak rotation (Sabbadin et al. 1984) and a low efficiency
of mechanisms building-up large-scale fields, the diffuse, extended emission
shows strong signatures of regular magnetic fields.
About 40% of the area with detectable polarization is polarized by more
than 20%.
A polarized (up to 50% in peaks) ridge was found to run along a dense
portion of the H I shell surrounding the north-east and
eastern disk boundary (Hunter et al., priv. comm.).
The polarized emission forms also fans around bright star-forming regions in
the disk centre, with little polarization at their positions.
The mean equipartition value of the regular field,
averaged either over central fans or the outer polarized ridge amounts
to 9±3 µG (errors include uncertainties of assumptions).
In highly polarized regions the regular field may reach 11 µG.
The polarization B-vectors show large-scale, ordered domains with
similar magnetic pitch angles, making impression of some fragments of a spiral
structure.
However, a clear global spiral pattern is not obvious in our data
(Fig. 1).
On the other hand, the orientation of B-vectors shows some association
with large-scale gas dynamics.
Along the north-east and eastern edge the B-vectors run nearly
azimuthally, parallel to the H I shell with local wiggles
close to bright H II regions.
In the vicinity of the galaxy centre the regular field is directed radially
outwards from star-forming regions, running parallel to the system of
Halpha filaments (Sabbadin & Bianchini 1979).
[Click here to see Fig. 1!]
3. Discussion
The structure of galactic magnetic fields results probably from a competition
of two mechanisms: a Coriolis force-driven dynamo, giving rise to
galaxy-scale field symmetries, as well as a passive stretching and compression
by large-scale gas flows.
In rapidly rotating spirals the formation of global structures may be fast
enough to dominate the resultant field symmetry.
In dwarf irregulars the global field generation may work considerably slower,
allowing the magnetic field to be more strongly modified by gas flows and the
formation of shells around star-forming regions.
At present we cannot clearly state what kind of input magnetic field is subject
to modifications by the gas flows.
In case of a unidirectional field (the only able to produce Faraday rotation),
some low-Coriolis-force version of the dynamo can be still at work.
In a contrary case of a stretched and compressed random magnetic field, other
mechanisms amplifying efficiently strong random fields in large disk volumes
away from star-forming regions have to be considered.
These possibilities can be distinguished after obtaining the planned Faraday
rotation data.
Acknowledgments.
KTCh and MU are indebted to Professor Richard Wielebinski for his kind
invitation and support enabling them to visit the MPIfR, where a large part
of this work has been done.
We are also grateful to all our colleagues from MPIfR and Astronomical
Observatory in Krakow for their valuable remarks and comments.
This work was supported by a grant from the Polish Research Committee (KBN),
grant no. PB 962/P03/97/12.
References
- Bajaja E., Huchtmeier W.K., Klein U., 1994, A&A 285, 385
- Beck R., Brandenburg A., Moss D., Shukurov A., Sokoloff D., 1996,
ARA&A 34, 155
- Klein U., Hummel E., Bomans D.J., Hopp U., 1996, A&A 313, 396
- Sabbadin F., Bianchini A., 1979, PASP 91, 280
- Sabbadin F., Ortolani S., Bianchini A., 1984, A&A 131, 1
- Wielebinski R., Krause F., 1993, A&AR 4, 691
Links (back/forward) to:
| First version: | 10th | August, | 1998
|
| Last update: | 29th | September, | 1998
|
Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)