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Astron. Astrophys. 355, 128-137 (2000)

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5. Summary and conclusions

We performed a total power and polarization study of the dwarf irregular galaxy NGC 4449 at 8.46 and 4.86 GHz using VLA in its D-configuration. The object rotates slowly and chaotically, thus no large-scale regular magnetic fields were expected. To reach the maximum sensitivity to extended structures we combined our VLA data with the Effelsberg ones at 10.55 GHz and 4.85 GHz, respectively. Despite the slow and chaotic rotation of NGC 4449, unfavourable for dynamo-induced magnetic fields, we found it to possess strong regular, galaxy-scale fields.

The following results were obtained:

  • NGC 4449 shows a large, partly polarized halo extending from its main plane up to 3.5 kpc, more than the isophotal major axis radius at [FORMULA].

  • The radio-brightest peaks coincide with strongly star-forming regions. These regions show increased thermal fractions (up to 80%), however the nonthermal emission is enhanced there as well.

  • The galaxy possesses regular magnetic fields reaching locally 6 - 8 µG, comparable to those in rapidly rotating spiral galaxies.

  • NGC 4449 shows large domains of non-zero Faraday rotation measures indicating a genuine galaxy-scale regular magnetic field rather than random anisotropic ones with frequent reversals of their direction.

  • The magnetic field structure consists of two basic elements: radial "fans" stretching away from the central star-forming complex and a magnetic ring at the radius of about 2.2 kpc. The magnetic field in the ring shows clear characteristics of a magnetic spiral with a substantial radial component signifying dynamo action.

  • Both the radial "fans" and the polarized ring can still be explained in terms of a combination of sky-projected poloidal and toroidal dynamo-generated fields, taking into account the smaller size of NGC 4449 compared to normal massive spirals. Alternatively, magnetic "fans" could result from the gas outflow from the central star-forming complex. Even in this case a large-scale coherence of the magnetic field subject to stretching by outflows is required. Thus, some kind of dynamo action is needed, with a preference of non-standard (e.g. buoyancy-driven) dynamos. Whether and how this process can produce classical dynamo-like magnetic fields in a complex and chaotic velocity field of NGC 4449 remains yet unknown.

The detection of regular magnetic fields in spiral galaxies is important for understanding processes like turbulence, turbulent diffusion and the magnetic field generation in astrophysical plasmas; this is also of importance for plasma physics in general. It demonstrated that even in a highly turbulent medium large-scale regular fields can persist and grow quite efficiently. This has already boosted the development of dynamo theories applicable not only to a variety of astrophysical objects from planets to clusters of galaxies but also to laboratory plasmas. On the other hand, there was a widespread prejudice that all the mentioned concepts are restricted solely to rapidly rotating plasma bodies. Against these expectations we show that strong regular fields can also arise in slowly and chaotically rotating systems. Their dynamical role in dwarf irregulars, especially in processes of star-formation triggered by magnetic instabilities, filament formation and confinement or even accelerating galactic winds via cosmic ray pressure exerted on MHD waves (Breitschwerdt et al. 1991), cannot be further neglected. NGC 4449 is the irregular galaxy with the best studied magnetic field so far. We believe that further progress needs more detailed models for such objects. Further detailed observations of the radio polarization of a larger number of irregulars with various morphological characteristics are also required.

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000