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

1. Introduction

The generation of large-scale galactic magnetic fields from small-scale field perturbations caused by turbulence (as postulated by the dynamo concept) requires a preferred sense of twisting of turbulent gas motions, called the -effect (Wielebinski & Krause 1993). In normal spiral galaxies it is determined by Coriolis forces caused by the disk rotation giving rise to strong dynamo action and to the observed spiral-like regular magnetic fields (Beck et al. 1996b). To make the dynamo process work, either the differential rotational shear or the galaxy's angular speed (in case of rigid rotation) must exceed certain threshold values (Ruzmaikin et al. 1988).

Dwarf irregulars are small, low-mass galaxies with a patchy distribution of star-forming regions. Though they exhibit a large variety of rotation curves (Hunter et al. 1998a) many of them show slow rotation with much less rotational shear than in normal spirals. Some dwarf irregulars show complex velocity fields with chaotic motions comparable in speed to the overall rotation. Even if the dynamo could still work in such conditions, the generation time scales of the magnetic fields estimated from classical dynamo theory would be very long and strong large-scale magnetic fields are not expected. Their observational detection would mean that the dynamical role of global magnetic fields in gas dynamics and star formation in irregular galaxies has to be reconsidered.

Signatures of a global magnetic field were already detected in the Large Magellanic Cloud (LMC, Klein et al. 1993). However, this galaxy still shows a significant degree of differential rotation (Luks & Rohlfs 1992) so that, like in normal spirals, the standard dynamo process could be at work. In this paper we present a sensitive radio polarization study of the dwarf irregular galaxy NGC 4449 which exhibits only weak signs of global rotation (cf. also Sabbadin et al. 1984, Hartmann et al. 1986). The radial velocities in NGC 4449 relative to the systemic one reach - 30 km/s. However, the analysis of the high-resolution HI data cube (kindly supplied by Dr D. Hunter) does not show the classical picture of a global rotation. Instead, NGC 4449 shows velocity jumps and gradients along both the major and minor axis with centroids not coincident with the optical centre. They are intermixed with chaotic velocity variations with an amplitude of about 10 - 15 km/s. These very complex and chaotic kinematics, partly due to the interactions with DDO125 (Hunter at al. 1998b) and possibly also to a high star formation rate, make NGC 4449 an interesting target to investigate the magnetic field structure under conditions very difficult for the classical galactic dynamo. The basic parameters of NGC 4449 are summarized in Table 1.

Table 1. Basic properties of NGC 4449

A low-resolution detection of polarized emission (Klein at al. 1996) showed that the magnetic field in NGC 4449 is running across its bright star-forming body, very different from that in normal galaxies. In this work we present a total power and polarization study of this galaxy with a resolution and sensitivity several times better than that in the work of Klein et al. (1996). The use of two frequencies (8.46 and 4.86 GHz) enables us to determine the distribution of Faraday rotation over the disk of NGC 4449, allowing to discriminate between the galaxy-scale uniform fields and those passively stretched and compressed in the gas flows powered by huge star-forming regions.

© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000
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