1996A&A...313..396K

The large-scale distribution of the radio emission and the magnetic field of the bright irregular galaxy NGC 4449 is investigated. The galaxy possesses a low-frequency halo, with an extent of ∼7kpc. Its radio spectrum steepens towards the galaxy's periphery, with extreme values of the spectral index α=-0.7±0.1 (S_ν~ν^α). The synchrotron halo may have been produced by a galactic wind, driven by the star formation and associated supernova activity pervading the body of this galaxy. The fraction of thermal radio emission at 1GHz is f_th1=10±3%. Using an Hα image, we have made an attempt to separate the thermal and nonthermal emission across NGC 4449. The results are consistent with what is derived from the integral radio spectrum of the galaxy. The spectrum of the synchrotron radiation is found to vary considerably across the galaxy, with α_nth~-0.5 in the central regions, indicating on-going particle acceleration, and α_nth~-0.7 in the halo regime. While the overall nonthermal spectral index is identical to that of the Large Magellanic Cloud (LMC), the relative amount of thermal emission, though somewhat higher than in normal spirals, is not at all as high as in the LMC. A magnetic field, which in the halo has ordered components on kpc scales, is found to pervade NGC 4449, with degrees of linear polarization exceeding 40% on scales of 2-3kpc. It is mostly tangled within the central (disk) region of the galaxy. There is evidence for a coherent magnetic field structure to emerge from a chain of HII complexes, and stretching into the synchrotron halo. Together with the LMC, there are now two known examples of (low-mass) irregular galaxies with detected linear radio polarization.