The role of the magnetic field in the interstellar medium of the post-starburst dwarf irregular galaxy NGC1569

NGC1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10-40 Myr ago. We present observations that reach surface brightnesses 2-80 times fainter than previous radio continuum observations and the first radio continuum polarization observations of this galaxy at 20cm, 13cm, 6cm, and 3cm. These observations allow us to probe the relationship of the magnetic field of NGC1569 to the rest of its interstellar medium (ISM). We confirm the presence of an extended radio continuum halo at 20cm and see for the first time the radio continuum feature associated with the western Hα arm at wavelengths shorter than 20cm. Although, in general, the spectral indices derived for this galaxy steepen as one moves into the halo of the galaxy, there are filamentary regions of flat spectral indices extending to the edge of the galaxy. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. There is strong polarized emission at 3cm and 6cm and weak polarized emission at 20cm and 13cm. We estimate that the thermal fraction is 40%-50% in the center of the galaxy and falls off rapidly with height above the disk. Using this estimate, we derive a total magnetic field strength of 38 μG in the central regions and 10-15 μG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ∼ 3-9 μG and is strongest in the halo. Using our total magnetic field strength estimates and the results of previous observations of NGC1569, we find that the magnetic pressure is the same order of magnitude but, in general, a factor of a few less than the other components of the ISM in this galaxy. The uniform magnetic field in NGC1569 is closely associated with the Hα bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. Based on our pressure estimates and the morphology of the magnetic field, the outflow of hot gas from NGC1569 is clearly shaping the magnetic field, but the magnetic field in turn may be aiding the outflow by channeling gas out of the disk of the galaxy. Dwarf galaxies with extended radio continuum halos like that of NGC1569 may play an important role in magnetizing the intergalactic medium.