THE ROLE OF THE MAGNETIC FIELD IN THE INTERSTELLAR MEDIUM OF THE POST-STARBURST DWARF IRREGULAR GALAXY NGC 1569

Abstract

(abridged) NGC 1569 is a nearby dwarf irregular galaxy which underwent an intense burst of star formation 10 to 40 Myr ago. We present observations that reach surface brightnesses two to eighty times fainter than previous radio continuum observations and the first radio continuum polarization observations. These observations allow us to probe the relationship of the magnetic field of NGC 1569 to the rest of its interstellar medium. We confirm the presence of an extended radio continuum halo at 20 cm and see for the first time the radio continuum feature associated with the western Halpha arm at wavelengths shorter than 20cm. The spectral index trends in this galaxy support the theory that there is a convective wind at work in this galaxy. We derive a total magnetic field strength of 38 microG in the central regions and 10-15 microG in the halo. The magnetic field is largely random in the center of the galaxy; the uniform field is ~3-9 microG and is strongest in the halo. 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 interstellar medium in this galaxy. The uniform magnetic field in NGC 1569 is closely associated with the Halpha bubbles and filaments. We suggest that a supernova-driven dynamo may be operating in this galaxy. The outflow of hot gas from NGC 1569 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 NGC 1569 may play an important role in magnetizing the intergalactic medium.