The radio continuum structure of the edge-on spiral galaxy NGC 4945

Continuum observations of the edge-on spiral galaxy NGC 4945 with the Australia Telescope National Facility's Compact Array (ATCA) and the Parkes 64-m radio telescope are presented. The ATCA maps are currently the highest resolution wide-band radio continuum maps of this galaxy. A compact, non-thermal core of less than 60 pc diameter has been detected. The luminosity of the central 90 pc is 2 x 10(39) erg s(-1), equivalent to almost half the total value of the galaxy. 90 per cent of the galaxy's lambda=3 cm radio emission originates from the nuclear bulge (central 4 kpc), the radio disc measuring 23 +/- 2 kpc in diameter. The nuclear bulge of the galaxy has a spectral index of alpha = -0.57 +/- 0.04 (S proportional to v(alpha)), similar to the core value, The disc emission of the galaxy has a much steeper spectral index of alpha = -1.1 +/- 0.1, indicative of either a high-energy cut-off to the disc population of cosmic rays or differing source energetics in the disc, The large-scale radio morphology resembles that of the majority of edge-on spirals in that no radio halo has been detected. The average total equipartition magnetic field strength of the disc is 13 mu G, whilst our polarization results indicate that the regular component of the magnetic field is about 3 mu G. We conclude that NGC 4945 is one of a number of edge-on spiral galaxies with a magnetic field (and hence cosmic rays) confined to the disc.