Magnetic fields and hot gas in the spiral galaxy NGC 1566 as derived from ATCA radio polarization and ROSAT X-ray observations

NGC 1566, a southern galaxy with an exceptionally regular spiral pattern, was observed in radio continuum at lambda 3.5, 6.2, 12.7 and 21.7 cm wavelengths with the Australia Telescope Compact Array (ATCA(1)), and in the optical Ha line with the MSSSO 1.0-m telescope. The total radio emission has a smooth nonthermal component and a thermal component associated with Ha! emission. Even at lambda 3.5 cm the thermal emission dominates only in the western spiral arm. The nonthermal intensities require an equipartition magnetic field of 13 +/- 2 mu G average strength, and similar or equal to 15 - 20 mu G in spiral arms. For the first time linearly polarized radio emission has been detected in NGC 1566. At lambda 6.2 cm the highest polarized intensities with a degree of polarization p similar or equal to 30% emerge from interarm regions, possibly a result of magnetic field compression between two spiral arms. No such phenomenon has been observed for any other spiral galaxy. As Faraday rotation is low, the B-vectors at lambda 6.2 cm indicate the orientations of the magnetic field lines which almost perfectly follow the optical spiral structure. No systematic variations of the field pitch angles from spiral arm to interarm regions were found, in contrast to density-wave models. The polarized emission at lambda 12.7 cm is 2.5-10 times lower than at lambda 6.2 cm. If this is due to internal Faraday dispersion in the halo of NGC 1566, we would need electron densities of n(e) greater than or similar to 0.03 cm(-3). We observed NGC 1566 in X-rays with the Position Sensitive Proportional Counter (PSPC) aboard the Rontgen Satellit (ROSAT). The bright nuclear region with a luminosity of L(x)(0.1-2.4 keV)= 1.0 . 10(41) erg s(-1) can best be fitted with a power-law spectral model of index 2.3 that is in good agreement with the mean index for Seyfert type I galaxies. Besides the nuclear source we detected point sources in the spiral arms of NGC 1566 and extended emission (L(x) = 1.4 . 10(40) erg s(-1)) surrounding the nucleus. The extent of the radio continuum emission is similar to that of the extended X-ray emission, suggesting a link between hot gas and magnetic field. Assuming that the extended soft X-ray emission is due to a hot gaseous component, we derive an electron density less than or equal to 1.0 . 10(-3)/root eta cm(-3) With an unknown volume filling factor eta. This is too small to explain the observed depolarization at lambda 12.7 cm, indicating that the hot gas is far from radiative equilibrium, or that another component of ionized gas with lower temperature exists in the halo.