Magnetic fields in the spiral galaxy NGC 2997

We made multi-band radio observations of NGC 2997, including total power mapping at λ20 with the Very Large Array (VLA), sensitive polarization observations at λ13 cm with the Australia Telescope Compact Array (ATCA) and images at λ6 and λ3 cm with the VLA. The detected regular magnetic field extends from the centre to the outer disk. In the central part (galactic radius R ≤ 0.5′), the dominated field has the form of an axisymmetric spiral in the nearby layer above the galactic plane. Such a field might be the result of an α − α dynamo. In the inner disk (0.5′ < R ≤ 1.5′), the total power of radio emission is stronger in the regions of the spiral arms, but the polarized emission is detected mainly along the inner edge of the arms, as expected from compressed field due to shocks from density waves. The RM data on the northeastern arm indicate that the field there might be coherent and that the field points inwards, opposite to the central field. This is the first case of a field reversal between the central region and the inner disk detected in any external galaxy. In the outer disk (R > 1.5′), we found two so-called “magnetic spiral arms” starting near the major bifurcations of the optical arms. They are offset from the optical arms, extending from the inner to the outer disk and are up to 50% polarized. The magnetic arms might result from an interaction between the dynamo and the density wave. Alternatively, we propose that they may be associated with interarm gaseous features generated by perturbations of the spiral pattern.