Discovery of a Strong Spiral Magnetic Field Crossing the Inner Pseudoring of Ngc4736

We report the discovery of a coherent magnetic spiral structure within the nearby ringed Sab galaxy NGC4736. High sensitivity radio polarimetric data obtained with the VLA at 8.46GHz and 4.86GHz show a distinct ring of total radio emission precisely corresponding to the bright inner pseudoring visible in other wavelengths. However, unlike the total radio emission, the polarized radio emission reveals a clear pattern of ordered magnetic field of spiral shape, emerging from the galactic centre. The magnetic vectors do not follow the tightly-wrapped spiral arms that characterize the inner pseudoring, but instead cross the ring with a constant and large pitch angle of about 35. The ordered field is thus not locally adjusted to the pattern of star-formation activity, unlike what is usually observed in granddesign spirals. The observed asymmetric distribution of Faraday rotation suggests the possible action of a large-scale MHD dynamo. The strong magnetic total and regular field within the ring (up to 30μG and 13μG, respectively) indicates that a highly efficient process of magnetic field amplification is under way, probably related to secular evolutionary processes in the galaxy. Subject headings: galaxies: individual (NGC 4736) — galaxies: magnetic fields — MHD