Discovery of a strong spiral magnetic field crossing the inner pseudoring of NGC 4736

TL;DR

This paper reports the discovery of a coherent spiral magnetic field crossing the inner pseudoring of NGC 4736, revealing a large-scale ordered magnetic structure that is not aligned with star-forming regions, indicating an efficient magnetic field amplification process.

Contribution

It presents the first detailed observation of a large-scale ordered magnetic spiral crossing the inner pseudoring in NGC 4736, suggesting a dynamo process independent of star formation patterns.

Findings

Magnetic field exhibits a spiral pattern crossing the pseudoring with ~35° pitch angle.

Strong total and regular magnetic fields up to 30μG and 13μG, respectively.

Asymmetric Faraday rotation implies large-scale MHD dynamo activity.

Abstract

We report the discovery of a coherent magnetic spiral structure within the nearby ringed Sab galaxy NGC 4736. 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 35deg. The ordered field is thus not local adjusted to the pattern of star-formation activity, unlike what is usually observed in grand-design 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 30microG and 13microG, respectively) indicates that a highly efficient process of magnetic field amplification is under way, probably related to secular evolutionary processes in the galaxy.