2008/03/04 by K. T. Chyży, Krzysztof T. Chyzy, R. Buta +1 · 1 citation
Physics and Astronomy · #Astrophysics and Star Formation Studies #Galaxies: Formation, Evolution, Phenomena #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/587958
Accepted for publication in the Astrophysical Journal Letters, 5 pages, 5 color figures
arxiv created 2008/03/04 · openalex publication_date 2008/03/17 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
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 8.46 and 4.86 GHz 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 center. 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 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 30 and 13 μG, respectively) indicates that a highly efficient process of magnetic field amplification is under way, probably related to secular evolutionary processes in the galaxy.