Strong magnetic spiral pattern in a ringed galaxy NGC4736

TL;DR

This study uses high-sensitivity radio polarimetric observations to reveal a coherent spiral magnetic field pattern in galaxy NGC4736, contrasting with its ringed morphology and suggesting a dynamo origin.

Contribution

It provides the first detailed radio polarization map showing a spiral magnetic field in a ringed galaxy, highlighting magnetic structures independent of visible gas morphology.

Findings

Magnetic field forms a coherent spiral pattern across the galaxy.

Magnetic vectors cross the starburst ring and dust lanes.

Magnetic field may originate from a dynamo process, not gas motions.

Abstract

We present high sensitivity radio polarimetric (VLA) observations of a galaxy with strong orbital resonances - NGC4736. The total radio intensity at 8.4GHz covers smoothly the whole galaxy bulge and reveals a distinct ring of radio emission closely related to the ring morphology visible in infrared, CO and Halpha emission. However, the magnetic field reveals a very coherent spiral pattern. The magnetic field vectors are crossing the inner starbursting ring, the dust lanes within the ring and other rather circularly shaped features visible in other gas traces. Either the magnetic field uncovers the pattern of gas motions not seen in other spectral ranges, or the spiral magnetic field is of a pure dynamo origin, ignoring the ringed morphology of the galaxy.