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arXiv · 2609.22811

The magnetic field in the multi-phase interstellar medium of NGC 6946

Amir Yarahmadi·Fatemeh Tabatabaei·Golshan Ejlali·Rainer Beck·Enrique Lopez-Rodriguez·Alejandro Borlaff
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Abstract

Magnetic fields regulate the multi-phase interstellar medium (ISM) of spiral galaxies, yet their behavior across ISM phases remains poorly constrained observationally. We present a comparative polarimetric study of the nearby spiral galaxy NGC 6946 using far-infrared (FIR) data at 154 $μ$m from SOFIA/HAWC+ and radio continuum data at 6 cm from the VLA and Effelsberg telescopes, which probe magnetic fields in the cold dust-emitting and diffuse synchrotron-emitting phases of the ISM, respectively. The synchrotron polarization reveals a coherent large-scale spiral magnetic field, with high polarization fractions in interarm regions and strong depolarization in star-forming arms. The FIR polarization fraction is lower on average and shows stronger spatial variations. Both tracers decrease systematically with increasing star formation activity, indicating that star-formation-driven turbulence reduces magnetic field ordering; the synchrotron polarization fraction is strongly suppressed in the central kiloparsec but stable across the mid disk, with a tendency to increase beyond $R \approx 7$ kpc, while the FIR polarization fraction remains low ($\sim$2-3%) and approximately constant over the radial range where it can be reliably measured ($R \lesssim 5.4$ kpc). Our results indicate that NGC 6946 hosts a globally ordered magnetic field locally modulated by turbulence. This study highlights the complementary nature of FIR and radio polarimetry in probing magnetic fields across ISM phases and provides quantitative constraints on how magnetic field ordering varies between the cold and diffuse ISM components.

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Amir Yarahmadi, Fatemeh Tabatabaei, Golshan Ejlali, Rainer Beck, Enrique Lopez-Rodriguez, Alejandro Borlaff. 2026-09-19. The magnetic field in the multi-phase interstellar medium of NGC 6946. https://arxiv.org/abs/2609.22811

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