High resolution polarization observations of spiral galaxies in far-infrared are enabling for the first time to study galactic magnetic fields deep into the cold dark molecular disks, where the most complex gas kinematics and star formation take place. Taking advantage of HAWC+, the far-infrared polarization imaging instrument installed in the Stratospheric Observatory for Infrared Astronomy (SOFIA), we study the polarized thermal emission of magnetically aligned dust grains at 155um of the grand design spiral galaxy M51. We compare quantitatively the magnetic spiral structure obtained from the dust polarization maps of M51 with the archival radio observations of the 3 cm and 6 cm synchrotron emission of the diffuse interstellar medium of M51.
We found that the spiral magnetic field is wrapped tighter at shorter wavelengths, showing significantly lower values for the magnetic pitch angle in far-infrared than in radio observations. In addition, the spiral arm closer to the the companion galaxy shows a significant distortion in the far-infrared magnetic pitch angle profile which is not observable in the radio polarization maps.
Moreover, we find a strong negative correlation of the far-infrared polarization fraction with the intensity, pointing to an effect of turbulence in the magnetic fields on higher-density environments. These two parameters clearly separate the galactic core, arms, and the interarm regions of the galactic disk. These regions show also different magnetic pitch angles, suggesting that small-scale turbulent fields are dynamically different in each region.
Our analysis of the M51 galactic system reveal a previously unknown and very complex scenario: Radio observations do not trace the same magnetic field than far-infrared polarization observations, being related to different depths in the galactic disk. This lack of homogeneity with environment density is confirmed by the polarization fraction maps, revealing a limiting effect of magnetic turbulence on the densest regions. Multi-wavelength polarization observations are the key to disentangle the interlocked relation between star formation, magnetic fields and gas kinematics in the different phases of the interstellar medium.