Gradient Era for Tracing Magnetic Field in Spiral Galaxies

Magnetic fields in multiple-phase are crucial in understanding the spiral galaxy’s formation. To map the magnetic fields in nearby galaxies M51, NGC 1068, NGC 1097, NGC 3627, NGC 4826 with high resolution, we apply the velocity gradient technique (VGT) to the ALMA and PAWS CO isotopologs data. Our magnetic field maps are globally compatible with the existing VLA synchrotron and HAWC+ dust polarization. The agreement of the VGT and VLA measurements in M51, NGC 1097, and NGC 3627 suggest an active star formation process, and the synchrotron is trapped in the galactic disk’s vicinity. The discrepancy between the VGT and dust polarization in NGC1097 suggests that the dust lanes strongly feed the inner-bar area. We find preferentially tangential magnetic fields associated with the galactic dynamo activity in spiral arms. The inner-bar and spiral arm transition regions of NGC 1097, NGC 1068, and NGC 3627, however, are dominated by radial fields.