Astronomy Object of the Month: 2025, January

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Multiphase magnetic fields in the galaxy NGC 3627

Magnetic fields play an important role in the formation and evolution of a galaxy, but it is challenging to measure them by observation. Leo Triplet member NGC3627 was a subject of determining magnetic field orientation via Velocity Gradient Technique (VGT) using spectroscopic data. The agreement of the VGT-CO and polarization suggests that the magnetic fields associated with synchrotron emission also percolate through star-forming regions. The VGT-H_α measurement reveals the magnetic fields in the warm ionized medium that permeates the disc and disc’s vicinity so that it exhibits less agreement with polarization.

Magnetic fields are pervasive in galaxies and play a vital role in a number of important astrophysical processes. They can provide supports against their gravitational collapse and regulate cosmic rays acceleration and transport. Magnetic fields are also believed to be able to exert strong torques, providing an efficient way of transporting gas from a galactic nuclear ring inward to feed an active galactic nuclei (AGH). Studying the role of magnetic fields is therefore essential for understanding overall galaxy evolution.

In the past decades, our understanding of the galactic magnetic field has been significantly advanced by measurements such as dust polarization or synchrotron polarization. However, these techniques have limitations, especially while dealing with the complex multiphase interstellar medium. The measurements of galactic magnetic fields by any of the techniques are biased towards a particular component of this galactic interstellar medium. Therefore, the synergy of different measurements is essential for understanding of the actual magnetic field structure.

We also know that turbulent phenomena are common in galaxies. One of the most important properties of such turbulence is its scale-dependent anisotropy induced by magnetic fields. These inspire a novel method, i.e. the Velocity Gradient Technique (VGT), to trace galactic magnetic fields. Its physical basis stems from the anisotropy properties of MHD turbulence: due to turbulent reconnection, turbulent eddies are not resisted if they rotate in the direction perpendicular to the local magnetic field that percolates the eddies. This makes the small eddies aligned as needles along the local magnetic field. Therefore, the gradient of turbulent velocity is perpendicular to the eddy rotational axis and thus the direction of local magnetic fields we want to trace. Moreover, VGT’s ability in tracing molecular- and atomic-gas-associated magnetic fields has been widely tested in numerical simulations.

In this work, we study NGC 3627: a barred spiral galaxy with Seyfert 2-type nucleus, located 11 Mpc away, in the Leo Triplet galaxy group, within the Leo constellation. NGC 3627 is a popular target for study due to several interesting features. Previous studies suggest that a tidal interaction between NGC 3627 and the neighbouring galaxy NGC 3628 some 800 myriads of years ago caused intense compression of the interstellar medium in the western region of NGC 3627, resulting in a very bright polarization emission in its western spiral arm. Meanwhile, at the south-eastern end of the galactic bar, magnetic fields determined by synchrotron polarization are separated from the optical dust lane. This unusual magnetic field configuration may be the result of a collision between NGC 3627 and its companion dwarf galaxy a few tens of Myr ago. NGC 3627 has been observed by many spectroscopic measures including the high-resolution PHANGS surveys, which makes it a good target of the VGT. In this paper, we mapped the magnetic fields in its warm-phase media using the PHANGS-MUSE H_α emission. The VGT measurements are compared with the magnetic fields inferred from synchrotron polarization emission observed with the Very Large Array at 8.46 GHz. The alignment and misalignment of the VGT and VLA polarization can have very important implications on many physical processes, e.g. star formation, and cosmic ray propagation.

The previously-determined good agreement between the results of the VGT-CO (based on CO (2–1) emission line) and polarization measuring methods suggested the VGT’s validity. In our study, magnetic field orientations probed with the VGT–CO show a better alignment with the synchrotron polarization than the VGT-H_α. It suggests that the magnetic fields associated with synchrotron emission in the galaxy also percolate through its star-forming regions. The new VGT-H_α and VGT-CO measurement reveals the magnetic fields in the warm ionized medium that permeates the disc and disc’s vicinity so that it exhibits less agreement with polarization. We find prominent radial fields measured by synchrotron polarization appear in the transition regions from the spiral arms to the galactic bar, while such morphology is less apparent in the VGT-CO and VGT-H_α measured magnetic fields. These radial fields imply that the magnetic torque is important in removing orbiting gas’ angular momentum to fuel the Seyfert nucleus.

We also notice that magnetic fields inferred from the dust polarization, VGT-CO, and synchrotron polarization methods are different in the part of the east arm, which is suspected to be a collision site of the galaxy with a dwarf galaxy. We interpret this difference as arising from the fact that the three measurements are tracing the magnetic fields associated with pre-collision, the mixture of pre-collision and post-collision, and post-collision flows, respectively.








Original publication: Mingrui Liu, Yue Hu, A. Lazarian, Siyao Xu, Marian Soida, Multiphase magnetic fields in the galaxy NGC 3627, MNRAS 519, 1068–1079 (2023).


The research was conducted at the Department of Radioastronomy and Space Physics of the Jagiellonian University’s Astronomical Observatory (OA UJ). The work was supported by National Aeronautics and Space Administration (NASA).



Contact:

Marian Soida Astronomical Observatory Jagiellonian University M.Soida [at] oa.uj.edu.pl