Resolved magnetic structures in the disk-halo interface of NGC 628

Abstract

Context. Magnetic fields are essential to fully understand the interstellar medium and its role in the disk-halo interface of galaxies is still poorly understood. Star formation is known to expel hot gas vertically into the halo and these outflows have important consequences for mean-field dynamo theory in that they can be efficient in removing magnetic helicity. Aims. We aim to probe the vertical magnetic field and enhance our understanding of the disk-halo interaction of galaxies. Studying a face-on galaxy is essential so that the magnetic field components can be separated in 3D. Methods. We perform new observations of the nearby face-on spiral galaxy NGC 628 with the Karl G. Jansky Very Large Array (JVLA) at S -band (2.6-3.6 GHz effective bandwidth) and the Effelsberg 100-m telescope at frequencies of 2.6 GHz and 8.35 GHz with a bandwidth of 80 MHz and 1.1 GHz, respectively. Owing to the large bandwidth of the JVLA receiving system, we obtain some of the most sensitive radio continuum images in both total and linearly polarised intensity of any external galaxy observed so far. Results. The application of rotation measures synthesis to the interferometric polarisation data over this large bandwidth provides high-quality images of Faraday depth and polarisation angle from which we obtained evidence for drivers of magnetic turbulence in the disk-halo connection. Such drivers include a superbubble detected via a significant Faraday depth gradient coinciding with a H I hole. We observe an azimuthal periodic pattern in Faraday depth with a pattern wavelength of 3.7 ± 0.1 kpc, indicating Parker instabilities. The lack of a significant anti-correlation between Faraday depth and magnetic pitch angle indicates that these loops are vertical in nature with little helical twisting, unlike in IC 342. We find that the magnetic pitch angle is systematically larger than the morphological pitch angle of the polarisation arms which gives evidence for the action of a large-scale dynamo where the regular magnetic field is not coupled to the gas flow and obtains a significant radial component. We additionally discover a lone region of ordered magnetic field to the north of the galaxy with a high degree of polarisation and a small pitch angle, a feature that has not been observed in any other galaxy so far and is possibly caused by an asymmetric H I hole. Conclusions. Until now NGC 628 has been relatively unexplored in radio continuum but with its extended H I disk and lack of active star formation in its central region has produced a wealth of interesting magnetic phenomena. We observe evidence for two drivers of magnetic turbulence in the disk-halo connection of NGC 628, namely, Parker instabilities and superbubbles.

Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges

“…The largest difference is found in M 31. For individual magnetic arms, p o was found to be larger than p s by about 20 • in M 83 [173], about 8 • in M 101 [174], and 12 • − 23 • for the three main arms in M 74 [171]. This could indicate that anisotropic turbulent fields, responsible for a significant fraction of the polarized emission, have a systematically larger pitch angle than the regular field.…”

Evolving galactic dynamos and fits to the reversing rotation measures in the halo of NGC 4631

“…Similar results have been seen in the disk of the face-on galaxy, NGC 628, as shown in Figs. 18 and 26 of Mulcahy et al (2017) and also more recently in the disk of the edge-on galaxy, NGC 4666 (Stein et al 2019). For the latter galaxy, the field direction also flips across the major axis of the galaxy.…”

“…However it is becoming common place to give the Faraday depth by RM synthesis for nearly face-on galaxies (e.g. Beck 2015b; Mulcahy et al 2017). Thus in this section we give a preliminary RM screen analysis of essentially the same model used in the previous section.…”

Study of the Vertical Magnetic Field in Face-on Galaxies Using Faraday Tomography

“…On the other hand, we take B coh as a free parameter, varying its value to see the effects on the Faraday spectrum. Recent RM studies of face-on external galaxies such as IC 342 (Beck 2009) and NGC 628 (Mulcahy et al 2017) indicated absolute values of the RM up to ∼100 rad m −2 , which corresponds to up to ∼6 μG if we assume that the average thermal electron density along the LOS is 0.02 cm −3 and the path length through the thermal gas is 1 kpc. So, we set B coh = 0-5 μG.…”