CHANG-ES XXI. Transport processes and the X-shaped magnetic field of NGC 4217: off-center superbubble structure

TL;DR

This study investigates the magnetic field structure and cosmic ray transport in NGC 4217, revealing an X-shaped magnetic field, a helical outflow, and advection-dominated cosmic ray transport through multi-frequency radio and X-ray data.

Contribution

It provides new insights into the magnetic field configuration and cosmic ray transport mechanisms in NGC 4217, highlighting the dominance of advection over diffusion.

Findings

X-shaped magnetic field structure covering most of the galaxy

Helical outflow extending nearly 7 kpc into the halo

Advection is the primary cosmic ray transport process

Abstract

In order to gain a better understanding of the influence of cosmic rays (CRs) and magnetic fields in the disk-halo interface of edge-on spiral galaxies, we investigate the radio continuum halo, the magnetic field, and the transport processes of the CRs of the edge-on spiral galaxy NGC 4217 using CHANG-ES radio data at two frequencies, 6 GHz (C-band) and 1.5 GHz (L-band), and supplemental LOFAR data at 150 MHz and X-ray Chandra data. NGC 4217 shows a large-scale X-shaped magnetic field structure, covering a major part of the galaxy with a mean total magnetic field strength in the disk of 9 micro Gauss (via equipartition). Using rotation measure synthesis (RM-synthesis) at C-band, we found that the direction of the disk magnetic field is pointing inward. A helical outflow structure is furthermore present in the northwestern part of the galaxy, which is extended nearly 7 kpc into the halo. More polarized emission is observed on the approaching side of the galaxy. With a simplified galaxy disk model, we are able to explain that finding and predict that roughly 75% of edge-on spiral galaxies will show higher polarized intensity on the approaching side. Many loop and shell structures are found throughout the galaxy in total intensity at C-band. A superbubble-like structure is prominent in total and polarized intensity, as well as in Halpha and optical dust filaments, being a possible result of concentrated star formation in the disk. The flux density contribution of the disk in comparison to the halo decreases toward lower frequencies. Total intensity profiles at the three radio frequencies were fit with two-component exponential functions. The frequency dependence of the resulting scale heights between C-band and L-band suggests advection to be the main CR transport process. The 1D CR transport modeling (SPINNAKER) shows that advection appears to be more important than diffusion.