CHANG-ES XXXV: Cosmic Ray Transport and Magnetic Field Structure of NGC 3556 at 3 GHz

TL;DR

This study uses VLA radio observations to analyze the magnetic field structure and cosmic ray transport in galaxy NGC 3556, revealing advection-dominated cosmic ray propagation, a toroidal magnetic field, and a star formation-driven superwind bubble.

Contribution

First detailed cosmic ray transport modeling and magnetic field mapping of NGC 3556 using multi-frequency radio data from CHANG-ES survey.

Findings

Advection better explains cosmic-ray propagation than diffusion.

Magnetic field shows a toroidal configuration with mean strength ~8.3 μG in the disk.

Detection of a star formation-driven superwind bubble extending into the halo.

Abstract

Radio halos of edge-on galaxies are crucial for investigating cosmic ray propagation and magnetic field structures in galactic environments. We present VLA C-configuration S-band (2--4 GHz) observations of the spiral galaxy NGC 3556, a target from the Continuum Halos in Nearby Galaxies - an EVLA Survey (CHANG-ES). We estimate the thermal contribution to the radio emission from a combination of the H$\alpha$ and mid-IR data, and employ Rotation Measure Synthesis to reveal the magnetic field structures. In our data, NGC 3556 exhibits a box-like radio halo extending nearly 7 kpc from the galactic plane. The scale height of the total S-band intensity in the halo is $1.68\pm 0.29$ kpc, while that of the non-thermal intensity is $1.93\pm 0.28$ kpc. Fitting the data to a 1-D cosmic-ray transport model, we find advection to describe the cosmic-ray propagation within the halo better than diffusion, with advection speeds of $245 \pm 15$ km s$^{-1}$ and $205 \pm 25$ km s$^{-1}$ above and below the disk, respectively. The magnetic field is detected patchily across the galaxy, displaying a toroidal configuration in the rotation measure map. The mean equipartition magnetic field strength is approximately $8.3\ \mu$G in the disk and $4.5\ \mu$G in the halo. In addition, a bubble-like structure extends nearly 3~kpc into the southern halo, aligned with the polarized intensity and H$\alpha$ image, suggestive of superwinds generated by recent star formation feedback in the nuclear region.