CHANG-ES XIV: Cosmic-ray propagation and magnetic field strengths in the radio halo of NGC 4631

Abstract

NGC 4631 is an interacting galaxy that exhibits one of the largest, gaseous halos observed among edge-on galaxies. We aim to examine the synchrotron and cosmic-ray propagation properties of its disk and halo emission with new radio continuum data. Radio continuum observations of NGC 4631 were performed with the Karl G. Jansky Very Large Array at C-band (5.99 GHz) in the C and D array configurations, and at L-band (1.57 GHz) in the B, C, and D array configurations. Complementary observations of NGC 4631 with the Effelsberg telescope were performed at 1.42 and 4.85 GHz. The interferometric total intensity data were combined with the single-dish Effelsberg data in order to recover the missing large-scale total power emission. The thermal and nonthermal components of the total radio emission were separated by estimating the thermal contribution through the extinction-corrected H$\alpha$ emission. The H$\alpha$ radiation was corrected for extinction using a linear combination of the observed H$\alpha$ and 24 $\mu$m data. NGC 4631 has a global thermal fraction at 5.99 (1.57) GHz of 14$\pm$3% (5.4$\pm$1.1%). The mean scale heights of the total emission in the radio halo (thick disk) at 5.99 (1.57) GHz are $1.79\pm 0.54$ kpc ($1.75\pm 0.27$ kpc) and have about the same values for the synchrotron emission. The total magnetic field of NGC 4631 has a mean strength of $⟨{\mathrm{B}}_{\mathrm{eq}}⟩\simeq 9\mu \mathrm{G}$ in the disk, and a mean strength of $⟨{\mathrm{B}}_{\mathrm{eq}}⟩\simeq 7\mu \mathrm{G}$ in the halo. We also studied a double-lobed background radio galaxy southwest of NGC 4631, which is an FR~II radio galaxy according to the distribution of spectral index across the lobes. From the halo scale heights we estimated that the radio halo is escape-dominated with convective cosmic ray propagation, and conclude that there is a galactic wind in the halo of NGC 4631.

15 pages, 13 figures, accepted to be published in A&A

References in corpus (13)

• Revised equipartition & minimum energy formula for magnetic field strength estimates from radio synchrotron observations
• The Radio Spectral Energy Distribution and Star Formation Rate Calibration in Galaxies
• Cosmic rays and the magnetic field of the nearby starburst galaxy NGC 253
• High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission
• Winds, Clumps, and Interacting Cosmic Rays in M82
• The Westerbork SINGS Survey: I. Overview and Image Atlas
• Radio haloes in nearby galaxies modelled with 1D cosmic-ray transport using SPINNAKER
• Synchrotron spectral index and interstellar medium densities of star-forming galaxies
• CHANG-ES XV: Large-scale magnetic field reversals in the radio halo of NGC 4631
• The NOD3 software package: A graphical user interface-supported reduction package for single-dish radio continuum and polarisation observations
• Detection of a ~20 kpc coherent magnetic field in the outskirt of merging spirals: the Antennae galaxies
• CHANG-ES VIII: Uncovering Hidden AGN Activity in Radio Polarization
• Exploring the Dust Content of Galactic Winds with Herschel. I. NGC 4631

Cited by in corpus (19)

• CHANG-ES XXII: Coherent Magnetic Fields in the Halos of Spiral Galaxies
• CHANG-ES XV: Large-scale magnetic field reversals in the radio halo of NGC 4631
• Nearby galaxies in LoTSS-DR2: insights into the non-linearity of the radio-SFR relation
• CHANG-ES XXIII: Influence of a galactic wind in NGC 5775
• FEASTS: IGM cooling triggered by tidal interactions through the diffuse HI phase around NGC 4631
• The radio continuum perspective on cosmic-ray transport in external galaxies
• CHANG-ES XXVI: Insights into cosmic-ray transport from radio halos in edge-on galaxies
• CHANG-ES XXV: HI Imaging of Nearby Edge-on Galaxies -- Data Release 4
• HI Vertical Structure of Nearby Edge-on Galaxies from CHANG-ES
• An X-ray View of the Hot Circum-Galactic Medium
• CHANG-ES XXIX: The Sub-kpc Nuclear Bubble of NGC 4438
• CHANG-ES XXXII: Spatially Resolved Thermal/Nonthermal Separation from Radio Data Alone -- New Probes into NGC 3044 and NGC 5775
• CHANG-ES XXXIV: a 20 kpc radio bubble in the halo of the star-forming galaxy NGC 4217
• CHANG-ES: XXXVI. The thin and thick radio discs
• Radio Halo of NGC 4631: Comparing Observations and Simulations
• Starburst-Driven Galactic Outflows -- Unveiling the Suppressive Role of Cosmic Ray Halos
• CHANG-ES XXXVII. Effects of spectral aging on radio scale heights
• The universality of the relation between magnetic fields and star formation in galaxies
• Magnetohydrodynamic turbulence and the associated spatial diffusion tensor of cosmic rays in dynamical galactic halos