Abstract
Context. The observation of total and linearly polarized synchrotron radiation of spiral galaxies in the radio continuum reveals the distribution and structure of their magnetic fields. By observing these, information about the proposed dynamo processes that preserve the large-scale magnetic fields in spiral galaxies can be gained. Additionally, by analyzing the synchrotron intensity, the transport processes of cosmic rays into the halo of edge-on spiral galaxies can be investigated. Aims. We analyze the magnetic field geometry and the transport processes of the cosmic rays of the edge-on spiral starburst galaxy NGC 4666 from CHANG-ES radio data in two frequencies; 6 GHz (C-band) and 1.5 GHz (L-band). Supplementary X-ray data are used to investigate the hot gas in NGC 4666. Methods. We determine the radio scale heights of total power emission at both frequencies for this galaxy. We show the magnetic field orientations derived from the polarization data. Using rotation measure (RM) synthesis we further study the behavior of the RM values along the disk in C-band to investigate the large-scale magnetic-field pattern. We use the revised equipartition formula to calculate a map of the magnetic field strength. Furthermore, we model the processes of cosmic-ray transport into the halo with the 1D SPINNAKER model. Results. The extended radio halo of NGC 4666 is box-shaped and is probably produced by the previously observed supernova-driven superwind. This is supported by our finding of an advective cosmic-ray transport such as that expected for a galactic wind. The scale-height analysis revealed an asymmetric halo above and below the disk as well as between the two sides of the major axis. A central point source as well as a bubble structure is seen in the radio data for the first time. Our X-ray data show a box-shaped hot halo around NGC 4666 and furthermore confirm the AGN nature of the central source. NGC 4666 has a large-scale X-shaped magnetic field in the halo, as has been observed in other edge-on galaxies. The analysis furthermore revealed that the disk of NGC 4666 shows hints of field reversals along its radius, which is the first detection of this phenomenon in an external galaxy.
Highlights
Strong star formation leads to outflow of matter over the whole disk of a galaxy (e.g., Habe et al 1981)
The total power map (Stokes I) of L-band is shown in Fig. 1 with total intensity contours overlayed on the optical Sloan Digital Sky Survey (SDSS)4 image made from the ugr filters using
It is evident that the extent of the radio continuum radiation beyond the star-forming disk only occurs in the vertical direction of the galaxy
Summary
Strong star formation leads to outflow of matter over the whole disk of a galaxy (e.g., Habe et al 1981). The radio halo of the edge-on spiral starburst galaxy NGC 4666 is investigated to obtain further clues about the magnetic fields and processes in spiral galaxies with high star formation rates (SFRs). The Hα image and the radio continuum maps (Dahlem et al 1997) show a homogeneous distribution of many star-forming regions across the disk of NGC 4666 This is different from a nuclear starburst galaxy with a 1 kpc diameter of high star formation in the center. Based on BeppoSAX and XMM-Newton data, Persic et al (2004) found that the star burst, which extends over most of the disk, and AGN activities coexist in NGC 4666 They found a prominent emission line from Fe-Kα at ≈6.4 keV from the nuclear region.
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