CHANG-ES. XXXII. Spatially Resolved Thermal–Nonthermal Separation from Radio Data Alone—New Probes into NGC 3044 and NGC 5775

Abstract

We have carried out spatially resolved thermal–nonthermal separation on two edge-on galaxies, NGC 3044 and NGC 5775, using only radio data. Narrowband imaging within a frequency band that is almost contiguous from 1.25 to 7.02 GHz ( L band, S band, and C band) has allowed us to fit spectra and construct thermal, nonthermal, and nonthermal spectral index maps. This method does not require any ancillary H α and IR data or rely on dust corrections that are challenging in edge-on galaxies. For NGC 3044, at 15″ resolution, we find a median thermal fraction of ∼13% with an estimated uncertainty in this fraction of ∼50% at 4.13 GHz. This compares well with the H α mixture method results. We uncovered evidence for a vertical outflow feature reaching at least z ∼ 3.5 kpc in projection above the plane, reminiscent of M82's starburst wind. For the higher star formation rate galaxy, NGC 5775, at 12″ resolution, we find a median thermal fraction of 44% at 4.13 GHz with an estimated error on this fraction of 17%. Both galaxies show a change of slope (flattening) in L band. These results suggest that a radio-only method for separating thermal from nonthermal emission is not only feasible, but able to reveal new features that might otherwise be obscured in edge-on disks.

Keywords

• Radio continuum emission
• Nonthermal radiation sources
• Spiral galaxies