Abstract
Context. Cosmic-ray electrons (CREs) originating from the star-forming discs of spiral galaxies frequently form extended radio haloes that are best observable in edge-on galaxies, where their properties can be directly investigated as a function of vertical height above the disc.Aims. For the present study, we selected two nearby edge-on galaxies from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES), NGC 891 and 4565, which differ largely in their detectable halo extent and their star-formation rates (SFRs). Our aim is to figure out how such differences are related to the (advective and/or diffusive) CRE transport in the disc and in the halo.Methods. We use wide-band 1.5 and 6 GHz Very Large Array (VLA) observations obtained in the B, C, and D configurations, and combine the 6 GHz images with Effelsberg observations to correct for missing short spacings. After subtraction of the thermal emission, we investigate the spatially resolved synchrotron spectral index distribution in terms of CRE spectral ageing. We further compute total magnetic field strengths assuming equipartition between the cosmic-ray (CR) energy density and the magnetic field, and measure synchrotron scale heights at both frequencies. Based on the fitted vertical profiles of the synchrotron intensity and on the spectral index profile between 1.5 and 6 GHz, we create purely advective and purely diffusive CRE transport models by numerically solving the 1D diffusion–loss equation. In particular, we investigate for the first time the radial dependence of synchrotron and magnetic field scale heights, advection speeds, and diffusion coefficients, whereas previous studies of these two galaxies only determined global values of these quantities.Results. We find that the overall spectral index distribution of NGC 891 is mostly consistent with continuous CRE injection. In NGC 4565, many of the local synchrotron spectra (even in the disc) feature a break between 1.5 and 6 GHz and are thus more in line with discrete-epoch CRE injection (Jaffe–Perola (JP) or Kardashev–Pacholczyk (KP) models). This implies that CRE injection time-scales are lower than the synchrotron cooling time-scales. The synchrotron scale height of NGC 891 increases with radius, indicating that synchrotron losses are significant. NGC 891 is probably dominated by advective CRE transport at a velocity of ≳150 km s−1. In contrast, NGC 4565 is diffusion-dominated up toz = 1 kpc or higher, with a diffusion coefficient of ≥2 × 1028 cm2 s−1.
Highlights
Over the past four decades, evidence has accumulated that the majority of star-forming spiral galaxies show haloes emitting radio continuum, which often extend up to several kpc from the disc in vertical direction
We present the vertical profiles of the non-thermal spectral index αnth,1.5−6 GHz Effelsberg Thermal (6 GHz) and the non-thermal intensity Inth,1.5 GHz at each major-axis position, along with our best-fitting advection and diffusion models, in Figs. 12 and 13 for our preferred solutions
We corrected the total emission for the thermal contribution estimated from Hα and IR data and applied short-spacing corrections at 6 GHz using Effelsberg observations
Summary
Over the past four decades, evidence has accumulated that the majority of star-forming spiral galaxies show haloes emitting radio continuum, which often extend up to several kpc from the disc in vertical direction. The NRAO is a facility of the National Science Foundation operated under agreement by Associated Universities, Inc. an EVLA Survey (CHANG-ES, Irwin et al 2012) showed that spiral galaxies without significant extraplanar radio emission are, if anything, a rare exception (Wiegert et al 2015). Radio continuum emission in haloes is primarily synchrotron radiation of cosmic-ray electrons (and a tracer of extraplanar magnetic fields), which have been accelerated to relativistic energies in shock fronts of supernova (SN) remnants in the disc. Disc–halo interactions have largely been investigated by considering
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