CHANG-ES XVI: An in-depth view of the cosmic-ray transport in the
  edge-on spiral galaxies NGC 891 and NGC 4565
  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. For the present study we selected two nearby edge-on galaxies
from the CHANG-ES survey, NGC 891 and 4565, which differ largely in halo size
and SFR. To figure out how such differences are related to the CRE transport in
disc and halo, we use wide-band 1.5 and 6 GHz 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. We study the spatially
resolved non-thermal spectral index distribution in terms of CRE spectral
ageing, compute total magnetic field strengths assuming energy equipartition
between CRs and magnetic fields, and also determine synchrotron scale heights.
Based on the vertical profiles of synchrotron intensity and spectral index, 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 in
these two galaxies. We find the spectral index distribution of NGC 891 to be
mostly consistent with continuous CRE injection, while in NGC 4565 the local
synchrotron spectra are more in line with discrete-epoch CRE injection (JP or
KP models). This implies that CRE injection timescales are lower than the
synchrotron cooling timescales. The 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
$\gtrsim150\,\mathrm{km\,s^{-1}}$. In contrast, NGC 4565 is diffusion-dominated
up to $z=1$ kpc or higher, with a diffusion coefficient of
$\geq2\times10^{28}\,\mathrm{cm^2\,s^{-1}}$.
