Relativistic AGN jets III. Synthesis of synchrotron emission from
  Double-Double Radio Galaxies
  The class of Double-Double Radio Galaxies (DDRGs) relates to episodic jet
outbursts. How various regions and components add to the total intensity in
radio images is less well known. In this paper we synthesize synchrotron images
for DDRGs based on special relativistic hydrodynamic simulations, making
advanced approximations for the magnetic fields. We study the synchrotron
images for: Three different radial jet profiles; Ordered, entangled or mixed
magnetic fields; Spectral ageing from synchrotron cooling; The contribution
from different jet components; The viewing angle and Doppler (de-)boosting; The
various epochs of the evolution of the DDRG. To link our results to
observational data, we adopt to J1835+6204 as a reference source. In all cases
the synthesized synchrotron images show two clear pairs of hotspots, in the
inner and outer lobes. The best resemblance is obtained for the piecewise
isochoric jet model, for a viewing angle of approximately $\vartheta \sim
-71^{\circ}$, i.e. inclined with the lower jet towards the observer, with
predominantly entangled ($\gtrsim 70$ per cent of the magnetic pressure) in
turbulent, rather than ordered fields. The effects of spectral ageing become
significant when the ratio of observation frequencies and cut-off frequency
$\nu_{\rm obs}/\nu_{\infty,0} \gtrsim 10^{-3}$, corresponding to $\sim 3 \cdot
10^2$ MHz. For viewing angles $\vartheta \lesssim -30^{\circ}$, a DDRG
morphology can no longer be recognized. The second jets must be injected within
$\lesssim$ 4 per cent of the lifetime of the first jets for a DDRG structure to
emerge, which is relevant for Active Galactic Nuclei feedback constraints.
