Double-double radio galaxies: further insights into the formation of the
  radio structures
  Double-double radio galaxies (DDRGs) offer a unique opportunity for us to
study multiple episodes of jet activity in large-scale radio sources. We use
radio data from the Very Large Array and the literature to model two DDRGs,
B1450+333 and B1834+620, in terms of their dynamical evolution. We find that
the standard Fanaroff-Riley II model is able to explain the properties of the
two outer lobes of each source, whereby the lobes are formed by ram-pressure
balance of a shock at the end of the jet with the surrounding medium. The inner
pairs of lobes, however, are not well-described by the standard model. Instead
we interpret the inner lobes as arising from the emission of relativistic
electrons within the outer lobes, which are compressed and re-accelerated by
the bow-shock in front of the restarted jets and within the outer lobes. The
predicted rapid progression of the inner lobes through the outer lobes requires
the eventual development of a hotspot at the edge of the outer lobe, causing
the DDRG ultimately to resemble a standard Fanaroff-Riley II radio galaxy. This
may suggest that DDRGs are a brief, yet normal, phase of the evolution of
large-scale radio galaxies.
