Relativistic jet models for two low-luminosity radio galaxies: evidence
  for backflow?
  We show that asymmetries in total intensity and linear polarization between
the radio jets and counter-jets in two lobed Fanaroff-Riley Class I (FR I)
radio galaxies, B2 0206+35 (UGC 1651) and B2 0755+37 (NGC 2484), can be
accounted for if these jets are intrinsically symmetrical, with decelerating
relativistic outflows surrounded by mildly relativistic backflows. Our
interpretation is motivated by sensitive, well-resolved Very Large Array
imaging which shows that both jets in both sources have a two-component
structure transverse to their axes. Close to the jet axis, a centrally-darkened
counter-jet lies opposite a centrally-brightened jet, but both are surrounded
by broader collimated emission that is brighter on the counter-jet side. We
have adapted our previous models of FR I jets as relativistic outflows to
include an added component of symmetric backflow. We find that the observed
radio emission, after subtracting contributions from the extended lobes, is
well described by models in which decelerating outflows with parameters similar
to those derived for jets in plumed FR I sources are surrounded by backflows
containing predominantly toroidal magnetic fields. These return to within a few
kpc of the galaxies with velocities of roughly 0.25c and radiate with a
synchrotron spectral index close to 0.55. We discuss whether such backflow is
to be expected in lobed FR I sources and suggest ways in which our hypothesis
can be tested by further observations.
