Unveiling the cause of hybrid morphology radio sources (HyMoRS)
  Hybrid morphology radio sources (HyMoRS) are a rare group of radio galaxies
in which differing Fanaroff & Riley morphologies (FR I/II) are observed for
each of the two lobes. While they potentially provide insights into the
formation of lobe structure, particle acceleration, and the FR dichotomy,
previous work on HyMoRS has mainly been limited to low-resolution studies,
searches for new candidates, and milliarcsecond-scale VLBI observations of the
core region. In this paper, we use new multi-array configuration Very Large
Array (VLA) observations between 1 and 8 GHz to determine the morphology of
HyMoRS on arcsecond scales and perform the first well-resolved spectral study
of these unusual sources. We find that while the apparent FR I lobe is
centre-brightened, this is the result of a compact acceleration region
resembling a hotspot with a spectrum more consistent with an FR II
("strong-flavour") jet. We find that the spectra of the apparent FR I lobes are
not similar to their classical counterparts and are likely the result of
line-of-sight mixing of plasma across a range of spectral ages. We consider
possible mechanisms that could lead to the formation of HyMoRS under such
conditions, including environment asymmetry and restarted sources, concluding
through the use of simple modelling that HyMoRS are the result of orientation
effects on intrinsically FR II sources with lobes non-parallel to the inner
jet.
