Jet in jet in M87
  New high-resolution Very Long Baseline Interferometer observations of the
prominent jet in the M87 radio galaxy show a persistent triple-ridge structure
of the transverse 15-GHz profile with a previously unobserved ultra-narrow
central ridge. This radio structure can reflect the intrinsic structure of the
jet, so that the jet as a whole consists of two embedded coaxial jets. A
relativistic magnetohydrodynamic model is considered in which an inner jet is
placed inside a hollow outer jet and the electromagnetic fields, pressures and
other physical quantities are found. The entire jet is connected to the central
engine that plays the role of a unipolar inductor generating voltage between
the jets and providing opposite electric currents, and the charge neutrality
and current closure together with the electromagnetic fields between the jets
can contribute to the jet stabilization. The constant voltage is responsible
for the similar widening laws observed for the inner and outer jets. This
jet-in-jet structure can indicate simultaneous operation of two different
jet-launching mechanisms, one relating to the central supermassive black hole
and the other to the surrounding accretion disc. An inferred magnetic field of
80 G at the base is sufficient to provide the observed jet luminosity.
