Evidence of internal rotation and a helical magnetic field in the jet of
  the quasar NRAO150
  The source NRAO150 is a very prominent millimeter to radio emitting quasar at
redshift z=1.52 for which previous millimeter VLBI observations revealed a fast
counterclockwise rotation of the innermost regions of the jet. Here we present
new polarimetric multi-epoch VLBI-imaging observations of NRAO150 performed at
8, 15, 22, 43, and 86GHz with the VLBA and GMVA between 2006 and 2010. All new
and previous observational evidence are consistent with an interpretation of
the source behavior where the jet is seen at an extremely small angle to the
line of sight, and the high frequency emitting regions in NRAO150 rotate at
high speeds on the plane of the sky with respect to a reference point that does
not need to be related to any particularly prominent jet feature. The observed
polarization angle distribution at 22, 43, and 86 GHz during observing epochs
with high polarization degree suggests that we have detected the toroidal
component of the magnetic field threading the innermost jet plasma regions.
This is also consistent with the lower degree of polarization detected at
progressively poorer angular resolutions, where the integrated polarization
intensity produced by the toroidal field is explained by polarization
cancellation inside the observing beam. All this evidence is fully consistent
with a kinematic scenario where the main kinematic and polarization properties
of the 43 GHz emitting structure of NRAO150 are explained by the internal
rotation of such emission regions around the jet axis when the jet is seen
almost face on. A simplified model developed to fit helical trajectories to the
observed kinematics of the 43 GHz features fully supports this hypothesis. This
explains the kinematics of the innermost regions of the jet in NRAO150 in terms
of internal jet rotation.
