Evidence of internal rotation and a helical magnetic field in the jet of the quasar NRAO 150^⋆,⋆⋆

¹ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
e-mail: smolina@iaa.es; jlgomez@iaa.es
² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston MA 02215-1401, USA
³ Current Address: Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
e-mail: agudo@jive.nl
⁴ Max-Planck-Institut für Radioastronomie, auf dem Hügel, 69, 53121 Bonn, Germany
⁵ Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden

Received: 21 January 2014
Accepted: 5 April 2014

Abstract

The source NRAO 150 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 NRAO 150 performed at 8, 15, 22, 43, and 86 GHz with the Very Long Baseline Array (VLBA), and the Global Millimeter VLBI Array (GMVA) between 2006 and 2010. All new and previous observational evidence – i.e., spectral index maps, multi-epoch image cross-correlation, and low level of linear polarization degree in optically thin regions – 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 NRAO 150 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 NRAO 150 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 NRAO 150 in terms of internal jet rotation.