Spatially resolved origin of millimeter-wave linear polarization in the nuclear region of 3C 84^⋆

¹ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: jykim@mpifr-bonn.mpg.de
² Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 022, USA
³ Astronomical Institute, St. Petersburg State University, Universitetskij Pr. 28, Petrodvorets, 198504 St. Petersburg, Russia
⁴ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
⁵ Instituto de Radio Astronomía Millimétrica, Avenida Divina Pastora, 7, Local 20, 18012 Granada, Spain
⁶ Korea Astronomy and Space Science Institute, 776 Daedeokdae-ro, Yuseong-gu, Daejeon 30455, Korea
⁷ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
⁸ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint-Martin-d’Hères, France
⁹ Observatorio de Yebes (IGN), Apartado 148, 19180 Yebes, Spain
¹⁰ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹¹ Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

Received: 28 February 2018
Accepted: 16 November 2018

Abstract

We report results from a deep polarization imaging of the nearby radio galaxy 3C 84 (NGC 1275). The source was observed with the Global Millimeter VLBI Array (GMVA) at 86 GHz at an ultrahigh angular resolution of 50 μas (corresponding to ∼200R_s). We also add complementary multiwavelength data from the Very Long Baseline Array (VLBA; 15 and 43 GHz) and from the Atacama Large Millimeter/submillimeter Array (ALMA; 97.5, 233.0 and 343.5 GHz). At 86 GHz, we measured a fractional linear polarization of ∼2% in the VLBI core region. The polarization morphology suggests that the emission is associated with an underlying limb-brightened jet. The fractional linear polarization is lower at 43 and 15 GHz (∼0.3−0.7% and <0.1%, respectively). This suggests an increasing linear polarization degree toward shorter wavelengths on VLBI scales. We also obtain a large rotation measure (RM) of ∼10^5–6 rad m² in the core at ≳43 GHz. Moreover, the VLBA 43 GHz observations show a variable RM in the VLBI core region during a small flare in 2015. Faraday depolarization and Faraday conversion in an inhomogeneous and mildly relativistic plasma could explain the observed linear polarization characteristics and the previously measured frequency dependence of the circular polarization. Our Faraday depolarization modeling suggests that the RM most likely originates from an external screen with a highly uniform RM distribution. To explain the large RM value, the uniform RM distribution and the RM variability, we suggest that the Faraday rotation is caused by a boundary layer in a transversely stratified jet. Based on the RM and the synchrotron spectrum of the core, we provide an estimate for the magnetic field strength and the electron density of the jet plasma.