Magnetic Fields Confine the Torus at the Core of Cygnus A

By Enrique Lopez-Rodriguez, Kassandra Bell, and Joan Schmelz (USRA)

Paper: The Highly Polarized Dusty Emission Core of Cygnus A
Lopez-Rodriguez, Enrique, et al., 2018, ApJL, 861, L23.

Observations from SOFIA reveal that magnetic fields are trapping and confining the obscuring dust near the center of the active galaxy, Cygnus A, and feeding material onto the supermassive black hole. The unified model of active galaxies maintains that the core is surrounded by a dusty torus that absorbs radiation at all wavelengths and re-emits it in the infrared. How this obscuring structure is created and sustained has never been clear, but these new results from SOFIA confirm that the magnetic field plays a crucial role.

The presence of collimated jets that emanate from the core of active galaxies like Cygnus A implies strong accretion onto the supermassive black hole. The jets may be launched by extracting rotational energy from the black hole itself. This process requires strong magnetic fields in the nucleus of the galaxy that can help convert the rotational energy to a launching energy sufficient to overcome the enormous force of gravity near the black hole’s event horizon. In fact, the fundamental difference between radio-loud galaxies like Cygnus A and their radio-quiet siblings may indeed be due to the presence or absence of a strong, coherent magnetic field surrounding the black hole. 

Although magnetic fields are notoriously difficult to observe, polarimetric observations of the infrared emission from aligned dust grains has proven to be a powerful technique. SOFIA recently observed the central 20 pc (65.2 light-years) of Cygnus A with the High-resolution Airborne Wideband Camera (HAWC+) at 53 and 89 microns with an angular resolution of 5” and 9”. These observations are sensitive to temperatures of 30-50 K and show highly polarized infrared emission dominated by a well-aligned dusty structure. The polarization vectors indicate that the most powerful radio-loud active galaxy in the sky, with its iconic large-scale jets, is able to confine the obscuring torus that feeds the supermassive black hole using a strong magnetic field. 

Cygnus A is in the perfect location to learn about the role magnetic fields play in confining the tori of the unified model, channeling material into supermassive black holes, and launching jets at relativistic speeds because it is the closest and most powerful active galaxy. More observations of different types of active galaxies are necessary to get the full picture of how magnetic fields affect the evolution of the environment surrounding supermassive black holes. If, for example, a HAWC+ survey reveals highly polarized infrared emission from the centers of radio-loud active galaxies but not from radio-quiet galaxies, it would imply that magnetic fields play a crucial role in the accretion process and jet formation.

SOFIA is a Boeing 747SP jetliner modified to carry a 106-inch diameter telescope. It is a joint project of NASA and the German Aerospace Center, DLR. NASA’s Ames Research Center in California’s Silicon Valley manages the SOFIA program, science and mission operations in cooperation with the Universities Space Research Association headquartered in Columbia, Maryland, and the German SOFIA Institute (DSI) at the University of Stuttgart. The aircraft is based at NASA’s Armstrong Flight Research Center Hangar 703, in Palmdale, California.

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