ALMA 3 mm polarimetry of radio-quiet active galactic nuclei
Journal article, 2025
The compact millimeter emission ubiquitously found in radio-quiet active galactic nuclei (RQ AGNs) exhibits properties consistent with synchrotron radiation from a small region (≤1 light day) and undergoing self-absorption below ∼100 GHz. Several scenarios have been proposed for its origin, including an X-ray corona, a scaled-down jet, or outflow-driven shocks, which can be tested via millimeter polarimetry. In the optically thin regime, synchrotron emission is expected to show polarization up to ∼70%, but disordered magnetic fields and Faraday rotation reduce this to a few percent for jets and outflows, while an X-ray corona is likely to result in complete depolarization. To investigate this, we conducted the first ALMA Band 3 full-polarization observations of three RQ AGNs – NGC 3783, MCG 5–23–16, and NGC 4945. No polarized signal was detected in any of the AGNs, with an upper limit of 0.5–1.5%, supporting the X-ray corona scenario. However, we detected a compact source with 17% polarization in NGC 3783, 20 pc away from the AGN, co-spatial with the millimeter and narrow-line outflow, likely linked to a shock propagating through the outflowing material. Additionally, combining our data with archival ALMA observations, we found typical millimeter variability in RQ AGNs by a factor of 2.
submillimeter: galaxies
techniques: polarimetric
galaxies: active
Author
E. Shablovinskaia
Diego Portales University
Max Planck Society
C. Ricci
Beijing University of Technology
Diego Portales University
R. Paladino
Istituto nazionale di astrofisica (INAF)
Ari Laor
Technion – Israel Institute of Technology
Chin Shin Chang
Atacama Large Millimeter-submillimeter Array (ALMA)
D. Belfiori
Istituto nazionale di astrofisica (INAF)
Taiki Kawamuro
Osaka University
E. Lopez-Rodriguez
Kavli Institute for Particle Astrophysics and Cosmology
D. J. Rosario
Newcastle University
Susanne Aalto
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Michael J. Koss
Eureka Scientific
Space Science Institute
Richard Mushotzky
University of Maryland
G. Privon
University of Virginia
National Radio Astronomy Observatory
College of Liberal Arts and Sciences
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 703 A82Subject Categories (SSIF 2025)
Atom and Molecular Physics and Optics
Fusion, Plasma and Space Physics
Astronomy, Astrophysics, and Cosmology
DOI
10.1051/0004-6361/202555796