ALMA 3 mm polarimetry of radio-quiet active galactic nuclei

¹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn D-53121, Germany
³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
⁴ INAF, Istituto di Radioastronomia di Bologna, Via Piero Gobetti 101 40129 Bologna, Italy
⁵ Physics Department, Technion, Haifa 32000, Israel
⁶ Joint ALMA Observatory, Avenida Alonso de Cordova 3107, Vitacura, Santiago 7630355, Chile
⁷ Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama, Toyonaka 560-0043, Osaka, Japan
⁸ Kavli Institute for Particle Astrophysics & Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA
⁹ School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
¹⁰ Department of Space, Earth and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
¹¹ Eureka Scientific, 2452 Delmer Street Suite 100, Oakland, CA 94602-3017, USA
¹² Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
¹³ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁴ Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA
¹⁵ National Radio Astronomy Observatory, Charlottesville VA 22903, USA
¹⁶ Department of Astronomy, University of Florida, Gainesville, FL 32611, USA
¹⁷ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA

^⋆ Corresponding author: elena.shablovinskaia@mail.udp.cl

Received: 3 June 2025
Accepted: 5 September 2025

Abstract

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.