ALMA 3 mm polarimetry of radio-quiet active galactic nuclei

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.