ALMA polarimetry of radio-quiet AGNs

TL;DR

This study used ALMA polarimetry to investigate the origin of mm emission in radio-quiet AGNs, finding no polarization support for jets or outflows and suggesting an X-ray corona as the source.

Contribution

First ALMA full-polarization observations of RQ AGNs, providing new constraints on emission mechanisms and magnetic field structures.

Findings

No polarized signal detected in the AGNs, supporting the X-ray corona hypothesis.

Detected a polarized source near NGC 3783 likely linked to an outflow shock.

Found typical mm variability in RQ AGNs, but data sparsity limited detailed analysis.

Abstract

The compact mm emission ubiquitously found in radio-quiet active galactic nuclei (RQ AGN) exhibits properties consistent with synchrotron radiation from a small region ($\leq$1 light day) and undergoing self-absorption below $\sim$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 mm polarimetry. In the optically thin regime, synchrotron emission is expected to show polarization up to $\sim$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 AGN - NGC 3783, MCG 5-23-16, and NGC 4945. No polarized signal was detected in any of the AGN, 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 mm 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 mm variability in RQ AGN by a factor of 2; however, the sparsity of the data prevented a more detailed analysis of the total flux variability.