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ALMA polarimetry of radio-quiet AGNs

E. Shablovinskaia, C. Ricci, R. Paladino, A. Laor, C-S. Chang, D. Belfiori, T. Kawamuro, E. Lopez-Rodriguez, D. J. Rosario, S. Aalto, M. Koss, R. Mushotzky, G. C. Privon

TL;DR

This study uses ALMA Band 3 full-polarization observations of three nearby radio-quiet AGN to probe the origin of compact mm emission. No polarization is detected in the AGN cores (PD limits ~0.5–1.5%), supporting strong Faraday depolarization consistent with an X-ray corona origin, while a bright, offset polarized source in NGC 3783 suggests a shock in a nearby outflow. Archival data indicate mm flux variability by a factor of around 2, and the authors derive size-scale estimates for the emitting region under equipartition, finding a compact core with possible extended structures tens to thousands of gravitational radii away. The results imply that corona-dominated Faraday depolarization likely suppresses mm polarization in the cores, with extended polarized features possibly tracing outflow-related shocks, and highlight the need for higher-frequency and higher-spectral-resolution polarimetry to constrain RM and the underlying physics.

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.

ALMA polarimetry of radio-quiet AGNs

TL;DR

This study uses ALMA Band 3 full-polarization observations of three nearby radio-quiet AGN to probe the origin of compact mm emission. No polarization is detected in the AGN cores (PD limits ~0.5–1.5%), supporting strong Faraday depolarization consistent with an X-ray corona origin, while a bright, offset polarized source in NGC 3783 suggests a shock in a nearby outflow. Archival data indicate mm flux variability by a factor of around 2, and the authors derive size-scale estimates for the emitting region under equipartition, finding a compact core with possible extended structures tens to thousands of gravitational radii away. The results imply that corona-dominated Faraday depolarization likely suppresses mm polarization in the cores, with extended polarized features possibly tracing outflow-related shocks, and highlight the need for higher-frequency and higher-spectral-resolution polarimetry to constrain RM and the underlying physics.

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 (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 mm 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 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.

Paper Structure

This paper contains 19 sections, 9 equations, 6 figures, 9 tables.

Table of Contents

  1. Introduction
  2. Expected levels of mm polarization
  3. Beam depolarization
  4. Faraday depolarization
  5. Faraday rotation in mm emitting sources
  6. Sample, observations and data reduction
  7. Results
  8. NGC 3783
  9. MCG 5--23--16
  10. NGC 4945
  11. Discussion
  12. Extended mm structure and polarization
  13. What causes depolarization of AGN mm emission?
  14. Variability in mm
  15. The size of the emitting region
  16. ...and 4 more sections

Figures (6)

  • Figure 1: ALMA observations of NGC 3783. Upper panel: total intensity with the contours corresponding to the 2, 4, 6, 18, 50$\sigma$ levels (1$\sigma$ = 0.007 mJy/beam) in the box $\sim$0.9$\times$0.9$"$. The location of the AGN according to Gaia DR3 gaia_dr3 is marked with a black cross. Middle panel: polarized intensity with the overplotted total intensity contours. Bottom panel: polarization degree in per cent. The areas with a signal-to-noise ratio $<$ 5 on the total intensity map are masked.
  • Figure 2: ALMA observations of MCG 5--23--16. Upper panel: total intensity with the contours corresponding to the 4, 6, 19, 37$\sigma$ levels (1$\sigma$ = 0.010 mJy/beam) in the box $\sim$1$\times$1$"$. The location of the AGN according to Gaia DR3 gaia_dr3 is marked with a black cross. Middle panel: polarized intensity with the overplotted total intensity contours. Bottom panel: polarization degree in per cent. The areas with a signal-to-noise ratio $<$ 5 on the total intensity map are masked.
  • Figure 3: ALMA observations of NGC 4945. Upper panel: total intensity with the contours corresponding to the 31, 46, 55, 92, 184$\sigma$ levels (1$\sigma$ = 0.016 mJy/beam) in the box $\sim$1.2$\times$1.2$"$. The location of the AGN, where mm emission peaks, is marked with a black cross. Middle panel: polarized intensity with the overplotted total intensity contours. Bottom panel: polarization degree in per cent. The areas with a signal-to-noise ratio $<$ 5 on the total intensity map are masked.
  • Figure 4: Map of the in-band spectral index $\alpha$ (upper panel) and its associated error (bottom panel). Dashed contours represent the total intensity (see Fig. \ref{['fig:3783']}). The position of the polarized source is highlighted by a thick black contour. Note that the very steep slope in the outer regions is likely an artifact caused by the slightly narrower beam size at higher frequencies.
  • Figure 5: Archival ALMA Band 3 data. Only observations with a resolution higher than 03 were used. For all data, the flux error is assumed to be 5%.
  • ...and 1 more figures