Scattered synchrotron emission and a giant torus revealed in polarized light in the nearest radio galaxy Centaurus A^⋆

¹ Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 11 Rue de l’université, 67000 Strasbourg, France
² Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19c, B5c, 4000 Liège, Belgium
³ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026, China
⁴ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
⁵ University of California, Physics Department, Santa Barbara, Broida Hall, Santa Barbara, CA 93106-9530, USA
⁶ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁷ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
⁸ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, E-38205 La Laguna, Tenerife, Spain
⁹ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
¹⁰ European Southern Observatory (ESO), Alonso de Córdova 3107, Casilla 19, Santiago 19001, Chile

^⋆⋆ Corresponding author; frederic.marin@astro.unistra.fr

Received: 18 October 2024
Accepted: 7 February 2025

Abstract

Context. Centaurus A (Cen A) is the closest radio galaxy and a prime example of a low-luminosity active galactic nucleus (AGN), exhibiting complex emissions across the electromagnetic spectrum. The nature of its continuum emission, particularly the mechanisms powering it, has been a subject of considerable debate due to the fact that the AGN is deeply buried in dust and therefore not directly observable.

Aims. This study aims to elucidate the origin of the continuum emission in Cen A and determine the geometrical arrangement of matter in the nuclear region by the mean of optical and near-infrared spectropolarimetry.

Methods. We obtained spectropolarimetric data of Cen A using the VLT/FORS2, covering the 6100−10 300 Å spectral range with an effective resolving power of about 1000. Analysis was conducted on several regions near the obscured AGN, examining total and polarized fluxes, in order to find signatures of the AGN in scattered light.

Results. The analysis revealed a region showing strong and narrow emission lines associated with AGN activity. After correction for interstellar polarization in the dust lane (but not for starlight), the intrinsic polarization of the scattered AGN light exhibits a polarization degree of 2−4%, decreasing from optical to near-infrared, associated with a polarization position angle perpendicular to the radio jet axis. We exclude the presence of a hidden broad line in our polarized flux spectrum at ≥99% probability. Narrow emission lines are found to be strongly polarized and orthogonal to the jet position angle. We demonstrate that a beamed synchrotron jet, scattering onto the narrow line region (NLR) best fits all the observational properties reported in this paper and the literature. In this model, the base of the NLR is obscured by a giant (≥10 pc) circumnuclear region and can only become visible through perpendicular scattering onto the outermost part of the NLR, naturally producing high polarization degrees and polarization angles perpendicular to the radio structure.

Conclusions. This study provides strong evidence that Cen A defines a new class of hidden-NLR AGNs in which two other objects naturally find their place (NGC 4258 and 3C 270) and this supports old predictions that beamed synchrotron jets can be observed in reflection. Future surveys should focus on identifying similar hidden-NLR AGNs, especially among misdirected BL Lac AGNs.