Forgotten treasures in the HST/FOC UV imaging polarimetric archives of active galactic nuclei

IV. 5 Orphaned AGNs

¹ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
² Department of Physics & Astronomy, University of South Carolina, Columbia, SC 29208, USA
³ Kavli Institute for Particle Astrophysics & Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA

^⋆ Corresponding author: thibault.barnouin@astro.unistra.fr

Received: 24 June 2025
Accepted: 4 August 2025

Abstract

Context. The Faint Object Camera (FOC) on board the Hubble Space Telescope (HST) acquired high-resolution spatially resolved polarimetric images of nearby active galactic nuclei (AGNs) in the near-ultraviolet (near-UV) band. Eight of the 25 individual targets in the polarized archives had no published analysis until the beginning of this series of papers. We describe the last 5 targets here.

Aims. In this paper, we finalize the publication of near-UV imaging polarimetry of AGNs in the HST/FOC archives. We render available spatially resolved polarization maps of the [OIII] emission lines for Mrk 3 and Mrk 78, as well as near-UV continuum polarization maps for Mrk 3, NGC 3862, Cygnus A, and 3C 109.

Methods. We used the generalized reduction pipeline presented in the first paper in this series to homogeneously analyze the five remaining polarized observations of AGNs in the FOC archives.

Results. The polarization pattern in Mrk 3 and Mrk 78in the narrow-line regions is consistent with scattering from an obscured nucleus. For NGC 3862, we confirm marginal UV polarization parallel with the inner radio jet that is related to synchrotron emission. In Cygnus A, we report spatially resolved centro-symmetric polarization patterns in the two opposite outflows, which highlights the scattering origin of the polarized light. Finally, 3C 109 shows high nuclear polarization that is consistent with AGN-dominated emission and parallel with the radio axis, but differs from the polarization from dichroic absorption invoked by previous authors.

Conclusions. The imaging polarimetry we obtained for the narrow-line region and the extended scattering medium surrounding the obscured AGNs is aligned with the predictions of the unified AGN model and demonstrates the power of spatially resolved polarimetric observation to decipher the complex morphologies at work in AGNs.