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

Context. The Mrk 463 system is known to host two powerful sources that are separated by about 4 kpc, which were both identified as active galactic nuclei (AGN). This makes the Mrk 463 system a unique laboratory for studying the geometry and dynamics of galaxy merging and its relation to AGN duty cycles. Aims. The eastern nucleus, Mrk 463E, is the brighter of the two and thus a prime target for a polarimetric study. It is classified as a Seyfert 2 galaxy, meaning that high polarization degrees from scattering off electrons and dust in the polar winds are expected. Methods. In the continuity of our series of papers, we reduced archived and previously unpublished polarization observations obtained with the Faint Object Camera on board the Hubble Space Telescope to obtain a high-resolution near-ultraviolet polarization map of the Mrk 463E nuclei. We coupled this map to near-infrared and X-ray observations to obtain a clear picture of the geometric arrangement of matter around the core of Mrk 463E. Results. We found that the nucleus location is farther south of the optical peak flux than previously estimated. The strongly polarized conical wind has a half-opening angle of ∼15° and displays three main periods of mass ejection. Its polarization allowed us to estimate the AGN inclination toward the observer (∼55°). Finally, our maps revealed a streamer connecting Mrk 463E and Mrk 463W, with a tentative detection of a large kiloparsec-scale ordered magnetic field connecting both galaxies. Conclusions. This unpublished observation finally offered more than the original proposal asked for and allowed us to derive tight geometric and dynamical constraints for Mrk 463E. High-resolution radio maps and infrared polarimetry are now necessary to further study the jet and the newly discovered streamer.