Forgotten treasures in the HST/FOC UV imaging polarimetric archives of active galactic nuclei. IV. 5 Orphaned AGNs

TL;DR

This paper completes the homogeneous re-analysis of five previously unpublished HST/FOC near-UV imaging polarimetry observations of AGNs, yielding spatially resolved polarization maps for Mrk 3, Mrk 78, NGC 3862, Cygnus A, and 3C 109. Using a common pipeline, the study finds NLR-centered, centro-symmetric polarization patterns in Mrk 3, Mrk 78, and Cygnus A consistent with scattering from an obscured nucleus, while NGC 3862 shows marginal UV polarization aligned with the inner jet indicative of synchrotron emission, and 3C 109 exhibits strong nuclear polarization aligned with the radio axis, suggestive of a dust-obscured quasar-like nucleus with multiple scattering components. The results reinforce the unified AGN model and demonstrate the diagnostic power of spatially resolved UV polarimetry for deciphering complex AGN geometries, winds, and scattering media. Collectively, these mappings illustrate how archival UV polarimetry, when recalibrated, can provide core insights into AGN morphology and the interplay between jets, outflows, and the surrounding medium.

Abstract

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. 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. 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. The polarization pattern in Mrk 3 and Mrk 78 in 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. 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.

Figures (11)

• Figure 1: HST/FOC polarization maps of Mrk 3 through filter F502M. Left: Intensity map overlaid with the integration regions used in Table \ref{['tab:Mrk3']}. Right: Debiased polarization degree. Darker pixels have $\left[S/N\right]_P < 1$. Both maps are at spatial binning of 0.10, smoothed using a 0.15 FWHM Gaussian kernel. 30 pixels have $\left[S/N\right]_P \geq 3$. The orange cross displays the estimated location for the obscured nucleus Kishimoto2002.
• Figure 2: HST/FOC total intensity map of NGC 3862, through filter F342W, cropped to the central 5$\times$5. Left: Intensity map at the Nyquist sampling resolution without smoothing. No polarization is detected among individual pixels. Right: Intensity map at a spatial binning of 0.10, smoothed using a 0.15 FWHM Gaussian kernel. 2 pixels have $\left[S/N\right]_P \geq 3$.
• Figure 3: HST/FOC polarization maps of Mrk 78 through filter F502M. Top: Intensity map overlaid with the integration regions used in Table \ref{['tab:MRK78']}. Bottom: Debiased polarization degree. Darker pixels have $\left[S/N\right]_P < 1$. Both maps are at a spatial binning of 0.05, smoothed using a 0.075 FWHM Gaussian kernel. 4 pixels have $\left[S/N\right]_P \geq 3$. The orange cross displays the estimated location for the obscured nucleus Whittle2004.
• Figure 4: HST/FOC polarization maps of Cygnus A, observation ID 6510 through filter F342W. Left: Intensity map overlaid with the integration regions used in Table \ref{['tab:CygnusA']}. Right: Debiased polarization degree. Darker pixels have $\left[S/N\right]_P < 1$. Both maps have a spatial binning of 0.10, smoothed using a 0.15 FWHM Gaussian kernel. 18 pixels have $\left[S/N\right]_P \geq 3$. The orange cross displays the estimated location for the obscured nucleus Jackson1998.
• Figure 5: HST/FOC total intensity maps of 3C 109, observation ID 6927 through filter F342W, cropped to the central 2$\times$2. Left: Intensity map at the native spatial resolution without smoothing. No polarization is detected among individual pixels. Right: Intensity map at a spatial binning of 0.10, smoothed using a 0.15 FWHM Gaussian kernel. 4 pixels have $\left[S/N\right]_P \geq 3$. The maps are zoomed in, the color-bar flux values and scale of the pixels differ between the two figures.