Spatially resolved BPT mapping of nearby Seyfert 2 galaxies

TL;DR

This study uses high-resolution HST imaging to map the ionization regions in Seyfert 2 galaxies, revealing complex structures including Seyfert, LINER, and star-forming zones, and emphasizing the importance of spatial diagnostics.

Contribution

It provides the first detailed spatially resolved BPT maps of Seyfert 2 galaxies, uncovering the prevalence of LINER cocoons around AGN regions.

Findings

Nuclear and ionization cones are dominated by Seyfert emission.

LINER cocoons extend up to 250 pc around nuclei.

Spatial diagnostics are essential for understanding excitation mechanisms.

Abstract

We present spatially resolved BPT mapping of the extended narrow line regions (ENLRs) of seven nearby Seyfert 2 galaxies, using HST narrow band filter imaging. We construct the BPT diagrams using $\leq$ 0.1" resolution emission line images of [O III]$\lambda$5007, H$\alpha$, [S II]$\lambda$$\lambda$6717,6731, and H$\beta$. By mapping these diagnostic lines according to the BPT classification, we dissect the ENLR into Seyfert, LINER, and star-forming regions. The nucleus and ionization cones are dominated by Seyfert-type emission, which can be interpreted as predominantly photoionization by the active galactic nucleus (AGN). The Seyfert nucleus and ionization cones transition to and are surrounded by a LINER cocoon, extending up to $\sim$ 250 pc in thickness. The ubiquity of the LINER cocoon in Seyfert 2 galaxies suggests that the circumnuclear regions are not necessarily Seyfert-type, and LINER activity plays an important role in Seyfert 2 galaxies. We demonstrate that spatially resolved diagnostics are crucial to understanding the excitation mechanisms in different regions and the AGN-host galaxy interactions.