High Resolution (<100 pc) Ionization Structures in the ISM of AGN ESO 137-G34

Abstract We present a detailed, spatially resolved Veilleux and Osterbrock (VO) diagram analysis of ESO 137-G034, a Compton-thick active galactic nucleus (AGN), based on narrowband Hubble Space Telescope imaging in [O III ], [S II ], H α , and H β emission lines. The narrowband optical emission exhibits a biconical morphology and traces the diffuse X-ray emission observed with Chandra. We dissect the interstellar medium (ISM) into Seyfert-, LINER-, and H II -dominated regions with a resolution of ∼0 . ″ 1 (∼20 pc at z = 0.009). To visualize the fine spatial structure of the ISM excitation, we introduce a new parameter: the Seyfert/LINER Index (SLI), defined as the perpendicular distance of each point in the VO diagram from the Seyfert/LINER division line. SLI mapping reveals fine spatial structures in the ISM excitation especially in the Seyfert ionization bicones where the statistical significance of our data is higher. Most of the emission within the bicones is Seyfert-like, with SLI values consistent with regions of the VO diagram that can be modeled with AGN photoionization. In the northwest Seyfert cone three radial SLI peaks suggest episodic nuclear outbursts, with estimated timescales of a few 10 3 yrs and 1000 km s −1 ) induced excitation. The northwest cone is surrounded by a smooth LINER “cocoon.” The LINER region in the southeast cone appears more irregular, with intercone LINER points likely shaped by both local ISM structure and shocks. Our findings highlight the complex interplay of different AGN feedback mechanisms in the ISM.