HST Observations of the [O iii] Emission-Line Region in Markarian 78

Abstract

We present the results of HST observations of Mrk 78, a Seyfert 2 galaxy. Observations were taken with the Faint Object Camera using the F550M, F502M (which includes the [O III] λλ4959, 5007 emission lines), and F130M filters. These are medium band filters centered at 5500, 5020, and 1300 A, respectively. The [O III] image shows a biconical shape with a full opening angle of 40^deg^, elongated in the direction of the radio structure, with its apex approximately coincident with the center of both the radio and optical continuum. A resolved (0.8" in extent) obscuring region is present close to the center of the cone. The UV image confirms the result that emission from the nucleus is not seen directly. Our results provide a strong observational support for the "occultation/reflection" model for Seyfert 2 galaxies. We discuss the relation between radio and emission-line regions and propose a pure photoionization origin for the [O III] emission. Finally, we test the occultation model by comparing our observations with the IRAS data: a strongly anisotropic nuclear source is required to meet the energy requirement derived from the analysis of the far-infrared emission.