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We present a detailed study of the Seyfert 2 galaxy Markarian 78, using continuum and emission-line images and multiaperture spectra from the Hubble Space Telescope (HST) and a deep 3.6 cm VLA image. Our overall aim is to study the interaction between the radio source and the emission-line gas, since ground-based data already indicate the presence of a strong bipolar jet-driven flow. First, in the wider context, Mrk 78 is probably a post-merger system, with a nuclear dust lane, approximate r1/4 continuum profile, and highly extended asymmetric gas distribution. The [O III] and radio images both show complex structures with many similarities but also important differences. A careful comparison shows convincing morphological evidence for jet-gas interaction: (1) the western inner radio jet terminates and flares at the position of a bright [O III] knot; (2) the weaker eastern radio jet changes direction as it encounters a large [O III] knot; (3) most [O III] features appear limb brightened on the upstream side, and flared on the downstream side; and (4) in the outer regions the radio components tend to lie between or adjacent to [O III] knots, indicating the radio and line emitting phases do not easily interpenetrate. In addition to evidence of jet-gas interaction, two features indicate the importance of a central ionizing radiation field: an inner fanlike structure to the east, with a straight kinematically quiet northern edge; and an approximately fanlike extended narrow-line region to the west, lying well outside the radio source. The [O III] line profiles from 10 Faint Object Spectrograph apertures provide further kinematic insight into the jet-gas interaction. (1) On the eastern side, where the radio source is deflected, the [O III] profile contains a highly redshifted component (~700 km s-1) that is also narrow (FWHM ≤ 200 km s-1), indicating significant coherent gas acceleration with almost no induced turbulence. (2) At this location, the [O III] profile is also double, suggesting lateral expansion away from a jet axis, as it burrows into the cloud complex. (3) In contrast, the bright western inner knot, which seems to disrupt the inner radio jet, has essentially undisturbed kinematics. (4) Across the outer complex western region, gas velocities are higher in the region center, decrease at the leading edge and may be highest where radio flows "blow out" of the region. Overall, morphology and kinematics suggest the western side is best described as an initially disrupted jet that then fills, accelerates, and leaks out of a complex incomplete "bubble" of ionized gas. The eastern side is best described as a large centrally illuminated fanlike gas structure that is penetrated, accelerated, and ultimately deflects the radio source. We use these different regions to construct a plausible evolutionary sequence: Initially, a dense (molecular?) cloud enters the jet flow and disrupts it (inner western knot); as time passes, the jet begins to penetrate, accelerate, and ablate the cloud (eastern knot complex); continued jet influence further disperses the cloud fragments, sweeping out channels and gaps (western lobe region). This process may repeat itself many times over the lifetime of the jet. In companion papers, we study the ionization mechanisms and jet properties in more detail using recently acquired Space Telescope Imaging Spectrograph spectra, taking a more complete account of the energetics of the radio source and the ionized gas.

Jet-Gas Interactions in Markarian 78. I. Morphology and Kinematics