Jet-Gas Interaction in Markarian 78. II. Ionization Mechanisms

Abstract

We investigate the ionization mechanisms in the Seyfert galaxy Markarian 78, chosen because its narrow-line region (NLR) is dominated by a strong jet-gas interaction. Our principal aim is to ascertain whether or not shock-related ionization is important. We use Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) optical and UV spectra from four slits, as well as HST Faint Object Spectrograph optical spectra from 10 apertures. Ultimately, several lines of evidence argue against the importance of shocks but in favor of central source photoionization, even for gas that is clearly accelerated by the jet flow. Following earlier work, we compare emission-line ratios with four classes of ionization model: photoionization of optically thick gas (standard and dusty U sequences), photoionization of optically thick and thin gas (Am/i sequence), and fast shocks. Traditional excitation measures are well matched by all three models, with -3.0