The Narrow‐Line Region of the Seyfert 2 Galaxy Mrk 78: An Infrared View

We report near-infrared spectroscopic data for the Seyfert 2 galaxy Mrk 78,taken with the LIRIS near-infrared camera/spectrometer at the William HerschelTelescope (WHT). The long-slit spectra clearly show extended emission. Theresolution and depth of the near-infrared spectra allows the examination of itsmorphology and ionization regions, and a direct comparison with similarly deepvisible spectra. The emission-line ratios obtained are used to derive theextinction towards the nucleus. The detection of strong features such as [FeII], H$_{2}$, hydrogen recombination lines and the coronal [SiVI]$\lambda$1.962 line is used to study the kinematics and excitationmechanisms occurring in Mrk 78, revealing that despite of the strong radio-jetinteraction present in this object, photoionization from the active nucleusdominates the narrow line region emission, while UV fluorescence is the sourceof the H$_{2}$ emission. Lines with extended emission yield velocitydistributions with an amplitude of about 600 km/s, the consequence of aneastern lobe moving away from us plus a western lobe with the oppositecontribution. We used the photoionization code CLOUDY to recreate a typicalnarrow line region region, to derive the ionization parameter, and to compareour spectral data with diagnostic diagrams.Comment: 18 pages, 12 figures, 4 tables. Accepted by Ap