Molecular hydrogen and [Fe II] in active galactic nuclei - III. Low-ionization nuclear emission-line region and star-forming galaxies

We study the kinematics and excitation mechanisms of H₂ and [Fe II] lines in a sample of 67 emission-line galaxies with Infrared Telescope Facility SpeX near-infrared (0.8-2.4 μm) spectroscopy together with new photoionization models. H₂ emission lines are systematically narrower than narrow-line region lines, suggesting that the two are, very likely, kinematically disconnected. The new models and emission-line ratios show that the thermal excitation plays an important role not only in active galactic nuclei but also in star-forming galaxies. The importance of the thermal excitation in star-forming galaxies may be associated with the presence of supernova remnants close to the region emitting H₂ lines. This hypothesis is further supported by the similarity between vibrational and rotational temperatures of H₂. We confirm that the diagram involving the line ratios H₂ 2.121 μm/Brγ and [Fe II] 1.257 μm/Paβ is an efficient tool for separating emission-line objects according to their dominant types of activities. We suggest new limits to the line ratios in order to discriminate between the different types of nuclear activities.