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

Abstract

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.