Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies

Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of H₂ and [FeII] lines in a sample dominated by Seyfert 2 galaxies. The spectra simultaneously cover the JHK bands, allowing us to compare line fluxes emitted in the interval 0.8-2.4 μm and avoiding aperture and seeing effects. The H₂ lines are systematically narrower than the narrow-line region lines, suggesting that, very likely, the H₂ does not originate from the same parcel of gas that forms the narrow-line region. Emission-line ratios between H₂ lines favour thermal excitation mechanisms for the molecular gas in active galactic nuclei. It was found that non-thermal excitation contributes, at most, 30 per cent of the observed H₂. Thermal excitation is also confirmed by the rather similar vibrational and rotational temperatures in the objects (~2000 K). The mass of hot H₂ ranges from 10² to 10³M_solar, with nearly half of objects showing values of <500 M_solar. It shows that the fraction of molecular mass present in the nuclear region and emitting in the near-infrared is a very small fraction of the warm molecular mass present in the centre. A diagnostic diagram composed of the line ratios H₂/Brγ and [FeII]/Paβ proves to be a useful tool in the near-infrared for separating emission-line objects by their degree of nuclear activity. We found that active galactic nuclei are characterized by H₂ 2.121 μm/Brγ and [FeII] 1.257 μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxies display line ratios <0.6 while low-ionization nuclear emission-line regions are characterized by values larger than 2 in either ratio.