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

Abstract

Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of H2 and [Fe II] 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 hotH2 ranges from 10 2 to 10 3Mʘ, with nearly half of objects showing values of <500Mʘ. 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 [Fe II]/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 [Fe II] 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.