Open AccessMolecular hydrogen and [Fe ii ] in active galactic nuclei – II. Results for Seyfert 2 galaxies
Author(s) -
RodríguezArdila A.,
Riffel R.,
Pastoriza M. G.
Publication year - 2005
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2005.09638.x
Subject(s) - physics , galaxy , active galactic nucleus , astrophysics , excitation , line (geometry) , spectral line , infrared , emission spectrum , excitation temperature , luminous infrared galaxy , hydrogen , thermal , mass fraction , astronomy , quantum mechanics , geometry , mathematics , meteorology , thermodynamics
Near‐infrared spectroscopy is used to study the kinematics and excitation mechanisms of H 2 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 H 2 lines are systematically narrower than the narrow‐line region lines, suggesting that, very likely, the H 2 does not originate from the same parcel of gas that forms the narrow‐line region. Emission‐line ratios between H 2 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 H 2 . Thermal excitation is also confirmed by the rather similar vibrational and rotational temperatures in the objects (∼2000 K). The mass of hot H 2 ranges from 10 2 to 10 3 M ⊙ , with nearly half of objects showing values of <500 M ⊙ . 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 H 2 /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 H 2 2.121 μm/Brγ and [Fe ii ] 1.257 μm/Paβ flux ratios between 0.6 and 2. Starburst/H ii galaxies display line ratios <0.6 while low‐ionization nuclear emission‐line regions are characterized by values larger than 2 in either ratio.