X-rays as the dominant excitation mechanism of [Fe II] and H2 emission lines in active galaxies

Abstract

We investigate the excitation mechanisms of near-infrared [Fe II] and H<SUB>2</SUB> emission lines observed in active galactic nuclei (AGNs). We built a photoionization model grid considering a two-component continuum: one component accounts for the 'big bump' component peaking at 1Ryd and another represents the X-ray source that dominates the continuum emission at high energies. Photoionization models considering as ionizing source a spectral energy distribution obtained from photometric data of Seyert 2 Mrk1066 taken from the literature were considered. Results of these models were compared with a large sample of observational long-slit and Integral field Unit (IFU) spectroscopy data of the nuclear region for a sample of active objects. We found that the correlation between the observational [Fe II]λ 1.2570μm/Paβ and H<SUB>2</SUB>λ2.1218μm/Brγ is well reproduced by our models, as are the relationships that involve the H<SUB>2</SUB> emission-line ratios observed in the spectroscopic data. We conclude that heating by X-rays produced by active nuclei can be considered a common and very important mechanism of excitation of [Fe II] and H<SUB>2</SUB>.