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

TL;DR

This study demonstrates that X-ray irradiation from active galactic nuclei is a primary excitation mechanism for near-infrared [Fe ii] and H2 emission lines, supported by photoionization models matching observational data.

Contribution

The paper introduces comprehensive photoionization models incorporating X-ray sources that successfully explain observed emission line ratios in active galaxies.

Findings

X-ray heating reproduces observed emission line ratios.

X-ray excitation is a dominant mechanism in AGNs.

Models match observational data across multiple spectroscopic datasets.

Abstract

We investigate the excitation mechanisms of near-infrared [Fe ii] and H2 emission lines observed in Active Galactic Nuclei (AGNs). We built a photoionization model grid considering a two-component continuum, one 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 the Sy 2 Mrk 1066 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]{\lambda}1.2570 {\mu}m/Pa{\beta} vs. H2{\lambda}2.1218 {\mu}m/Br{\gamma} is well reproduced by our models as well as the relationships that involve the H2 emission line ratios observed in the spectroscopic data.We conclude that the heating by X-rays produced by active nuclei can be considered a common and very important mechanism of excitation of [Fe ii] and H2.