Molecular Hydrogen and [Fe II] in Active Galactic Nuclei III: LINERS and Star Forming Galaxies

TL;DR

This study investigates the kinematics and excitation mechanisms of H2 and [Fe II] emission lines in 67 emission-line galaxies, revealing the significance of thermal excitation in both active galactic nuclei and star-forming galaxies, and refining diagnostic tools for galaxy activity classification.

Contribution

The paper provides new insights into the excitation mechanisms of H2 and [Fe II] lines, demonstrating the role of thermal processes in star-forming galaxies and proposing improved line ratio criteria for galaxy activity classification.

Findings

H2 emission lines are narrower than NLR lines, indicating different kinematic origins.

Thermal excitation significantly influences H2 and [Fe II] lines in various galaxy types.

Refined line ratio thresholds improve the classification of nuclear activity.

Abstract

We study the kinematics and excitation mechanisms of H2 and [Fe II] lines in a sample of 67 emission-line galaxies with Infrared Telescope Facility SpeX near-infrared (NIR, 0.8-2.4 micrometers) spectroscopy together with new photoionisation models. H2 emission lines are systematically narrower than narrow-line region (NLR) lines, suggesting that the two are, very likely, kinematically disconnected. The new models and emission-line ratios show that the thermal excitation plays an important role not only in active galactic nuclei but also in star forming galaxies. The importance of the thermal excitation in star forming galaxies may be associated with the presence of supernova remnants close to the region emitting H2 lines. This hypothesis is further supported by the similarity between the vibrational and rotational temperatures of H2. We confirm that the diagram involving the line ratios H2 2.121/Br_gamma and [Fe II] 1.257/Pa_beta is an efficient tool for separating emission-line objects according to their dominant type of activity. We suggest new limits to the line ratios in order to discriminate between the different types of nuclear activity.