Tracing the origin of the gas emission lines in galaxies can prove challenging in certain cases. Buried active galactic nuclei (AGNs) and obscured young stellar clusters pose significant obstacles with respect to traditional optical diagnostic diagrams. Therefore, developing new tools to trace the excitation sources across the spectrum is a necessary effort for the advancement of the field. Our goal is to explore the full spectral range of the JWST--NIRSpec data, searching for alternative diagnostic diagrams in the less explored near-infrared (NIR) and to investigate the nature of the ionizing and heating source. We analyzed the high-resolution spectra of the circumnuclear regions of nine local (z < 0. 1) U/LIRGS, investigating potential emission-line ratios for tracing the excitation mechanisms acting on the line-emitting gas. We investigated these objects using the well-established ii, 1. 2570μm/Paβ versus H₂, 1--0, S (1) /Brγ diagram and attempted to correlate its classifications with other emission features across the spectrum. We then compared the empirical classifications with CLOUDY photoionization models to evaluate how accurately the data can be reproduced. Finally, we compared the line widths at 80% of the total flux (W₈0) of selected emission lines with the corresponding gas excitation mechanisms. We propose two line-excitation diagnostic diagrams based on i, 0. 9852μm/Paγ and H₂, 1--0, O (5) /PAH 3. 3, μm ratio, which we found to directly correlate with the hardness of the radiation field and, therefore, with the gas excitation mechanisms. Both the empirical ii, 1. 2570μm/Paβ versus H₂, 1--0, S (1) /Brγ diagram and our proposed i, 0. 9852μm/Paγ versus H₂, 1--0, S (1) /Brγ diagnostic diagram are well reproduced by the photoionization models. In addition, the line-kinematics analysis shows that the W₈0 values of regions excited by star formation are, on average, slightly lower than those of AGN-excited regions, while shock-excited regions display distinctively higher W₈0 values compared to the other two groups. Our JWST--NIRSpec results reinforce reports in previous studies indicating that the H₂ emission in the central regions of AGN hosts is complex and likely produced by multiple excitation mechanisms.
Costa-Souza et al. (Mon,) studied this question.