“MECANISMOS DE EXCITAÇÃO DO GÁS EM LINERS: UMA PERSPECTIVA NO INFRAVERMELHO PRÓXIMO”

Abstract

Low-Ionization Nuclear Emission-line Regions (LINERs) show ambiguities regarding the gas excitation mechanisms, often sharing characteristics with classical active galactic nuclei (AGNs). In this work, we proposed and applied near-infrared (NIR) diagnostic diagrams to characterize, for the first time, a local sample of LINERs. The main goal was to identify the dominant ionization mechanism in 47 active galaxies observed with the GNIRS spectrograph on the Gemini North telescope. Spectral synthesis was performed with Starlight, and emission line fitting was carried out with the Pyspeckit package, providing fluxes, line ratios, and estimates of interstellar reddening. We used combinations of line ratios sensitive to different mechanisms, such as H2 2.122μm/Brγ, [Fe,ii] 1.257μm/Paβ, [C,i]/Paβ, and [S,iii]/Paγ. The application of the Fe2H2 diagram showed that, among the galaxies for which all required lines were measurable, more than half (about 53%) fall within the typical LINER region. Four galaxies previously classified as Seyfert were reclassified as LINERs in the NIR, highlighting the potential of this spectral range to refine classifications in obscured environments. The comparison with other NIR diagnostic diagrams revealed that LINERs are concentrated in regions compatible with excitation by low-luminosity AGNs rather than by shocks. We also investigated interstellar reddening from Paβ/Brγ and [Fe,ii] 1.257/1.644μm ratios, finding that, in general, E(B-V)[FeII] is equal to or lower than the value obtained from hydrogen lines. However, since Brγ is weak and difficult to detect in most objects—especially in LINERs—this trend must be confirmed with higher signal-to-noise data. Nevertheless, it suggests that [Fe,ii] emission arises from more external and less obscured regions, predominantly photoionized by the central source. Finally, we detected a significant fraction of Type 1 LINERs (with broad components in Paβ, Paγ, and especially He i), supporting the scenario of weak AGNs with a detectable BLR. This study demonstrates that NIR diagnostics are effective tools to refine classifications and investigate the physical structure of obscured active nuclei.