Exploring the hardness of the ionizing radiation with the infrared softness diagram: II. Bimodal distributions in both the ionizing continuum slope and the excitation in active galactic nuclei

Aims. After exploring a version of the infrared (IR) softness diagram to characterize the hardness of the incident ionizing radiation in star-forming regions, we exploit the availability of high- excitation lines in the same spectral regime to explore its potential use for studying the narrow-line regions of active galactic nuclei (AGNs). Methods. We adapted the IR softness diagram to consider very high-excitation lines, such as [Ne V]14.3, 24.3 ensuremathμm or [O IV]25.9 ensuremathμm. The measured emission-line ratios were included as inputs for the code HCM-TEFF-IR in order to provide a quantification for the ionization and the ensuremathα$_OX$. The latter is sensitive to the spectral shape of the incident continuum in AGNs. We applied this code to a large AGN sample that included different spectral types with available Spitzer/IRS, Herschel/PACS, and/or SOFIA/FIFI-LS mid-IR spectroscopic observations. Results. The combination of the ([Ne II]12.8 ensuremathμm+[Ne III]15.6 ensuremathμm)/[Ne V]14.3 ensuremathμm and [O III]52, 88 ensuremathμm/[O IV]25.9 ensuremathμm emission line ratios is a robust proxy for the shape of the ionizing continuum in AGNs. Given the difficulties to observe the [O III]52, 88 ensuremathμm lines, an alternative based on the [S III]18.7 ensuremathμm+[S IV]10.5 ensuremathμm lines can be used. The inclusion of very high-excitation lines in the input ratios for HCM-TEFF-IR to derive both ensuremathα$_OX$ and U for the studied sample points to a bimodal distribution of galaxies. One of the peaks is characterized by relatively harder values of ensuremathα$_OX$ around ‑1.4 in combination with low values for log U around ‑2.4, while the other peak shows a softer ensuremathα$_OX$ ensuremath∼ ‑1.7 and high values of log U ensuremath∼ ‑1.5. This result is consistent with the existence of two very distinct AGN populations, where one has a softer ionizing continuum, possibly dominated by a radiatively efficient accretion disk in bright Seyfert nuclei. In contrast, we observed a harder radiation field in low-luminosity AGNs, where the accretion disk is expected to recede.