Exploring the hardness of the ionizing radiation with the infrared softness diagram

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 μm or [O IV ]25.9 μm. The measured emission-line ratios were included as inputs for the code HC M -T EFF -IR in order to provide a quantification for the ionization and the α 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 μm+[Ne III ]15.6 μm)/[Ne V ]14.3 μm and [O III ]52, 88 μm/[O IV ]25.9 μm 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 μm lines, an alternative based on the [S III ]18.7 μm+[S IV ]10.5 μm lines can be used. The inclusion of very high-excitation lines in the input ratios for HC M -T EFF -IR to derive both α 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 α OX around −1.4 in combination with low values for log U around −2.4, while the other peak shows a softer α OX ∼ −1.7 and high values of log U ∼ −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.