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 <ns0:sc>V</ns0:sc>]14.3, 24.3 μm or [O <ns0:sc>IV</ns0:sc>]25.9 μm. The measured emission-line ratios were included as inputs for the code HC<ns0:sc>M</ns0:sc>-T<ns0:sc>EFF</ns0:sc>-IR in order to provide a quantification for the ionization and the αOX. 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 <ns0:sc>II</ns0:sc>]12.8 μm+[Ne <ns0:sc>III</ns0:sc>]15.6 μm)/[Ne <ns0:sc>V</ns0:sc>]14.3 μm and [O <ns0:sc>III</ns0:sc>]52, 88 μm/[O <ns0:sc>IV</ns0:sc>]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 <ns0:sc>III</ns0:sc>]52, 88 μm lines, an alternative based on the [S <ns0:sc>III</ns0:sc>]18.7 μm+[S <ns0:sc>IV</ns0:sc>]10.5 μm lines can be used. The inclusion of very high-excitation lines in the input ratios for HC<ns0:sc>M</ns0:sc>-T<ns0:sc>EFF</ns0:sc>-IR to derive both αOX and U for the studied sample points to a bimodal distribution of galaxies. One of the peaks is characterized by relatively harder values of αOX around −1.4 in combination with low values for log U around −2.4, while the other peak shows a softer αOX ∼ −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.