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

¹ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
² Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Plaza San Juan 1, 44001 Teruel, Spain
³ Unidad Asociada CEFCA-IAA, CEFCA, Unidad Asociada al CSIC por el IAA, Plaza San Juan 1, 44001 Teruel, Spain
⁴ ARAID Foundation, Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, 44001 Teruel, Spain

^⋆ Corresponding author; epm@iaa.es

Received: 3 December 2024
Accepted: 9 March 2025

Abstract

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 HCM-TEFF-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 HCM-TEFF-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.