@article{2025A&A...696A.229P,
 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 \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.},
 adsnote = {Provided by the SAO/NASA Astrophysics Data System},
 adsurl = {https://ui.adsabs.harvard.edu/abs/2025A&A...696A.229P},
 archiveprefix = {arXiv},
 author = {Pérez-Montero, E. and Fernández-Ontiveros, J.~A. and Pérez-Dı́az, B. and V\ĺchez, J.~M. and Amor\',́ R.},
 doi = {10.1051/0004-6361/202453276},
 eid = {A229},
 eprint = {2503.09267},
 journal = {åp},
 keywords = {galaxies: active, galaxies: star formation, Astrophysics of Galaxies},
 month = {April},
 pages = {A229},
 primaryclass = {astro-ph.GA},
 title = {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},
 volume = {696},
 year = {2025}
}