Issue |
A&A
Volume 696, April 2025
|
|
---|---|---|
Article Number | A229 | |
Number of page(s) | 12 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202453276 | |
Published online | 25 April 2025 |
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
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 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 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 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.
Key words: galaxies: active / galaxies: star formation
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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