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
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arXiv
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| Natura: | Preprint |
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2025
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| _version_ | 1866915265344700416 |
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| author | Pérez-Montero, E. Fernández-Ontiveros, J. A. Pérez-Díaz, B. Vílchez, J. M. Amorín, R. |
| author_facet | Pérez-Montero, E. Fernández-Ontiveros, J. A. Pérez-Díaz, B. Vílchez, J. M. Amorín, R. |
| contents | After exploring the infrared 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 use for studying the narrow-line regions in AGN. We adapted the IR softness diagram to consider very high-excitation lines, such as [Ne V] 14.3,24.3 $μ$m or [O IV] 25.9$μ$m. The measured emission-line ratios were included as inputs for the code HCm-Teff-IR, in order to provide a quantification for both the ionization and the $α_{\rm OX}$ parameters. The latter is sensitive to the spectral shape of the incident continuum in AGN. We applied this to a large AGN sample including different spectral types with available Spitzer/IRS, Herschel/PACS and/or SOFIA/FIFI-LS mid-IR spectroscopic observations. The combination of the ([Ne II] 12.8 $μ$m+[Ne III] 15.6$μ$m)/[Ne V] 14.3$μ$m and [O III] 52,88 $μ$m/[O IV] 25.9$μ$m emission line ratios is a robust proxy for the shape of the ionizing continuum in AGN. An alternative based on the [S III] 18.7 $μ$m+[S IV] 10.5$μ$m lines can be used instead. The inclusion of very high-excitation lines in HCm-Teff-IR, to derive both $α_{\rm OX}$ and $U$ for the studied sample, points to a bimodal distribution of galaxies. One of the peaks is characterized by relatively harder values of $α_{\rm OX}$ around -1.4 in combination with low values for log $U$ around -2.4, while the other peak shows a softer $α_{\rm OX}$ around -1.7 and high values of log $U$ around-1.5. This result is consistent with the existence of two very marked AGN populations., one with a softer ionizing continuum, possibly dominated by a radiatively efficient accretion disk in bright Seyfert nuclei. In contrast, we observe a harder radiation field in low-luminosity AGN, where the accretion disk is expected to recede. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2503_09267 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | 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 Pérez-Montero, E. Fernández-Ontiveros, J. A. Pérez-Díaz, B. Vílchez, J. M. Amorín, R. Astrophysics of Galaxies After exploring the infrared 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 use for studying the narrow-line regions in AGN. We adapted the IR softness diagram to consider very high-excitation lines, such as [Ne V] 14.3,24.3 $μ$m or [O IV] 25.9$μ$m. The measured emission-line ratios were included as inputs for the code HCm-Teff-IR, in order to provide a quantification for both the ionization and the $α_{\rm OX}$ parameters. The latter is sensitive to the spectral shape of the incident continuum in AGN. We applied this to a large AGN sample including different spectral types with available Spitzer/IRS, Herschel/PACS and/or SOFIA/FIFI-LS mid-IR spectroscopic observations. The combination of the ([Ne II] 12.8 $μ$m+[Ne III] 15.6$μ$m)/[Ne V] 14.3$μ$m and [O III] 52,88 $μ$m/[O IV] 25.9$μ$m emission line ratios is a robust proxy for the shape of the ionizing continuum in AGN. An alternative based on the [S III] 18.7 $μ$m+[S IV] 10.5$μ$m lines can be used instead. The inclusion of very high-excitation lines in HCm-Teff-IR, to derive both $α_{\rm OX}$ and $U$ for the studied sample, points to a bimodal distribution of galaxies. One of the peaks is characterized by relatively harder values of $α_{\rm OX}$ around -1.4 in combination with low values for log $U$ around -2.4, while the other peak shows a softer $α_{\rm OX}$ around -1.7 and high values of log $U$ around-1.5. This result is consistent with the existence of two very marked AGN populations., one with a softer ionizing continuum, possibly dominated by a radiatively efficient accretion disk in bright Seyfert nuclei. In contrast, we observe a harder radiation field in low-luminosity AGN, where the accretion disk is expected to recede. |
| 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 |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2503.09267 |