[Paper Review] Mid-Infrared line diagnostics of Active Galaxies -- A spectroscopic AGN survey with ISO-SWS

E. Sturm, D. Lutz+6 more|ArXiv.org|Jul 18, 2002

Astrophysics and Star Formation StudiesPhysics and Astronomy2 references130 citations

TL;DR

This paper presents medium-resolution (R ~ 1500) ISO-SWS 2.4–45 μm spectra of 29 active galaxies, using mid-infrared fine structure line ratios to distinguish AGN-dominated from starburst-dominated emission. It introduces new diagnostic diagrams—such as [O IV]25.9 μm/[Ne II]12.8 μm and two-dimensional extinction-resistant diagrams—that enable robust identification of composite (AGN + starburst) sources and improve diagnostics in dusty, high-extinction environments.

ABSTRACT

We present medium resolution (R approx. 1500) ISO-SWS 2.4--45 micron spectra of a sample of 29 galaxies with active nuclei. This data set is rich in fine structure emission lines tracing the narrow line regions and (circum-)nuclear star formation regions, and it provides a coherent spectroscopic reference for future extragalactic studies in the mid-infrared. We use the data set to briefly discuss the physical conditions in the narrow line regions (density, temperature, excitation, line profiles) and to test for possible differences between AGN sub-types. Our main focus is on new tools for determining the propertibes of dusty galaxies and on the AGN-starburst connection. We present mid-IR line ratio diagrams which can be used to identify composite (starburst + AGN) sources and to distinguish between emission excited by active nuclei and emission from (circum-nuclear) star forming regions. For instance, line ratios of high to low excitation lines like [O IV]25.9um/[Ne II]12.8um, that have been used to probe for AGNs in dusty objects, can be examined in more detail and with better statistics now. In addition, we present two-dimensional diagnostic diagrams that are fully analogous to classical optical diagnostic diagrams, but better suited for objects with high extinction. Finally, we discuss correlations of mid-infrared line fluxes to the mid- and far-infrared continuum. We compare these relations to similar relations in starburst galaxies in order to examine the contribution of AGNs to the bolometric luminosities of their host galaxies. The spectra are available in electronic form from the authors.

Motivation & Objective

• To establish a coherent spectroscopic reference for mid-infrared extragalactic studies using high-quality ISO-SWS data.
• To investigate physical conditions in narrow line regions (density, temperature, excitation) of active galaxies.
• To develop new mid-infrared diagnostic tools for identifying composite sources (AGN + starburst) in high-extinction environments.
• To compare mid-infrared line fluxes with mid- and far-infrared continuum to assess AGN contributions to host galaxy bolometric luminosities.
• To test the applicability of classical optical diagnostic diagrams in the mid-infrared, adapted for high-extinction scenarios.

Proposed method

• Acquisition of medium-resolution (R ~ 1500) 2.4–45 μm spectra using the ISO-SWS instrument on 29 active galaxies.
• Measurement of fine structure line fluxes for ions such as [Ne II], [O IV], [Ne V], [Ne VI], [Fe II], [S IV], and [Si II] across the sample.
• Construction of mid-infrared line ratio diagrams, including [O IV]25.9 μm/[Ne II]12.8 μm and two-dimensional diagrams analogous to optical BPT diagrams.
• Correlation analysis of mid-infrared line luminosities with mid-IR (MIR) and far-IR (FIR) continuum luminosities to assess AGN contributions.
• Use of PAH features and high-ionization line ratios ([Ne VI]/[O IV], [Ne VI]/[Ne II]) as independent indicators for composite source identification.
• Application of linear mixing models to estimate AGN contributions to bolometric luminosities using [O IV]/[Ne II] and [Ne V]/[Ne II] ratios.

Experimental results

Research questions

• RQ1 Can mid-infrared line ratios such as [O IV]25.9 μm/[Ne II]12.8 μm effectively distinguish between AGN-dominated and starburst-dominated emission in dusty galaxies?
• RQ2 How do mid-infrared diagnostic diagrams compare to classical optical BPT diagrams in identifying composite sources under high extinction?
• RQ3 What is the relationship between mid-infrared line luminosities and the mid- and far-infrared continuum in AGN hosts?
• RQ4 Do the physical conditions (density, temperature, ionization) in the narrow line regions differ significantly between AGN sub-types?
• RQ5 To what extent can [Ne II]12.8 μm serve as a reliable reference line for comparing line ratios across different AGN and starburst sources?

Key findings

• The [O IV]25.9 μm/[Ne II]12.8 μm line ratio is a robust diagnostic tool for identifying composite sources, with higher ratios indicating stronger AGN contribution.
• Two-dimensional mid-infrared diagnostic diagrams—analogs to optical BPT diagrams—show improved performance in high-extinction environments and are applicable across varying redshift ranges.
• High-ionization lines like [O IV], [Ne V], and [Ne VI] are well correlated with both mid-IR and far-IR continuum, though the [Ne II]-to-FIR correlation is similar in both Seyfert and starburst galaxies, limiting direct use for quantifying AGN contribution.
• Low-ionization lines such as [Ne II] show a strong correlation with FIR luminosity in both AGN and starburst galaxies, supporting its use as a reference line for cross-source comparisons.
• All AGN in the sample with measurable [Ne V] emission have low narrow line region electron densities (a few 100 to a few 1000 cm⁻³), consistent with standard photoionization models.
• There is no significant difference in narrow line region density or ionization between different AGN sub-types within the statistical limits of the sample.