Far-infrared line spectra of active galaxies from the Herschel/PACS Spectrometer: the complete database

Fernández Ontiveros, J. A., Spinoglio, L., Pereira Santaella, M., Malkan, M. A., Andreani, P., Dasyra, K. M. 2016. Far-infrared line spectra of active galaxies from the Herschel/PACS Spectrometer: the complete database. Astrophysical Journal Supplement Series 226, 2, DOI: 10.3847/0067-0049/226/2/19

We present a coherent database of spectroscopic observations of far-IR fine-structure lines from the Herschel/Photoconductor Array Camera and Spectrometer archive for a sample of 170 local active galactic nuclei (AGNs), plus a comparison sample of 20 starburst galaxies and 43 dwarf galaxies. Published Spitzer/IRS and Herschel/SPIRE line fluxes are included to extend our database to the full 10-600 mu m spectral range. The observations are compared to a set of CLOUDY photoionization models to estimate the above physical quantities through different diagnostic diagrams. We confirm the presence of a stratification of gas density in the emission regions of the galaxies, which increases with the ionization potential of the emission lines. The new [O IV](25.9) (mu m/)[O III](88) (mu m) versus [Ne III](15.6) (mu m)/[Ne II](12.8 mu m) diagram is proposed as the best diagnostic to separate (1) AGN activity from any kind of star formation and (2) low-metallicity dwarf galaxies from starburst galaxies. Current stellar atmosphere models fail to reproduce the observed [O IV](25.9 mu m)/[O III](88 mu m) ratios, which are much higher when compared to the predicted values. Finally, the ([Ne III](15.6 mu m) + [Ne II](12.8 mu m))/([S IV](10.5 mu m) +[S III](18.7 mu m)) ratio is proposed as a promising metallicity tracer to be used in obscured objects, where optical lines fail to accurately measure the metallicity. The diagnostic power of mid-to far-infrared spectroscopy shown here for local galaxies will be of crucial importance to study galaxy evolution during the dust-obscured phase at the peak of the star formation and black hole accretion activity (1 < z < 4). This study will be addressed by future deep spectroscopic surveys with present and forthcoming facilities such as the James Webb Space Telescope, the Atacama Large Millimeter/submillimeter Array, and the Space Infrared telescope for Cosmology and Astrophysics.

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