Far-infrared line spectra of seyfert galaxies from the herschel-pacs spectrometer

We observed the far-IR fine-structure lines of 26 Seyfert galaxies with the Herschel-PACS spectrometer. Theseobservations are complemented with Spitzer Infrared Spectrograph and Herschel SPIRE spectroscopy.We used theionic lines to determine electron densities in the ionized gas and the [C i] lines, observed with SPIRE, to measurethe neutral gas densities, while the [Oi] lines measure the gas temperature, at densities below ∼104 cm−3. Usingthe [Oi]145 μm/63 μm and [S iii]33/18 μm line ratios, we find an anti-correlation of the temperature with the gasdensity. Various fine-structure line ratios show density stratifications in these active galaxies. On average, electrondensities increase with the ionization potential of the ions. The infrared lines arise partly in the narrow line region,photoionized by the active galactic nucleus (AGN), partly in Hii regions photoionized by hot stars, and partly inphoto-dissociated regions.We attempt to separate the contributions to the line emission produced in these differentregions by comparing our observed emission line ratios to theoretical values. In particular, we tried to separate thecontribution of AGNs and star formation by using a combination of Spitzer and Herschel lines, and we found thatbesides the well-known mid-IR line ratios, the line ratio of [O iii]88μm/[O iv]26 μm can reliably discriminate thetwo emission regions, while the far-IR line ratio of [C ii]157 μm/[O i]63 μm is only able to mildly separate thetwo regimes. By comparing the observed [Cii]157 μm/[N ii]205 μm ratio with photoionization models, we alsofound that most of the [C ii] emission in the galaxies we examined is due to photodissociation regions.