Far-infrared line spectra of Seyfert galaxies from the Herschel-PACS
  Spectrometer
  We present spectroscopic observations of FIR fine-structure lines of 26
Seyfert galaxies obtained with the Herschel-PACS spectrometer. These
observations are complemented by spectroscopy with Spitzer-IRS and
Herschel-SPIRE. The ratios of the OIII, NII, SIII and NeV lines have been used
to determine electron densities in the ionised gas regions. The CI lines,
observed with SPIRE, have been used to measure the densities in the neutral
gas, while the OI lines provide a measure of the gas temperature, at densities
below 10000 cm-3. Using the OI145/63um and SIII33/18um line ratios we find an
anti-correlation of the temperature with the gas density. Using various
fine-structure line ratios, we find that density stratification is common in
these active galaxies. On average, the electron densities increase with the
ionisation potential of the ions producing the NII, SIII and NeV emission. The
infrared emission lines arise partly in the Narrow Line Region (NLR)
photoionised by the AGN central engine, partly in HII regions photo ionised by
hot stars and partly in neutral gas in photo-dissociated regions (PDRs). We
attempt to separate the contributions to the line emission produced in these
different regions by comparing our emission line ratios to empirical and
theoretical values. In particular, we tried to separate the contribution of AGN
and star formation by using a combination of Spitzer and Herschel lines, and we
found that, besides the well known mid-IR line ratios, the mixed mid-IR/far-IR
line ratio of OIII88um/OIV26um can reliably discriminate the two emission
regimes, while the far-IR line ratio of CII157um/OI63um is only able to mildly
separate the two regimes. By comparing the observed CII157um/NII205um ratio
with photoionisation models, we also found that most of the CII emission in the
galaxies we examined is due to PDRs.
