Far-infrared Fine-structure Line Diagnostics of Ultraluminous Infrared Galaxies

We present Herschel observations of six fine-structure lines in 25 Ultraluminous Infrared Galaxies at z < 0.27. The lines, [O III]52μm, [N III]57μm, [O I]63μm, [N II]122μm, [O I]145μm, and [C II]158μm, are mostly single gaussians with widths <600 km s and luminosities of 10 − 10L⊙. There are deficits in the [O I]63/LIR, [N II]/LIR, [O I]145/LIR, and [C II]/LIR ratios compared to lower luminosity systems. The majority of the line deficits are consistent with dustier H II regions, but part of the [C II] deficit may arise from an additional mechanism, plausibly charged dust grains. This is consistent with some of the [C II] originating from PDRs or the ISM. We derive relations between far-IR line luminosities and both IR luminosity and star formation rate. We find that [N II] and both [O I] lines are good tracers of IR luminosity and star formation rate. In contrast, [C II] is a poor tracer of IR luminosity and star formation rate, and does not improve as a tracer of either quantity if the [C II] deficit is accounted for. The continuum luminosity densities also correlate with IR luminosity and star formation rate. We derive ranges for the gas density and ultraviolet radiation intensity of 10 < n < 10 and 10 < G0 < 10 , respectively. These ranges depend on optical type, the importance of star formation, and merger stage. We do not find relationships between far-IR line properties and several other parameters; AGN activity, merger stage, mid-IR excitation, and SMBH mass. We conclude that these far-IR lines arise from gas heated by starlight, and that they are not strongly influenced by AGN activity. Subject headings: galaxies: starburst – infrared: galaxies – galaxies: evolution – galaxies: active