FAR-INFRARED FINE-STRUCTURE LINE DIAGNOSTICS OF ULTRALUMINOUS INFRARED GALAXIES

Abstract

We present Herschel observations of six fine-structure lines in 25\nUltraluminous Infrared Galaxies at z<0.27. The lines, [O III]52, [N III]57, [O\nI]63, [N II]122, [O I]145, and [C II]158, are mostly single gaussians with\nwidths <600 km s-1 and luminosities of 10^7 - 10^9 Solar. There are deficits in\nthe [O I]63/L_IR, [N II]/L_IR, [O I]145/L_IR, and [C II]/L_IR ratios compared\nto lower luminosity systems. The majority of the line deficits are consistent\nwith dustier H II regions, but part of the [C II] deficit may arise from an\nadditional mechanism, plausibly charged dust grains. This is consistent with\nsome of the [C II] originating from PDRs or the ISM. We derive relations\nbetween far-IR line luminosities and both IR luminosity and star formation\nrate. We find that [N II] and both [O I] lines are good tracers of IR\nluminosity and star formation rate. In contrast, [C II] is a poor tracer of IR\nluminosity and star formation rate, and does not improve as a tracer of either\nquantity if the [C II] deficit is accounted for. The continuum luminosity\ndensities also correlate with IR luminosity and star formation rate. We derive\nranges for the gas density and ultraviolet radiation intensity of 10^1 < n <\n10^2.5 and 10^2.2 < G_0 < 10^3.6, respectively. These ranges depend on optical\ntype, the importance of star formation, and merger stage. We do not find\nrelationships between far-IR line properties and several other parameters; AGN\nactivity, merger stage, mid-IR excitation, and SMBH mass. We conclude that\nthese far-IR lines arise from gas heated by starlight, and that they are not\nstrongly influenced by AGN activity.\n