2009ApJ...701.1147A

The observed faintness of infrared fine-structure line emission along with the warm far-infrared (FIR) colors of ultraluminous infrared galaxies (ULIRGs) is a long-standing problem. In this work, we calculate the line and continuum properties of a cloud exposed to an active galactic nucleus (AGN) and starburst spectral energy distribution. We use an integrated modeling approach, predicting the spectrum of ionized, atomic, and molecular environments in pressure equilibrium. We find that the effects of high ratios of impinging ionizing radiation density to particle density (i.e., high-ionization parameters, or U) can reproduce many ULIRG observational characteristics. Physically, as U increases, the fraction of UV photons absorbed by dust increases, corresponding to fewer photons available to photoionize and heat the gas, producing what is known as a "dust-bounded" nebula. We show that high-U effects can explain the "[C II] deficit," the ∼1 dex drop in the [C II] 158 µm/FIR ratio seen in ULIRGs when compared with starburst or normal galaxies. Additionally, by increasing U through increasing the ionizing photon flux, warmer dust and thus higher IRAS F(60 µm)/F(100 µm) ratios result. High-U effects also predict an increase in [O I] 63 µm/[C II] 158 µm and a gradual decline in [O III] 88 µm/FIR, similar to the magnitude of the trends observed, and yield a reasonable fit to [Ne V] 14 µm/FIR ratio AGN observations.