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The observed faintness of infrared fine-structure line emission along withthe warm far-infrared (FIR) colors of ultraluminous infrared galaxies (ULIRGs)is a long-standing problem. In this work, we calculate the line and continuumproperties of a cloud exposed to an Active Galactic Nucleus (AGN) and starburstspectral energy distribution (SED). We use an integrated modeling approach,predicting the spectrum of ionized, atomic, and molecular environments inpressure equilibrium. We find that the effects of high ratios of impingingionizing radiation density to particle density (i.e. high ionizationparameters, or U) can reproduce many ULIRG observational characteristics.Physically, as U increases, the fraction of UV photons absorbed by dustincreases, corresponding to fewer photons available to photoionize and heat thegas, producing what is known as a "dust-bounded" nebula. We show that high Ueffects can explain the "[C II] deficit", the ~1 dex drop in the [C II] 158micron /FIR ratio seen in ULIRGs when compared to starburst or normal galaxies.Additionally, by increasing U through increasing the ionizing photon flux,warmer dust and thus higher IRAS F(60)/F(100) ratios result. High U effectsalso predict an increase in [O I]63 micron /[C II] 158 micron and a gradualdecline in [O III] 88 micron /FIR, similar to the magnitude of the trendsobserved, and yield a reasonable fit to [Ne V]14 micron /FIR ratio AGNobservations.

Dust-Bounded ULIRGs? Model Predictions for Infrared Spectroscopic Surveys