The [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited
NAVAL RESEARCH LAB WASHINGTON DC REMOTE SENSING DIV
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We present a study of the C II 157.74-micrometer fine-structure line in a sample of 15 ultraluminous infrared IR galaxies using the Long Wavelength Spectrometer LWS on the Infrared Space Observatory ISO. We confirm the observed order of magnitude deficit compared to normal and starburst galaxies in the strength of the C II line relative to the far-infrared FIR dust continuum emission found in our initial report, but here with a sample that is twice as large. This result suggests that the deficit is a general phenomenon affecting 4 out of 5 ULIRGs. We present an analysis using observations of generally acknowledged photodissociation region PDR tracers, which suggests that a high ultraviolet flux incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including, and 6.2-micrometer polycyclic aromatic hydrocarbon PAH emission, suggest that high PDRs alone cannot produce a self-consistent solution that is compatible with all of the observations. We propose that non-PDR contributions to the FIR continuum can explain the apparent C II deficiency.