The [C II] 158 micron line deficit in ultraluminous infrared galaxies revisited

We present a study of the [ C II] 157.74 mum fine- structure line in a sample of 15 ultraluminous infrared ( IR) galaxies ( IR luminosity L-IR greater than or equal to 10(12) L.; ULIRGs) 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 ([ C ii], [ O i] 63 and 145 mum, and FIR continuum emission), which suggests that a high ultraviolet flux G(0) incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including CO ( 1 - 0), [ C i] ( 1 - 0), and 6.2 mum polycyclic aromatic hydrocarbon ( PAH) emission, suggest that high G(0) = n 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. Here, unusually high G(0) and/ or n physical conditions in ULIRGs as compared to those in normal and starburst galaxies are not required to explain the [ C ii] deficit. Dust- bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [ C ii] emission, offer one possible physical origin for this additional nonPDR component. Such environments may also contribute to the observed suppression of FIR fine- structure emission from ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The implications for observations at higher redshifts are also revisited.