[ITAL]Infrared Space Observatory[/ITAL] Measurements of a [C [CSC]ii[/CSC]] 158 Micron Line Deficitin Ultraluminous Infrared Galaxies

We report measurements of the [C II] 157.74 μm fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) ( L IR > 10 12 L ☉ ) with the Long Wavelength Spectrometer on the Infrared Space Observatory . The [C II] line is an important coolant in galaxies and arises in interstellar gas exposed to far-ultraviolet photons ( h ν≥11.26 eV); in ULIGs, this radiation stems from the bursts of star formation and/or from the active galactic nuclei that power the tremendous infrared luminosity. The [C II] 158 μm line is detected in four of the seven ULIGs; the absolute line flux (about a few times 10 -20 W cm -2 ) represents some of the faintest extragalactic[C II] emission yet observed. Relative to the far-infrared continuum, the [C II] flux from the observed ULIGs is ~10% of that seen from nearby normal and starburst galaxies. We discuss possible causes for the [C II] deficit, namely (1) self-absorbed or optically thick [C II] emission, (2) saturation of the [C II] emission in photodissociated gas with high gas density n (≫3 × 10 3 cm -3 ) or with a high ratio of incident UV flux G 0 to n ( G 0 / n ≳ 10 cm 3 ), or (3) the presence of a soft ultraviolet radiation field caused, for example, by a stellar population deficient in massive main-sequence stars. As nearby examples of colliding galaxies, ULIGs may resemble high-redshift protogalaxies in both morphology and spectral behavior. If true, the suggested [C II] deficit in ULIGs poses limitations on the detection rate of high- z sources and on the usefulness of [C II] as an eventual tracer of protogalaxies.