Infrared Space Observatory measurements of a [C II] 158 micron line deficit in ultraluminous infrared galaxies

We report measurements of the [C II] 157.74 mu m fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) (L-1R > 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 (hv greater than or equal to 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 mu m 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 similar to 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 (much greater than 3 x 10(3) cm(-3)) or with a high ratio of incident W flux G(o) to n (G(o)/n greater than or similar to 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.