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 microns fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) 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; 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 micronline is detected in four of the seven ULIGs; the absolute line flux (about a few times 10(exp -20) W cm(expo -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 approx. 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 or with a high ratio of incident UV flux G(sub 0) to n 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.
Prepared in collaboration with NASA Goddard Space Flight Center, Greenbelt, MD; Department of Astronomy, University of Maryland, College Park, MD; Department of Physics, Towson University, Towson, MD; Institut d' Astrophysique Spatiale, Universite de Paris, France; IPAC, California Institute of Technology, Pasadena, CA; Harvard-Smithsonian Center for Astrophysics, Cambridge, MA; Astronomy Department, Cornell University, Ithaca, NY; and Queen and Mary Westfield College, University of London, London, UK. Published in The Astrophysical Journal, v504, pLL11-L15, 1 Sep 1998.