Infrared Space Observatory Measurements of a [C II] 158 micron Line Deficit in Ultraluminous Infrared Galaxies
NAVAL RESEARCH LAB WASHINGTON DC REMOTE SENSING DIV
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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 andor 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 10exp -20 W cmexpo -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 Gsub 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.