Ml. Luhman et al., Infrared Space Observatory measurements of a [C II] 158 micron line deficit in ultraluminous infrared galaxies, ASTROPHYS J, 504(1), 1998, pp. L11-L15
We report measurements of the [C II] 157.74 mu m fine-structure line in a s
ample of seven ultraluminous infrared galaxies (ULIGs) (L-1R > 10(12) L-.)
with the Long Wavelength Spectrometer on the Infrared Space Observatory. Th
e [C II] line is an important coolant in galaxies and arises in interstella
r 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 lumin
osity. 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 f
ar-infrared continuum, the [C II] flux from the observed ULIGs is similar t
o 10% of that seen from nearby normal and starburst galaxies. We discuss po
ssible causes for the [C II] deficit, namely (1) self-absorbed or optically
thick [C II] emission, (2) saturation of the [C II] emission in photodisso
ciated 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 simi
lar 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-seq
uence stars. As nearby examples of colliding galaxies, ULIGs may resemble h
igh-redshift protogalaxies in both morphology and spectral behavior. If tru
e, 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 trac
er of protogalaxies.